Iano's backfill

Complexities of Sustainability

LEED as the Definition of Sustainability
Can LEED Survive the Carbon-Neutral Era (Metropolis, November 2007) discusses the growing acceptance of the US Green Building Council LEED rating system while also considering future challenges to its relevance:

• USGBC claims 40,000 LEED-accredited professionals. The organization has certified only roughly 1000 buildings since its inception.
• A recent study by construction consultant Davis Langdon claims that LEED-certified buildings, at least up to the Gold certification level, need not cost more than conventionally designed buildings. Fiona Cousins of ARUP New York estimates it can cost $100,000 in service fees to document building performance for LEED.
• Until June of this year, buildings could achieve LEED certification without receiving any energy performance points associated with reductions in carbon emissions.
• ASHRAE, in conjunction with USGBC and others, is promoting its Advanced Energy Design Guides which target energy savings of 30% over current national  standards. ASHRAE intends to introduce similar mandatory standards by the year 2012. ASHRAE and the AIA are proposing national legislation that would require new buildings to be fully climate-neutral by the year 2020.

The article also discusses the pros and cons of the LEED "checklist" methodology for defining sustainability, in contrast to more ntegrated approaches to sustainable building design. And the article speculates on the possibility of LEED's broad definition of sustainability, which includes considerations of site and community development, materials and resources, and indoor air quality, being preempted in the future by the need to focus more narrowly on the conservation of water and energy.

Separately, The Battle for Green Building (Springfield Business Journal, 12/11/20067) discusses the Green Building Initiative's Green Globes sustainable building certification program, an alternative to the better know LEED. Though there are many similarities between these two programs, Green Globes is reportedly distinguished by its pending certification by the American National Standards Institute, and its lower implementation cost than LEED.

PVC as a Sustainable Material
The USGBC's February 2007 Assessment of the Technical Basis for a PVC-Related Materials Credit for LEED is the that organization's final report on the contentious issue of the use of PVC materials in building construction.

USGBC has been considering this issue since at least the year 2000. The Assessment looks at four common PVC applications: siding, drain/waste/vent piping, resilient flooring, and window frames. Each is compared with common alternatives, for example in the case of siding, with aluminum, wood, and fiber-cement. Materials are evaluated on a number of bases:

• Conventional life-cycle assessment in which all the resource and pollution inputs and outputs associated with the material-- beginning with its harvesting or extraction and ending with its reuse or disposal at the end of its service life--are considered. Impacts on both human health and the environment are included.
• Extended end-of-life analysis in which potential PVC dioxin emissions from backyard burning and accidental landfill fires are considered. Given the large uncertainties in the data for this scenario, upper, middle, and lower range estimates were evaluated.
• Risk assessments of the adverse human health effects due to exposure to toxic compounds generated throughout the life cycle of the materials.

The conclusion: No single material shows up as the best across all human health and environmental impact categories, nor as the worst.

The assessment's results resist simplistic conclusions. Material rankings vary depending on how environmental and human health impacts are prioritized. In other words, a choice of one material over another may benefit human health while increasing adverse effects for the environment, or vice-versa. Rankings also vary with the product category. Only in the resilient flooring category do PVC products rank consistently higher in both adverse human health and environmental effects than alternative materials (linoleum and cork).

Meanwhile, Schwarzenegger Bans PVC Additive In Toys (Healthy Building Network, October 25, 2007) reports that, despite claims made by the Vinyl Institute regarding the safety of PVC in children's toys, the state of California has passed legislation prohibiting the use of phthalates, a PVC plasticizer, in products intended for babies and children under three years of age. The article goes on to state:

Like the human carcinogens vinyl chloride and dioxin, phthalates are uniquely associated with PVC. It is this triple threat from PVC that distinguishes it as the worst plastic for environmental health and green building. Regrettably, there are still few restrictions on the use of vinyl in green buildings.

Evolving Measures of Material Sustainability
Shedding Light On The Pharos Project (Eco-Structure, December 2007) describes the Pharos Project, an ambitious building products rating program under development by the Healthy Building Network.

The Pharos Project is touted as a database of building materials intended to allow a more comprehensive and sophisticated evaluation of the sustainable attributes of materials than is currently offered by other rating systems. Its unique framework covers a broad range of health, environmental sustainability, and social justice criteria.The Project will also host a Wiki and online forums.

A visit to the Project's web site leaves one questioning whether the Project is alive and well. The most recently dated content appears to be from November of 2006. According to Eco-Structure, the next working version of the Project is scheduled for release in the spring of 2008.

Measures of Sustainable Buildings
Energy Performance Data Largely Lacking (ENR, November 12, 2007) reports that, despite the attention being given to green building design, there is a lack of standards for collecting and analyzing building energy performance data, and, though newer buildings may be designed to be more energy efficient than older buildings, building energy use overall continues to climb:

• According to the U.S. Department of Energy, commercial buildings consumed 18 quads (18 quadrillion BTUs) in 2004, and are projected to consume 25 quads--almost a 40 percent increase--by the year 2030. The largest part of this jump is attributed to increased use of electrical equipment and the increased cooling loads that result.
• Between 1980 and 2000, energy use per square foot in commercial buildings increased by roughly 25 percent.

LEED certification does not necessarily correlate with reduced building energy consumption. On the one hand, Seattle's LEED-Silver Alley24 mixed-used development, completed in 2006, is reportedly close to achieving a 50 percent targeted reduction in CO² emissions. On the other hand, Seattle's new City Hall, also LEED-Silver, completed in 2003, is separately reported as consuming significantly more energy than the larger, older building that it replaced.

WUFI Hygrothermic Modeling

WUFI-ORNL/IBP is a software program designed to model the dymanic movement of heat and moisture through building wall and roof assemblies. The software, developed jointly by Oak Ridge National Laboratory (ORNL) and Germany's Fraunhofer Institute of Bauphysics (IBP), is intended as a tool for researchers and building technologists to aid in the analysis and design of building envelope assemblies. This author's first impressions of the software after trying out a freely available research and education version of the program follow.

What is WUFI?

WUFI allows the user to model various building assemblies, run these assemblies through simulations of several years of typical climatic conditions for various locales (see the image at left), and analyze the performance of the assembly in terms of moisture flow, moisture accumulation, and other factors.

WUFI is a sophisticated yet relatively easy to use program. For example, it can simulate climatic conditions for different locals and account for differences in orientation of the building assembly. Assemblies themselves are modeled by selecting and arranging in the desired order components (such as sheetrock, plywood, vapor barrier sheet, etc.) from a database into which the relevant material properties have already been inputted. Once the initial conditions are established, simulations can be run and graphically observed at the push of a button. In this user's experience, an assembly could be modeled and run through a 2-year simulation in less than 5 minutes once the basic mechanics of the program have been mastered.

Some Sample Results

As an example of what WUFI can do, the results of three test scenarios are described below. All three are based on a wall modeled as follows: wood siding at the exterior, building paper, plywood sheathing, glass fiber batt insulation, and gypsum wallboard at the interior. The location is Seattle, Washington, using climate data for a relatively cold winter, with the wall assembly facing to the North. Interior conditions were set to moderate levels of relative humidity. The three scenarios differ in the placement of a polyethylene sheet vapor retarder, either close to the inside of the assembly, close to the outside of the assembly, or not included at all:

Scenario 1: Vapor retarder sheet behind the gypsum wallboard (close to interior, warm side of assembly). This is the conventionally "correct" location for a vapor retarder in the Seattle climate. To the left is a screen shot of the graphic results of the model after a 2-year simulation (click on the image to view a larger version). The exterior side of the assembly is to the left in the charts. The top chart records temperature data. The lower chart records relative humidity and moisture content. For example, the wide green band represents the range of relative humidity conditions encountered in various parts of the wall assembly throughout the two-year simulation. (Results are also presented by the program in tabulated and other graphic formats.)

