Iano's backfill

Storm-Resistant Home Design

Home Shapes And Roofs That Hold Up Best In Hurricanes (ScienceDaily, Jun. 21, 2007) reports on a research report from New Jersey Institute of Technology identifying design strategies for homes resistant to high winds and storm-driven funding. Among the findings:

• Hipped roofs with a 30-degree slope are the most resistant to wind uplift.
• Homes thats are roughly square in plan are more wind-resistant that those that are more rectangular.
• Roof overhangs should be limited to 20 inches deep.
• An elevated structure is less at risk from storm-driven flooding.

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

Plastic/Wood Composites

Polymers in various forms continue to make inroads into traditional wood product applications.

The New Forest of Man-Made Trim, Fine Homebuilding, August/September 2004, is a full-length article covering new alternatives to traditional sawn-lumber trim. Materials discussed include PVC, wood/plastic composites, compressed wood fiber, and engineered wood products that claim greater stability, reduced maintenance, and increased longevity. Some noteworthy points:

PVC trim is manufactured by extrusion, with the final product characteristics varying with the cooling process. One process, termed "celuka", results in a material with a dense, somewhat brittle outer layer and a less homogenous core. This product type is best suited to flat trim applications. The alternative "free-form" cooling process results in a material of uniform density throughout that is more suitable to applications requiring routing, complex cutting, or other more intricate work.

Wood/plastic composites are made from the mixing of phenolic resins or PVC plastic with wood fibers. Product characteristics vary with the various manufacturing processes. Some may be homogenous and exhibit workability similar to natural wood. Others may be acrylic clad with a less dense core.

Both solid plastic and wood/plastic composite products must be installed to allow for greater thermal expansion and contraction than natural wood products.

Compressed wood fiber products are also a mixture of phenolic resins and wood fibers, but with a higher proportion of wood fiber to resin material in comparison to composites. Similar to traditional hardboard (which has a history of failures when used in exterior applications), these newly formulated products are claimed to overcome earlier limitations. In general, product characteristics are similar to traditional hardboard or medium-density fiberboard, though qualities vary with individual manufacturer's proprietary material formulations and manufacturing processes. Given the higher wood fiber content, more care must be taken to protect these products from moisture absorption at cut ends and other vulnerable locations. Some product formulations also include chemicals to prevent decay or attack by insects.

Several products consisting of plywood or OSB panel material clad with resin-impregnated waterproof paper, similar to medium density overlay plywood, are also available for use as trim material.

Finger-jointed lumber offers greater dimensional uniformity and stability in comparison to traditional sawn-lumber. It can also be manufactured from readily renewable wood species. Improvements in finger jointing technology result in joints that are invisible in finished trim.

Many proprietary trim products are also available factory-primed and/or factory-finished, offering higher finish quality and consistency, and reduced installation costs.

Wood Design & Building reports that the University of Maine's Advanced Engineered Wood Composites Center has received a patent for a reinforced building panel that offers substantial increases in structural strength in comparison to traditional structural panel products. According to the article and information provided on the University of Maine's website, fiber-reinforced polymer reinforcing at the perimeter and other nailing surfaces of plywood and OSB panels results in increases in resistance to wind and earthquake forces of up to 20 percent in traditional stick-framed construction, or more in specially engineered systems.

The Construction Specifier, July 2004, reports on the 10-year successful track record of of boardwalk decking made from recycled/reclaimed plastic and waste wood. Installed in Spring Lake, New Jersey's boardwalk after a 1992 storm damaged the previous wood deck boardwalk, the new decking material has proved successful. Claimed benefits include: freedom from rot, no risk of splinters, no periodic refinishing or sealing requirements, and elimination of popped nail heads. Reportedly, the surface is also found to be more forgiving under foot to joggers.

Bobrick, manufacturer of washroom accessories, has announced its "Siera Series" toilet partitions made from Solid Color Reinforced Composite. So-called SCRC material is made from wood fibers suspended in high-density resins, and is claimed to offer superior resistance to abuse, easier repairs, and improved surface-burning characteristics in comparison to competing products.

More Info
Plastics in Construction are discussed on pages 694 through 697 in the textbook.

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.

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.

Anti-Pollution Paint

Smog-busting paint soaks up noxious gases, New Scientist, February 4, 2004, reports on Ecopaint, a coating product that asbsorbs nitrogen oxide (NOx) emissions.

