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.