Scenario 2: In this case, the vapor retarder sheet is (incorrectly) located on the exterior side of the exterior sheathing. In this configuration the vapor retarder is expected to trap condensed moisture in the plywood sheathing. Note the lower blue curve in the lower chart, representing moisture accumulation. As expected, in comparison to scenario 1, this configuration results in greater quantities of moisture accumulated in the exterior sheathing.

Scenario 3: No vapor retarder. In this scenario, the levels of moisture accumulated in the plywood sheathing fall in-between the two previous scenarios. Referring to the tabulated results provided by the software, water accumulated in the plywood sheathing for each of the scenarios is as follows: Scenario 1: 3.6 lb of water per cu. ft. of plywood Scenario 2: 8.4 lb / cu. ft. Scenario 3: 7.6 lb / cu. ft.

Interpreting WUFI Results

The WUFI anayses offer some interesting insights, and also raise additional questions.

For starters, the results seem to agree with conventional wisdom. That is, in a northern climate, a vapor retarder located toward the warm side (interior) of the assembly is beneficial in minimizing moisture accumulation in the wall: In the simulations above, Scenario 1 results in less than half the moisture accumulation of either of the other two assemblies.

A second question relates to the form of the data provided by the simulations. For example, is 3.6 lb of water per cu. ft. of plywood OK? How about 7.6 or 8.4 lb per cu. ft? To answer this question, one would have to convert these numbers to obtain moisture content as a percentage of the wood's oven dry weight, and then compare this to established standards which suggest that wood kept above a moisture content of approximately 15% to 20% is at risk of mold growth and decay. It would also be necessary to look at the moisture content data over time. For example, an occasional excess of moisture might be OK, but a long period of continuous excess moisture might not be.

On another point, this author was somewhat surprised by the results of Scenario 3 (no vapor retarder), which were almost as bad as Scenario 2. In the Seattle region, there is some question among the building technology community as to whether the use of vapor retarders is beneficial in this climate or not. The results of the Scenario 3 simulation seem to indicate that the absence of a vapor retarder is almost as detrimental as deliberately placing the retarder in the wrong part of the assembly. So the WUFI results seem somewhat in conflict with local practice.

Finally, there are some aspects of building assembly behavior that the WUFI program does not address at all. Two important ones are leakage of water due to construction defects and air flow through the assembly. In the real world, these two phenomena may very well be the most severe sources of moisture that an assembly will encounter, potentially contributing an order of magnitude more moisture to the assembly than those phenomena that WUFI does simulate. If so, to what extent are WUFI simulations useful at all? Based on conversations with other building scientists, the answer to this question seems to be that with proper use of the software, these effects can be simulated, and the results of WUFI analysis can be validly applied to actual building practice.

Despite these limitations, for those interested in the science of the building envelope, WUFI appears to be a useful tool for teaching and/or analysis.

More Info

• Oak Ridge National Laboratory WUFI website
• Fraunhofer Institute of Bauphysics WUFI website
• The WUFI Forum

Keeping Masonry Walls Warm and Dry

Recently, this author was reviewing proposed details for the renovation of an historic brick masonry building located in downtown Seattle. During this review, questions were raised regarding how much insulation to add to what has been until now, a massive but uninsulated solid masonry wall system, and whether the addition of a vapor retarder membrane to the wall system would be beneficial. After some research on the topic, our conclusions were not as obvious as one might expect.

Normally, building assemblies are designed with the understanding that insulating buildings is good practice, and that higher levels of insulation are implicitly better than lower levels. Building designers are taught this in architecture and engineering school; and this understanding is reinforced by energy codes that set minimum insulation values for buildings and green building guidelines that provide incentives for even greater reductions in the energy required to condition the spaces within our buildings.

However in the case of an existing historic structure, "more insulation is better" may be too simple an answer. While it is generally true for all buildings that higher insulation values will reduce building energy consumption, in the case of older buildings, this benefit must be traded off with the potentially harmful effects that adding insulation to an existing wall system may produce.

In the case of our building under consideration, the proposal was to add rigid foam insulation on the interior side of the 12-inch thick masonry walls. (Adding insulation to the interior would preserve the historic appearance of the building exterior.) However the concern became that the more insulation added to the interior side of the wall, the lower would be the temperature of the brick during the colder months of the year. And as discussed in a previous article on this site, Air, Moisture, and the Building Envelope, keeping a wall warm can be an effective strategy for keeping a wall dry as well.

So our concern was that, up to this point in the life of the building, the exterior walls had been kept relatively warm--and dry--by the building heating system. By changing this, would we risk creating a colder wall system that could become more vulnerable to the effects of greater temperature extremes and added moisture? After some research on the subject, our decision was to apply just one inch of rigid foam insulation (with an insulation value of R-5) to the interior side of the walls. Essentially this strategy aims to balance the greatest possible reduction in energy use with the least reduction in exterior wall temperatures.

Additionally, we decided to add a polyethylene sheet membrane vapor retarder to the interior side of the wall assembly. The intent of adding this component was to reduce moisture movement into the wall system from the building interior, both by water vapor difusion and by the direct passage of moisture-laden air. While the benefits of vapor retarder membranes in a relatively mild climate such as Seattle can be debated (such membranes can reduce wall drying in the warmer months of year), our judgement was that the overall effect would be a net positive.

More Info
Loadbearing masonry construction is discussed in Chapter 10 Masonry Loadbearing Wall Construction of the textbook.
In researching this topic, this author found Rehabilitation of Solid Masonry Walls (National Research Council of Canada) particularly informative. This Council's Rehabilitation of Masonry Assemblies page has a variety of additional articles and references.

Sustainability And The Highrise

Sustainability and highrise construction appears as a topic in a number of recent publications.

Innovation, a November 2004 supplement to Architectural Record magazine, is devoted to highrise design and includes considerations of sustainability from a variety of perspectives:

In lead editorial Aiming High, Robert Ivy discusses Norman Foster and Partner's new Swiss Re building (also known as 30 St. Mary Axe). Foster's building uses a dual-glazed skin to convert convective air flow into power, heat, and lighting for the building interior. As air within the curtainwall sandwich is warmed by solar heat gain and rises, it is captured by various mechanical systems and then turned into useful energy and heating.

In Do skyscrapers still make sense? the relationship of highrises to revived downtowns and the urban business environment is discussed:

_Three years after the attacks on the World Trade Center Towers, financial firms for the most part have decided to remain located in New York City's financial district, despite the costs. The benefits of the "human network"--face-to-face, close, personal interaction--outweigh the location's higher costs and risks.

_European cities are losing their traditional resistance to highrise development, recognizing the amenity and economic efficiency this building type can offer. London's new towers are characterized as modestly proportioned so as to minimize casting shadows on neighboring areas, and sustainably designed with daylighting, informal spaces for impromptu meetings, and individually controlled natural ventilation.