Ecopaint is scheduled to go on sale in Europe in March of this year. The coating relies on embedded particles of titanium dioxide and ultraviolet radiation from the sun to catalytically convert NOx gasses to nitric acid. The nitric acid is then either washed away by rain or neutralized by other embedded calcium carbonate particles. The success of the coating depends on the paint's unique polysiloxane base which, unlike other paint bases, is itself resistant to reacting with the titanium dioxide embedded in the coating. A 0.3-millimeter (12-mil) thick coating is expected to have a 5-year lifespan before exhausting the capacity of the calcium carbonate to neutralize the nitric acid. (Left unneutralized, the nitric acid may cause color changes in the coating.)

This article also discusses Europe's Photocatalytic Innovative Coverings Applications for Depollution Assessment program, and its experience with catalytic cement coatings for roadways that have demonstrated success at significantly reducing NOx gas levels as well. This program is part of EU-member countrys' efforts to reduce NOx levels to below an annual average of 21 parts per billion by the year 2010, a reduction from current levels of 10 times or more for some urban areas.

Very Airtight Construction

Fixing the Holes Where the Air Gets In, Journal of Light Construction, January 2004, is a detailed and well illustrated presentation of one contractor's techniques for minimizing air leakage through light wood frame constructed building envelopes. Techniques include:

• Overall building leakage rates are measured using a calibrated blower door. This contractor aims for passive rates of exchange of 1/10 air change per hour. Lessons learned from diagnosing leaky details through such testing are applied to future projects.
• Foundation sill plates are set on EPDM rubber gaskets. Rim joists are sealed to the sill with adhesive or sealant. The top edge of the rim joist is also sealed to the underside of the subfloor with construction adhesive.
• Neoprene gaskets are placed between the bottom of exterior wall frames and the subfloor.
• Sealing tape is applied to all horizontal exterior sheathing joints and other joints not backed up by solid framing.
• Holes in framing for plumbing or wiring are sealed with expanding foam caulk.
• The gap between door and window frames and rough opening framing is sealed with EPDM gaskets. Gaps too narrow for gaskets are sealed with silicone caulk.
• Gaps between firestopping and flues or chimneys are sealed with high-temperature silicone caulk.
• Ceiling end bays that intersect with ventilation channels at the roof eave are sealed with custom-cut rigid foam panels.
• Tops of interior partitions are sealed with sprayed expanding foam to seal the wall/ceiling framing juncture.
• Walls and ceilings are insulated with dry-blown cellulose. Where possible, insulation values are upgraded with 3/4- or 1-inch foil faced rigid foam insulation applied on the interior side of exterior walls (also with seams taped). Hard to reach areas are insulated with two-component polyurethane foam.
• Vapor barrier membrane is either Hanes Industry's 100% polypropylene or Certainteed's proprietary variable perm rate "MemBrain" product.
• Mechanical exhaust systems are sized for a minimum of 15 cubic feet of air change per hour per building occupant. Bathrooms fans can be manually or automatically boosted for high-humidity conditions. Heat recovery ventilation systems are also sometimes installed to increase energy efficiency.

While the techniques illustrated in this article may not be suitable for every builder or every project, this article illustrates field-tested, effective techniques for achieving low-leakage rate residential building envelopes.

Residential Wildfire Safety

In An Oasis of Fire Safety Planning Stands Out, the New York Times (November 2, 2003) reports on the higher survival rate in November's California wildfires of homes and residential communities designed for resistance to fire hazard. Fire-resistant design elements include:

• Wide streets
• Fire-resistant roofs
• Moist landscaping
• Double-glazed windows resistant to heat and breakage
• Stucco eaves to block the passage of sparks into attics
• Oversized address markers for easy visual identification
• Swimming pools equipped with valves allowing firefighters to draw water for fighting fires

For more about building for wildfire safety:
FEMA's Protecting Your Property from Fire provides recommendations and links to additional resources regarding mitigating wildfire risks.
University of California's Forest Products Laboratory provides resources related to wildfire mitigation, building standards for construction in urban/wildland interface zones, and fire performance testing.
Firewise.org provides educational information for homeowners living in wildfire prone areas.
The Cedar Shake and Shingle Bureau Certiguard page provides information on fire-retardant cedar shakes and shingles.

06 - Exterior Finishes for Wood Light Construction Links

This article contains external links to resources on the Web relevant to Chapter 6 Exterior Finishes for Wood Light Construction.

Western Red Cedar Lumber Association

Technical and educational information about Western Red Cedar lumber products.

Stucco- Portland Cement Plaster

Portland Cement Association's web site devoted to cement plaster/stucco finish.