_Holistic approaches to building physics, more common in Europe than the US, use air buoyancy effects, diurnal temperature cycles, natural ventilation, breathable skins, localized control of heating and cooling, and passive shading to strategically reduce building energy requirements. Systems that rely less on mass air movement than traditional HVAC approaches can also result in reduced floor-to-floor heights bringing additional economies.

The thrust of this article is perhaps best summed up by Craig Schwitter, structural engineer with Buro Happold in New York, quoted as saying, "I don't believe the challenges are in making towers bigger, but more livable."

In Green grows up...and up and up and up the recent surge in design of green highrises for New York City is discussed, including projects such as:

• _Renzo Piano Building Workshop and Fox & Fowle Architect's New York Times Tower
• _Cook + Fox Architect's One Bryan Park
• _Foster and Partner's Hearst Tower
• _SOM's Freedom Tower (in regard to the proposed wind turbines)
• _Cesar Pelli & Associate's  211 Murray Street residential building

Numerous sustainable design strategies are discussed, such as passive solar screens, daylighting, rainwater collection and reuse, underfloor air distribution, cogeneration, photovoltaics, adaptive reuse of existing building shells, slab-integrated cooling and heating, and more.

Building Safety Bulletin's High-Efficiency High-Rise Breaks Ground (November 2004) discusses New York City's Bank of America Tower, the first US highrise aiming for USGBC Platinum LEED rating. Sustainable technologies include double-wall skin, translucent insulating glazing, natural daylighting, automatic dimming of artificial lighting, planted green roof, on site cogeneration, ice-based thermal storage, underfloor HVAC with individual floor air handling units, and more.

New Yorker Magazine's Green Manhattan (October 18, 2004) is a highly readable article espousing the green benefits of living in Manhattan. The short story: If Manhattan were the fifty-first state, it would be the country's 12th largest based on population, but its energy use on a per capita basis would be the lowest of all. Read the article to learn more :)

New Moisture Barrier Products

In response to the design industry's increased awareness of the risks of mold growth in buildings, manufacturers are responding with new or updated products claiming improved performance at preventing the accumulation of moisture within building assemblies.

Breathable Waterproof Membrane
Henry Company's Blueskin Breather membrane: This self-adhering bituminous membrane is unusual in claiming both legitimate waterproofing capabilities (as opposed to water repellency or dampproofing) and high vapor permeability (37 perms). This company's web site also offers some well-presented background information on the design and performance of the building envelope.

High-Performance Building Wraps
Proctor Group's VaproShield building wrap/underlayment material: This woven high-density polypropylene fabric is water-resistant and has exceptionally high permeability. For example its "WallShield" product lists a vapor transmission rating of 212 perms. (In comparison, traditional 15-lb building felt has a perm rating of approximately 3, and Tyvek, a popular proprietary house wrap has perm rating of 50.

Pactiv Corporation's Raindrop Housewrap: This woven, non-perforated fabric is slightly thicker than 1/8-inch and incorporates closely-spaced vertical drainage channels creating a drainage plane facilitating the removal of water from behind cladding or siding.

Benjamine Obdyke's Home Slicker: This 1/4-inch thick, 3-dimensional nylon matting is another product used to create a drainage plane behind cladding or siding. The company's Home Slicker Plus Typar product provides the same drainage mat material prebonded to a commercial grade housewrap product.

Caution Advised
The topic of moisture movement into and out of the wall assembly is not a simple one. For an introduction to key aspects of this topic, see this site's previous article Air, Moisture, and the Building Envelope. Designers and specifiers are also advised to approach with caution manufacturers' claims for the superior benefits of any particular product. Leakage of the building envelope remains one of the highest risk areas for design liability and thorough research is recommended when designing a specifying such systems.

More Information
_Housewraps and underlayments are discussed on page 206 of the textbook. The role of vapor migration in wall and ceiling assemblies is discussed on pages 604 - 606.

NY Time's Full-Scale Lighting Tests

A Day In The Light, Metropolis, May 2004, describes the New York Time's full-scale mockup of a portion of a typical floor of their planned 51-story Manhattan high rise. The mockup is being used to evaluate numerous aspects of the design, including:

• office furnishings
• lighting and controls
• constructability
• user reactions to the space.

Evaluating lighting strategies is one of the mockup's primary and most interesting purposes. The 4300 sf mockup simulates the southwest corner of a typical office floor, the quadrant most affected by exposure to direct sunlight. Daylight control is accomplished with a combination of exterior shading elements and computer controlled motorized shades. Two control systems are being evaluated. One responds to ambient exterior lighting conditions analyzed in combination with a database of seasonal and time of day lighting information. The second system monitors and responds to interior lighting levels. Each of these daylight control systems is paired with one of two separate systems for controlling interior artificial light levels. Lighting data from the mockup is being recorded at Lawrence Berkeley National Laboratories (LBNL) every 60 seconds around the clock, and the tests are being run from winter solstice to summer solstice to capture the fullest range of solar exposure conditions.

Since the mockup represents only one-sixth of a typical floor plate, lighting conditions for the remainder of the floor are being modeled in computer using LBNL's Radiance software. By synchronizing the computer model to the data coming from the mockup, results can be generated for the remainder of the floor with a high degree of confidence. Human factors surveys are also a part of the testing protocol.

Designed by Renzo Piano, the building will be located on Eight Avenue, between 40th and 41st Streets, in New York City.

More Info
Radiance software

Air, Moisture, and the Building Envelope

Perhaps one of the most debated aspects of building performance is the behavior and control of air and moisture moving through the envelope of a building. Most designers and builders have at least a rough appreciation of the vapor barrier and its role in limiting condensation within the exterior wall. However debates over making the envelope too tight, or not tight enough frequently recur. And few in the industry are prepared to speak comprehensively regarding the various ways in which moisture may move through a wall, and the various roles played by air barriers, vapor retarders, and moisture barriers in providing an energy efficient, dry, and healthful building envelope.

One of the better discussions of these issues this author has recently came across on this topic is Building Science Corporation's Insulation, Sheathings and Vapor Diffusion Retarders. For teachers, students, and professionals interested in gaining a better understanding of these issues, this is recommended reading.

For starters, the following are some important points to keep in mind when considering these issues. The following assumes the reader is already familiar with the basic concepts of water vapor and condensation, and their interaction with insulation and vapor retarders in building walls. A review of these topics can be found in the textbook on pages 604 through 606.

Moisture Vapor
Moisture vapor is water in gas form--in other words, humidity. There are two main ways that moisture vapor can be transported through an exterior building wall, diffusion and air transport. Diffusion occurs in the direction from areas of greater vapor pressure to areas of lower vapor pressure, that is, from the warmer more humid side of the enclosure toward the cooler, dryer side. Diffusion can occur across an exterior wall even when the wall is perfectly air tight. The water vapor molecules can essentially slip directly through the various wall materials to reach the side of lower vapor pressure.

Water vapor can also be transported through an exterior wall by air transport. In this case, as air leaks through small gaps in the envelope construction, water vapor is carried along for the ride. Air transport occurs in the direction of high air pressure to low air pressure. For example, often the living areas of residential structures conditioned with conventional forced air heating are maintained at a slight positive pressure in relation to the exterior. In this case, air transport through gaps in the building envelope carries moisture from the interior through the wall assembly to the exterior. Note that it is not always the case that air transport and vapor diffusion will work in the same direction. It is possible for air transport and vapor diffusion to move moisture vapor in opposite directions simultaneously.

Condensation
With regard to transport of moisture vapor, the main concern is condensation. If moisture vapor moves by either diffusion or air transport to a condition where the relative humidity of the air reaches 100%, the vapor will condense from gas to liquid form. Think of your own exhaled breath in the winter. Warm air from your lungs is transported to the exterior, and the moisture vapor comes along for the ride. As this air cools, it reaches 100% relative humidity, and the excess moisture that the air can no longer hold in gas form condenses and forms fog. Likewise in a building, if moisture vapor moving across a wall assembly reaches a condition of 100% relative humidity (the dew point), condensation occurs and liquid moisture will collect at that point within the wall assembly.

Vapor Retarders
The role of a vapor retarder in an exterior assembly is to minimize the diffusion of water vapor through the assembly, and thereby minimize the risk of condensation within the assembly. Vapor retarders should always be located on the side of the assembly with higher vapor pressure. Typically this is the warm side of the wall, meaning the interior in northern climates, and the exterior in hot humid climates. With a careful analysis of relative humidity and temperatures across a wall assembly, it is also sometimes possible to determine locations partially within the depth of the assembly that may be acceptable locations for vapor retarders.

Vapor retarders are most needed where the vapor pressure differential across the assembly is high. Normal-use buildings in mild climates might not need any vapor retarder at all. However in more extreme cold or hot/humid climates, or in special use buildings with unusually higher interior moisture levels (such as natatoriums), vapor retarders are needed.

Air Barriers
As noted previously, moisture vapor can also move through an assembly by air transport, rather than diffusion. In residential construction, the roles of air barrier and vapor barrier are often performed by the same component within the wall, for example a sheet of polyethylene plastic installed behind the interior sheetrock. In this case the plastic acts both as an air seal by eliminating air gaps between framing and other components, and as a vapor barrier by its own ability to resist diffusion of vapor through the plastic material itself. However, air barriers and vapor retarders need not necessarily be located in the same location with the assembly. For example, in commercial construction, it is frequently easier to create an effective air barrier close to the exterior of a wall assembly, while vapor diffusion control may still need to be handled closer to the relatively warm, interior side. In a situation such as this, the air barrier and vapor retarder may be two separate components in the assembly.

Another important point is that where vapor condensation is a concern, air transport frequently has the potential for moving significantly more moisture through the assembly than vapor diffusion. In others words, eliminating air gaps in an exterior assembly may be much more important than maintaining a perfect vapor retarder. For example, a plastic sheet vapor retarder that is 95% complete may perform satisfactorily as a vapor retarder, but the 5% gaps may be sufficient to allow significant amounts of moisture to move through the wall due to air transport--thus the importance of carefully sealing seams and penetrations in the barrier at edges of the sheet, at electrical junction boxes, and at other points of potential air leakage.

Air Pressure
Vapor migration due to air transport can also be minimized by management of air pressure differentials. For example, in an interior swimming pool, with high interior humidity, the air handling system can be adjusted to create a slight negative pressure within the space relative to the outdoors. In this situation, where gaps due occur in the air barrier, relatively dry outdoor air will be drawn into the space without the risk of condensation within the wall, rather than humid air being driven out.

Wall Drying
Recently, this author has encountered more discussion of giving exterior walls the ability to dry out or expel moisture that does get into the assembly. In this regard, one important point to remember is that it is beneficial for a wall to be vapor permeable on at least one side, preferably the side with typically lower vapor pressure. Thus, if a vapor retarder is installed at the interior, building wraps or building paper used at the exterior should be relatively permeable to vapor diffusion. As another example, in hot/humid climates, where a vapor barrier may be located close to the exterior, impermeable interior materials, such as vinyl wallpaper, should definitely be avoided.

Another variation on this idea is vapor retarder or air barrier membranes that can adjust their permeability depending on moisture conditions within the wall. In theory, barrier materials that reduce their permeability when significant moisture accumulates in an assembly can enhance drying. See for example, the separate article on this site, Variable Perm Vapor Retarder.

Too Tight?
So have buildings become "too tight"? In this author's opinion, the answer to this question depends on what criteria are applied. Consider first, a traditional older home, uninsulated and with little in the way of protection against air leakage. Many such houses survive to this date with minimal evidence of damage due to moisture accumulation. So clearly, structures permeable to vapor diffusion and air movement can perform successfully at least under some circumstances.

What might explain the survival of such loosely sealed buildings? First, keep in mind that many old buildings that did experience significant water damage within the structure are no longer with us--they failed long ago. So when we look at old buildings today, we are seeing only the survivors, the better half of the lot. But discounting this consideration, it is also easy to argue that one great way to keep exterior assemblies dry and to minimize water damage is to blow heated air through these assemblies. Without much insulation in the walls, the risks of condensation or water accumulation are minimized. Instead, streaming warm air becomes an effective mechanism for removing moisture that may accumulate. And that is what happens in poorly insulated, loosely sealed buildings. As relatively large volumes of heated air flow through the assembly, moisture that arrives in the wall is easily removed. Of course there is a down-side to this solution and it is energy use. Uninsultated, loosely sealed buildings are expensive to operate from an energy-use perspective.

So we insulate and seal buildings in order not to throw away energy. And this does raise new concerns, among them indoor air quality and moisture accumulation within exterior assemblies. In this case the solution is not to unseal our buildings. The solution is to learn how to build with materials and assemblies that protect the interior air quality and effectively manage the movement of air and moisture through the structure.

More Info
Air barrier concepts often seem to be the least well understood. For one good discussion of approaches to air barriers in residential construction, see Air Sealing / Air Drywall Approach Details.
For energy conservation benefits of air-barrier use, see NIST's Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use.

Wind Loads, Part II: Component Wind Load Calculations

The first article in this series discussed some basic aspects of the 2003 International Building Code's requirements for wind load design. This article discusses several issues of note that arose for this author in the course of doing preliminary component wind load calculations for a project in the Midwest US that falls under the requirements of the 2003 IBC.

Caveat
This author is not a structural engineer, and the information provided in this series of articles is not intended as definitive instruction for those who need to make such calculations. For those who do, proper training or consultation with a qualified professional is advised.

Design Method
Using the IBC Simplified wind load method, for Components and cladding, Section 1609.6.1.2, appears on the surface as straightforward. Numbers are read from three separate tables and then multiplied together to obtain the design wind pressure for a given component:

• Adjustment factor for building height and exposure (Table 1609.6.2.1(4) accounts for differences in building height and in a building's exposure to winds. As one would expect, adjustment factors are greater for taller buildings, and for buildings less protected from the wind by surrounding topography.
• The importance factor (Table 1604.5) adjusts calculated wind loads according to the type of building occupancy. Buildings whose occupancies warrant a greater degree of survivability or protection, such as emergency centers, hospitals, high occupancy public buildings, and the like, are assigned a greater importance factor than buildings assumed to present a lower degree of hazard to occupants. For example the importance factor for a hospital is 1.15, whereas the importance factor for a residence is 1.
• The net design wind pressure table (Table 1609.6.2.1(2)) tabulates design pressures for a baseline building--one that is 30-feet tall, in Exposure B, with an Importance Factor of 1.

By reading from the net design wind pressure table, and adjusting for height, exposure, and importance, a final design pressure for the component in question can be determined. However reading from the net design wind pressure table requires several additional pieces of information that deserve additional comment.

Wind speed
Reading from the net design wind pressure table requires entering the table with a basic wind speed for your project location. For starters, basic wind speed can be read from Figure 1609 of the IBC which provides wind speed contour maps for the continental United States and Alaska. Indicated wind speeds range from a low of 85 miles per hour for major portions of West Coast states, to a high of 150 miles per hour at the southern tip of Florida. Much of the middle of the continental US is assigned a basic wind speed of 90 mph.

Before proceeding with the information taken from the basic wind speed map, the designer should also check with local building departments. Depending on local conditions, a building department may require use of a different wind speed than indicated in this map. This is also true for portions of the wind speed map indicated as "Special Wind Regions". Topographic features in these areas, such as mountains, river valleys, or gorges, can result in wind speeds significantly higher than indicated on the wind speed map, and must be assigned locally.

Use caution if you have reason to compare wind speeds from the 2003 IBC with other codes or standards to make sure you are making an apples-to-apples comparision. The IBC uses a wind speed scale called 3-second gust wind speeds. Some older standards, for example the 1997 Uniform Building Code, measure wind speed according to a different scale called fastest mile. If you do need to compare between such scales, you can use Table 1609.3.1 in the IBC to convert between the two.

For example, under the 1997 UBC, the City of Seattle requires a basic wind speed of 80 mph (fastest mile speed). According to the IBC conversion table, this is comparable to a 100 mph basic wind speed using the 3-second gust scale. Interestingly, in comparison to the 1997 UBC wind speed map, the 2003 IBC map appears to assign lower wind speeds to much of the central region of the continental US, even when speeds have been adjusted for comparison between the two scales.

Equivalent Area
The net design wind pressure table also requires entering the table with a factor termed effective area. For those who are not familiar with this factor (as this author was not), explanation is warranted. In many cases, the effective area of a component is simply the surface area of a cladding member, or tributary area of a framing member exposed to the force of the wind. When calculating component wind loads, the design pressure varies depending on the size of the effective area under consideration.

In what may at first seem to be a counter-intuitive result, design pressures for components with larger effective areas are smaller, and conversely, design pressures for smaller components are larger. For example, assuming a 100 mph basic wind speed acting on a component with an effective area of 10 square feet, the net design pressure is -19.5 pounds per square inch. If the component's effective area increases to 500 square feet, the net design pressure drops to -14.9 psf, a reduction of almost 25%. The rationale behind this reduction in pressure is that for larger areas, the pressure spikes caused by a wind gust do not act simultaneously on all parts of the component. While at any given moment some localized areas on the component will experience maximum pressures, others will not, resulting in an overall lower average pressure across the component's surface. For components with smaller effective areas, it is more likely that the entire surface of the component may simultaneously experience a maximum intensity pressure spike, and therefore, a higher design pressure is warranted.

In some cases, determining the effective area for a given building component can become a more difficult problem than just measuring its area. One complication is that the IBC increases the effective area of components that are relatively long and narrow in their proportions (see Section 1609.2 Defintions, Effective Area). Additionally, in some cases, the appropriate choice of effective area may not be immediately obvious. For example, in the case of reinforced masonry wall, is the effective area the total surface area of the wall, only the area supported by a vertically reinforced masonry cell, or perhaps the area spanning between window or other openings? This author found the ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02, referenced below, helpful in clarifying such areas.

Next: Applying component wind load calculations to the specification of doors and windows.

More Information
For those needing to perform component wind load calculations using the IBC's simplified method, the code itself provides sufficient information to complete such calculations. For more detailed guidance, ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02 provides a comprehensive set of examples of wind load calculations for various scenarios.

Wind Loads, Part I: The International Building Code

Recently, this author had to become familiar with provisions in the 2003 International Building Code for calculating wind loads on components of the building exterior such as windows, curtainwall, roofing, etc. This article is the first in a series that outlines that code's "simplified method" for calculating such component wind loads and discusses some noteworthy ramifications of this method. This article is not intended solely or primarily for structural engineers--many others in the building profession, such as architects, specifiers, fabricators, and others may at times have need to evaluate such loads.

Caveat
This author is not a structural engineer, and the information provided in this series of articles is not intended as definitive instruction for those who need to make such calculations. For those who do, proper training or consultation with a qualified professional is advised.

Wind Loads in IBC 2003
Wind loads are covered in Section 1609 of the International Building Code (IBC). This section starts off by stating that building wind loads should be determined according to the ASCE 7 standard. Separately, in Chapter 35 References, the code provides a specific reference to ASCE 7 as ASCE 7-02 Minimum Design Loads for Buildings and Other Structures.

So what is ASCE 7? ASCE is the American Society of Civil Engineers, an organization, over 150 years old, that represents the interests of civil engineers within the United States and internationally. Among its activities, ASCE develops voluntary standards which may be adopted by regulatory groups or model building codes such as the IBC. ASCE 7 is one such standard.

Through the consensus of the engineering profession, ASCE 7 has come to be accepted as one of a few definitive standards for the determination of building structural loads. Through its reference in the IBC, and the adoption of the IBC by various local building departments or other jurisdictions, designing to the ASCE 7-02 standard may then become a regulatory requirement.

It is also important to note the "-02" designation on the IBC's reference to this standard, signifying the version of this standard. There are significant differences between versions, and using a different version of the standard, such as ASCE 7-98, would not be compliant with the requirements of the 2003 IBC.

Following the reference to ASCE 7, the IBC lists several exceptions to this standard, one of which is the code's own Section 1609.6 Simplified wind load method. It is this simplified method which is the subject of the remainder of this article.

IBC Simplifed wind load method
The IBC's simplified wind load method is also derivied from ASCE 7, but offers two potential advantages. First, all the information needed to perform wind load calculations using this method are included in the IBC code book itself. (The full ASCE 7 standard is not included in the IBC code, and designers working to that standard must obtain it separately.) Second, as the name implies, this method simplifies the process of determining wind loads. In a typical scenario, a wind load calculaton using the simplified method requires little more than looking up figures in several tables and performing a straightforward multiplication.

The simplified wind load method is not suitable for all projects. This method can only be applied to:

• Buildings with a mean roof height not exceeding 60 feet, nor the length or width of the building;
• Buildings that are fully enclosed. For example, an airplane hanger which can at times be substantially open on one side cannot use the simplified method.
• Buildings not situated on the upper half of an isolated, steep hill or slope. (This restriction is more precisely described in paragraph 1609.6.1.)

Buildings that do not fall within these limitations must refer to the referenced ASCE 7 standard for determination of wind loads.

Primary Structure vs Secondary Structure
Wind load calculations are also divided into two sections depending on the part of the structure being evaluated. Calculation of wind loads acting on the building's primary structure are covered under requirements for Main windforce-resisting systems. Calculation of wind loads acting on secondary components or cladding elements are covered under requirements for Components and cladding. This remainder of the articles in this series look only at methods for such secondary elements, not primary structure.

Next: Component Wind Load Calculations

More Information
The ASCE 7-02 standard is available in both printed and cd-rom formats.
Guide to the Use of the Wind Load Provisions of ASCE 7-02 is ASCE's own companion handbook to the ASCE 7-02 standard.

Construction Innovation, December 2003

Canada's Institute for Research In Construction December 2003 Newsletter Construction Innovation includes the following items that may be of interest to users of Fundamentals of Building Construction:

• Fire researchers develop new tool for assessing fire resistance of wall assemblies using numerical modelling describes numerical modeling techniques being applied to the prediction of structural failure of residential building assemblies exposed to fire. This model is being verified experimentally, and when completed will allow assessment of the fire-resistance of assemblies for less cost than required by traditional testing techniques. For example, the buckling of a wood stud assembly under structural loads, due to fire's effects of heat, advance of char layer, and gradual joint openings can be predicted for untested assemblies.
• Realistic fire simulations will be used in fire-safety analysis and design describes the development of improved fire load modeling for use in the prediction of fire effects. Data developed from these simulations will allow the more realistic modeling of fire characteristics such as release of heat energy, size of fire, rate of spread, yield of products of combustion, and hot gas temperatures.
• Results of IRC's material properties studies now available describes IRC's database of over 100 building materials and their properites of heat capacity, thermal conductivity, water vapor permeance, equilibrium moisture content, liquid water diffusivity, and air permeance. The influence of relative humidity, water concentration, and air pressure differentials on these properites are also considered.
• IRC will host world building congress in Toronto details the upcoming CIB World Building Congress 2004, scheduled for May 2 to 7. This event is expected to draw approximately 700 participants from 40 countries and covers various aspects of building technology and construction, such as the construction process, trends in codes and regulatory systems, security in tall buildings, fire and structural safety, indoor air quality, energy conservation, and more.

Plastic Wrap for Buildings?

The Cleverest Building Material Around, The Sustainable Metropolis, describes SmartWrap, a thin, plastic membrane material under development for use as a complete building cladding system. Product developers James Timberlake and Stephen Kieren of KierenTimberlake Associates claim the material has the ability to change color and appearance, perform the functions of shelter and control of interior climate, and provide light and electricity. The membrane's heating, lighting, information display, and energy collection and storage capabilities can all be controlled by computer, either on- or off-site.

Finally, in an apparant hat trick, the material is claimed to be 100% recyclable.

Designing with Metals: Dissimilar Metals and The Galvanic Series

The galvanic series is a list of metals arranged in order of their relative electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604 of the textbook. When two metals are in contact in the presence of moisture, their locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion. The further apart the two materials on the list, the greater the risk of corrosion. The following are some guidelines for working with dissimilar metals and interpreting the galvanic series.

Avoid contact between metals far apart on the galvanic series.
Virtually every student of building technology is taught this most basic fact about the galvanic series. However, when presented in its usual list form, the galvanic series provides only minimal guidance on judging the relative differences between metals and evaluating their potential incompatibility. A more complete picture of the compatibility of metals can be constructed when numeric values for the metals' electrical potential are attached to the list as well.

This chart lists common architectural metals along with their ranges of relative electrical potential. As with the galvanic series, metals are arranged in order of increasing potential, but in this case, the relative differences between various metal types are more readily apparent.

For example, in the chart above consider the aluminum bronze alloy group and the next metals listed directly above and below. We can see that between the aluminum bronze alloys and the brass alloys directly below (naval, red, and yellow brasses), the relative difference between these metals is small. On the other hand, the difference between aluminum bronzes and mild steel, cast iron, and wrought iron directly above is many times greater. In fact, one must read down the list nine or more metals below aluminum bronze before the electrical potential difference is comparable to moving up only to the first metals above.

With quantified potential differences between metals, the galvanic series can also be used to estimate the compatibility of different metals under varying environmental conditions using the following rules of thumb:

• In coastal, very high humidity, or other harsh environments, galvanic metal pairs should be limited to those with a potential difference no greater than 0.15 volts.
• In moderate environments, metal pairs should have a potential difference no greater than 0.25 volts.
• In environments with controlled humidity and temperature, potential differences as great as 0.50 volts may be acceptable.

For example, consider again the aluminum bronze alloy group. In a harsh environment, the designer may opt to limit metals to be used in contact with this alloy group to other bronze alloys, brasses of various types, copper, tin, and 400 series stainless steel. On the other hand, in a controlled environment, aluminum bronze might safely be combined with any other metal listed on the chart, with the exception of zinc and galvanized steel.

Based on these rules of thumb, metals listed in the chart have been color-coded into groups that fall within potential difference ranges of roughly 0.20 volts. Metals within each of these groups may be considered least corrosion prone when used together in normal architectural conditions.

Avoid smaller anodes in contact with larger cathodes.
On the chart above, the more negative end of the potential scale is noted as anodic or active, and the more positive end of the scale as cathodic or passive. When different metals react galvanically, an exchange of electrons takes place between the two metals, with electrons flowing from the metal with greater negative potential (the anode) to the metal with lesser negative potential (the cathode). For example, if aluminum bronze and a 300 series stainless steel are used together, the aluminum bronze has a greater negative potential and will act as the anode, donating electrons to the less negative stainless steel, the cathode. On the other hand, if aluminum bronze is used with mild steel, mild steel has a greater negative potential and will act as the anode, donating electrons to the aluminum bronze, which in this case acts as the cathode. (A note on terminology: Literature on galvanic reactions often refers to cathodic metals as noble. These two terms are synonymous.)

To a chemist, the anode’s release of electrons is termed oxidation. In laymen’s terms this is known as corrosion. In other words, with any galvanic pair of metals, the anode corrodes as the galvanic reaction takes place. Controlling the rate of corrosion of the anodic metal is an important consideration in working with galvanic metal pairs. After consideration of the electrical potential difference between the two metals, the next most important factor governing the rate of corrosion of the anode is the relative surface area of the anode in comparision to the cathode. The smaller the surface area of the anode in relation to the cathode, the more concentrated the flow of electrons, and the faster the rate of corrosion. The larger the anode's surface area in relation the cathode, the more spread out the flow of electrons, and the less the corrosion. This principal, called the area ratio, often has important architectural implications.

For example, consider a sheet metal copper roof fastened with Type 304 stainless steel screws. The potential difference between the two metals is in the range 0.2 to 0.3 volts, so some corrosion effects may be expected under exterior conditions. In this galvanic pair, copper has a higher negative potential and will act as the anode and stainless steel will act as the cathode. However, since the surface area of the anode (the copper roof metal) is large in comparison to the surface area of the cathode (the stainless steel fasteners ), the corrosive effect on the copper is distributed over a relatively large area and greatly mitigated. In this case little if any long-term negative effect is anticipated. In fact, in practice, stainless steel screws are an accepted method of attachment for copper roofing.

As a counter example, consider a stainless steel sheet metal roof fastened with copper nails. In this case, the surface area of the anode (the copper fasteners) is very small in relation to the surface area of the cathode (the stainless steel roof metal), the flow of electrons from the anodes is highly concentrated, and rapid corrosion of the fasteners is expected.

In fact, the rate of corrosion of the anode in a galvanic metal pair is directly related to the numeric area ratio of the two metals. That is, if the surface area ratio of cathode to anode is doubled, the rate of corrosion of the anode is also doubled. Likewise, if the area ratio is halved, the rate of corrosion of the anode is halved.

Avoid Fasteners Acting as Anodes
The previous examples illustrate an important guideline, that fasteners should generally be selected to avoid taking on the role of anode in a galvanic reaction. Due to their normally small surface area in relation to the materials being fastened, such fasteners will be at risk of rapid corrosion. Thus when fastener and base metal differ, the fastener metal should be selected to be cathodic in relation to the base metal. When two dissimilar metals are joined with a third fastener, the fastener should be cathodic in relation to at least one of the other metals, so that it does not take on the role of anode in a galvanic reaction between the three.

There are many finer points regarding the selection of metal fasteners in relation to metals being joined, the building environment, and the particular application. This topic will be addressed more fully in a later article in this Designing with Metals series.

Avoid Rainwater Runoff From Cathode To Anode
Consider a sheet metal copper roof with a galvanized steel gutter. Rainwater flowing over the roof metal will pick up copper in solution and carry this dissolved metal into the gutter. When the dissolved copper and galvanized steel come into contact, these metals will react as a galvanic pair. Since the galvanized steel is anodic to copper, the gutter will be corroded. Alternatively, consider a galvanized steel roof and a copper gutter. In this case, dissolved zinc (from the galvanized coating on the steel) is carried into the copper gutter, where the zinc will act as the anode in the galvanic pair. In this case the copper gutter acts as the cathode and is not threatened with corrosion.

As a general rule with metal roof and wall systems, care should be taken that rainwater does not flow from metal surfaces that are relatively cathodic to others that are relatively anodic.

Treat Plated Metals According To Their Plating
When using galvanic series charts with plated metals, read from the position of the plating, not the base metal. For example, a cadmium plated mild steel fastener reacts according to the electrical potential of cadmium, not mild steel. Galvanized steel (a zinc metal coating on steel) reacts according to the electrical potential of zinc. Lead coated copper sheet will react according to the electrical potential of lead, not copper. Etc.

Understand Your Project Particulars
In practice, there are additional considerations that can influence the severity of the galvanic reaction between metals. For example, coastal environments tend to produce salt-laden precipitation that can significantly accelerate the reaction between galvanic metal pairs in comparison to non-coastal areas. Urban and industrial environments, with their relatively high concentrations of air pollutants, produce precipitation that is more acidic and conducive to corrosion than precipitation further from such areas. Perhaps less obvious is the potential for accelerated corrosion in some agricultural environments, where certain fertilizers are a known source of corrosive air pollutants.

The electrical potential of metals may vary depending on the medium in which the galvanic reaction takes place. In fact, most galvanic series, including the chart in this article, are based on the electrical potential of metals when immersed in flowing sea water. The designer should keep in mind that metals buried in soil, exposed to highly corrosive industrial solutions, or otherwise exposed to atypical environments may react quite differently from what is predicted by the standard galvanic series.

As one example, consider a 300 series stainless steel angle buried in mud. Due to a lack of free oxygen in such a soil condition, the stainless steel may not be able to maintain the passive layer that normally protects the underlying metal from corrosion. In this condition, the stainless steel surface can become more electrochemically active and assume a location in the galvanic series close to mild steel, a change in its electrical potential of approximately -0.5 volts. (For a discussion of stainless steel alloys and its active and passive conditions, see the related article Stainless Steel and Corrosion Resistance.) As another example, curiously, zinc may become cathodic to iron when immersed in hot tap water.

The particulars of a metal detail or assembly can also influence the rate of corrosion. For example, low-slope metal roofs, in which standing water can accumulate, may exhibit higher rates of corrosion in comparison to steeper roofs that shed water more rapidly and therefore remain dryer. Details that capture and trap water can also lead to accelerated corrosion in localized areas.

Approach Insulating Strategies Cautiously
One strategy for mitigating corrosion between metals is to insulate the metals from each other so that the electrochemical reaction can not take place. While this strategy is theoretically sound, in practice it must be approached with caution.

For example, consider a copper roof fastened with galvanized steel anchor clips. This is not a recommended assembly since the galvanized steel is anodic to the copper. Given the relatively small surface area of anchor clips in relation to roof metal, the anchor clips are expected to corrode rapidly. One way to attempt to overcome this problem might be to apply a non-conductive coating, such as asphalt mastic, to the anchor clips, thereby preventing electrical contact between the two metals. However, in practice, it must be assumed that some gaps will appear in the coating, either due to imperfect application or due to wear and tear over time as the metals expand and contract. In either case, where gaps occur, the galvanic reaction will proceed. Furthermore, given the even smaller area ratio between just these exposed areas on the anodes and the larger area of cathodic roof metal, the reaction will proceed in these areas at an even more accelerated rate than it would have otherwise.

In other words, applying insulating coatings to only the anode in a galvanic pair is strongly discouraged, as it may actually increase the risk of corrosion. When insulating coatings are used to prevent electrical conduction between galvanic pairs, the cathode should always be coated, whether the anode is coated or not.

As another example, consider a black rubber washer between a fastener and roof of different metals. If the washer contains a high percentage of carbon black (used to color the washer), it may be sufficiently electrically conductive to allow the galvanic reaction to proceed between the two metals. Furthermore, even with an insulating washer, the two metals remain in contact where the fastener penetrates the roof sheet, and the galvanic reaction can still proceed through this juncture.

Conclusion
The galvanic series is a powerful tool for evaluating the potential risk of corrosion between metals. However, to be used effectively, it must be applied knowledgeably and with consideration of factors that may influence the risk of corrosion between dissimilar metals. Wherever possible, past experience and local knowledge should be included in these considerations.

This is the second article in an occasional series on architectural metals.
Stainless Steel and Corrosion Resistance
Dissimilar Metals And The Galvanic Series (this article)
Next: Selecting Metallic Fasteners

For more information:
ASTM International's ASTM G 82 Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance includes the galvanic series, guidelines for its interpretation, and information on the deriviation of electrical potential values.
Corrosion Doctors and CorrosionSource.com web sites provide extensive information on many aspects of the corrosion of metals. See for example The Galvanic Series, Introduction To Design and Corrosion, Prevention of Galvanic Corrosion By Design, and Galvanic Compatibility
Department of Defense, Military Specification Finishes for Ground Based Electronic Equipment, MIL-F-14072D(ER) provides some useful guidelines and technical details for working with the galvanic series.

New ASTM Journal

As of January 2004 ASTM International is set to launch a new online technical publication. According to ASTM's web site, the Journal of ASTM International will cover the full range of ASTM's technical interests, including among other areas Materials Performance and Characterization, Civil Engineering and Building Materials, and General Methods and Instrumentation. The journal will be published ten times per year on a subscription basis.

Designing With Metals: Stainless Steel and Corrosion Resistance

The textbook, page 459, describes stainless steel as a steel alloy containing chromium and nickel, and being highly resistant to corrosion. Stainless steel is commonly used in architectural applications such as metal roofing, wall panels, railings, sanitary surfaces, flashing, shelf angles, lintels, masonry veneer anchors, and hardware and fasteners exposed to moisture. Stainless steel is 100 percent recyclable, and stainless steel made today typically contains 65 to 80 percent recycled content.

What is Stainless Steel?
Steel containing approximately 10 percent or more chromium qualifies as stainless. The chromium at the surface of the metal combines with oxygen from the atmosphere to form a thin, clear oxide film. Once formed, this oxide layer itself does not react with most corrosive elements and in effect forms a tight, protective seal around the metal. Under normal conditions, even if the oxide layer is scratched or damaged, the oxide layer will re-form, essentially healing itself and maintaining protection of the underlying metal. In the technical literature, this oxide layer is said to create a passive barrier on the surface of the steel.

Shown from front to back are flashing samples made from stainless steel, copper, and galvanized steel. Compared to copper, stainless steel is harder and stiffer, in these respects making it more difficult to work with as a flashing material. Copper has a more distinctive color and produces runoff that may cause staining. Galvanized steel is the least expensive of the three, but also is not as long lasting. (Hues in this digital image have been slightly intensified to accentuate the differences in color between the three metals.)

What Are the Common Types of Stainless Steel?
Different stainless steel alloys are distinguished by the varying amounts of chromium and other metals added to the steel. Within the range of common stainless steel alloys, the higher the percentages of chromium and nickel, generally speaking the more corrosion-resistant the alloy. In architectural applications, the most commonly specified stainless steel alloy is Type 304. Type 304 stainless steel has 18 to 20 percent chromium, 8 to 12 percent nickel, and smaller amounts of other elements. Chromium provides the base level of corrosion resistance. Nickel adds additional corrosion resistance and improves the ductility of the metal. (Steel alloyed with just chromium tends to be hard and brittle.)

In highly corrosive environments Type 316 stainless steel is recommended. This alloy differs from Type 304 in the addition of 2 to 3 percent molybdenum, and an increase in the percentage of nickel. The result is greater resistance to chlorides and other corrosives. This alloy is particularly noted for its resistance to a form of corrosion called pitting that is common in marine environments. Type 316 stainless steel is also more expensive than Type 304.

Where stainless steel requires heavy welding, variations on these two alloys may be used. At the high temperatures of welding, carbon in the steel combines chemically with the chromium, rendering the chromium unavailable for the formation of the protective oxide coating. Without the ability to form this coating, corrosion-prone areas form in the areas surrounding the weld. In Type 304L and Type 316L stainless steel, the carbon content is reduced from a maximum of 0.08 percent to less than 0.03 percent. The reduction in carbon ensures that chromium remains available for formation of the oxide coating. As a side effect, the yield strength of these alloys is also reduced, from approximately 30,000 psi (205 MPa) to 25,000 psi (170 MPa).

The most common stainless steels are also sometimes referred to as austenitic. This term describes the crystalline metal structure of these alloys (face-centered cubic). The austenitic stainless steels include the 300 series alloys as well as some less common 200 series, lower-nickel, alloys. In comparison to other alloys, austenitic stainless steels are higher in chromium and nickel, low in carbon, and highly corrosion resistant. They have rates of thermal expansion 30 to 50 percent greater than normal carbon steels. They are also distinguished by being non-magnetic, although some can exhibit mild magnetic properties after cold working.

One limitation of austenitic stainless steels is that they cannot be hardened by heat treatment. Type 410 stainless steel is a martensitic alloy containing roughly 12 to 14 percent chromium, little or no nickel, and up to 0.15 percent carbon. Type 410 stainless steel may be hardened, though it also has less corrosion resistance and is less ductile in comparison with the 300 series high nickel alloys. Case hardened Type 410 stainless steel may be used, for example, in the manufacture of self-drilling or self-tapping screws for fastening to steel or concrete, where Type 300 series alloys would lack sufficient hardness to cut through these dense materials.

Some architectural stainless steel may also be referred to as 18-8 stainless steel. This term refers to Type 304 and a few other closely related alloys, all of which have approximately 18 percent chromium and 8 percent nickel, and all of which share similar levels of corrosion resistance and other physical properties. In many cases the term 18-8 is used interchangeably with Type 304.

Two stainless steel fasteners from a manufacturer's catalog listing are shown. Note that the self-drilling screw for fastening into wood is made from 18-8 (Type 304) stainless steel, while the fastener designed to cut threads in much harder steel is made from hardened Type 410 stainless steel.

What Is the Difference Between "Active" and "Passive" Stainless Steel on the Galvanic Series?
The galvanic series is a list of metals arranged in order of their electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604, of the text. When two metals are in contact and in the presence of moisture, their relative locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion; the further apart the two materials on the list, the greater the risk of corrosion.

Many galvanic series lists show stainless steel in two locations, one for passive stainless steel and another for active. These terms refer to the presence or absence of the protective oxide coating that normally forms on the surface of the stainless steel, as discussed above. Under any normal circumstance, stainless steel used in architectural applications will exhibit this oxide coating and thus its galvanic properties should be referenced from its passive location within the galvanic series. While it is possible for the surface of stainless steel to become active under certain conditions, such circumstances are not common to architectural applications, and references to active stainless steel on the galvanic series should normally be ignored.

This is the first article in an occasional series on desining with architectural metals.
Next: Dissimilar Metals And The Galvanic Series

More information:
The textbook discusses architectural uses of metals on pages 458 - 460.
Why Is Stainless Steel Stainless? provides an easy to understand explanation of stainless steel basics, and links to additional informative sites.
Corrosion, Stainless Steel is a clear and technically detailed account of corrosion mechanisms in stainless steel.
Prevention of galvanic corrosion by design provides a brief summary of design strategies for avoiding galvanic corrosion between dissimilar metals, and lists links to related information.
Stainless Steel Information Center offers extensive reference information on the properties and uses of stainless steel.
Architectural Metals, by L. William Zahner (John Wiley & Sons, Inc., 1995), provides in-depth technical and design information on stainless steel and other architectural metals.

AR: Innovation

Innovation, a special supplement to the October 2003 issue of Architecture Record, provides good material for classroom discussion or further student research. Feature articles include:

Imagining the future asks "How will we build in 2030?" The future of steel, concrete, glass, and other construction materials, as well as developments in computational modeling of building form and structure are discussed.

Brave new solid-state, carbon-fiber world presents work of architects Peter Testa and Sheila Kennedy using this material. Testa and Devyn Weisers’ The Carbon Tower Prototype, an all-carbon 40-story high rise, is one example.

Seeking innovative alternatives showcases Kieran Timberlake and Associates School of Engineering and Applied Science at the University of Pennsylvania. The design for this building's ventilated curtainwall system was achieved, in part, through the firm's research into materials and methods funded by an AIA Latrobe Fellowship grant.

Technology transfer discusses professional's and school's increasingly looking outside of the traditional construction marketplace for technology solutions and alliances, for example to the aerospace and automotive industries.

Construction Innovation, September 2003

From the September 2003 issue of Canada's Institute for Research in Construction (IRC) Construction Innovation magazine:

Building Envelope Analysis Tool
The Institute's Building Envelope and Structure Program continues to develop hygIRC. This software models the distribution of temperature and relative humidity through a cross-section of the building envelope, accounting for exterior and interior conditions and specific factors such as temperature, relative humidity, solar radiation, wind, precipitation, air leakage, rising damp, thermal bridging, air pressure differences, etc. IRC claims that hygIRC can assist building designers in optimizing building envelope systems. For instructors and students involved in building envelope research, this might also be a tool worth taking a close look at.

Canadian Building Code Development
While new model codes duke it out for market share in the US, Canadian National Code documents continue to develop as well. The Canadian Commission on Building and Fire Codes this spring completed a public consultation process that it says was for the first time ever coordinated at the national, provincial, and territorial levels. The process was managed primarily through the web. Standing committees will meet this fall to begin considering comments received.

Web Resource
IRC says that its web site is becoming a more popular and useful resource. In addition to online versions of the Construction Innovation magazine, this site also archives their Construction Technology Updates, Canadian Building Digests, and more.

Tall Structures Conference
The 6th International Conference on Multipurpose Highrise Towers and Tall Buildings will be held in Toronto, May 2-7, 2004, concurrently with CIB World Building Congress 2004.
(c) Joe Iano 2003