The “Poly” Approach

The first approach for achieving a continuous air barrier is called the “Polyethylene Air-Vapor Barrier” (PAVB). It came to the United States from the Canadian R–2000 building program and from similar techniques used in factory-built houses in Sweden. In the PAVB approach, large sheets of polyethylene cover the inner surface of the exterior shell of the building -- except over doors and windows. At joints and seams the poly is carefully lapped and sealed.

Polyethylene sheets function as the air barrier and as the moisture diffusion retarder. Using the same material to serve two functions is supposed to save costs. To ensure continuity, “connector strips” of poly must be installed during rough-framing. Later, larger sheets covering the bulk of the walls and ceilings can be lapped and sealed to the strips already in place. The most common areas requiring connector strips are the intersections of the interior partitions with exterior walls and ceilings. The strips often take a beating during framing, plumbing, wiring and insulation.

Any penetrations in the PAVB have to be carefully treated. Because polyethylene is very flexible, it requires support or solid backing in order to seal around electrical boxes and duct penetrations. In addition, there are complex tricks involved in getting a good seal around window and door frames.

A number of builders have succeeded with this approach. However, many others found it too complicated. In spite of all the effort, many homes built with the technique are only slightly less leaky than they would have been without it. This result was less a fault of the PAVB system than of the lack of awareness and attention to the forced-air heating systems and the multitude of minor penetrations in the building envelope.

The PAVB system is more appropriate for factory-built houses where the materials can be assembled in a relatively clean area and protected from the weather. In outdoor construction, faced with rain, cold temperatures and wet sawdust, most builders have difficulty carrying out the air barrier details.

The Airtight Drywall Approach

The PAVB technique never became common practice for builders of energy efficient homes in the U.S. While the issues of energy conservation, indoor air quality and moisture control were recognized as important, most production-minded builders have been waiting for a better idea to come along. The first one arrived around 1984 and was called the “Airtight Drywall Approach” (ADA). It is a system that tries to bring air leakage control techniques to the mainstream production builder.

To overcomes the problems with the PAVB system, ADA splits the air barrier and diffusion-retarder functions into two construction components.

Moisture diffusion can be handled in a number of ways. A builder can use faced insulation, foil-backed gypsum wallboard, or vapor retarder paint to protect against vapor diffusion. Because a vapor retarders effectiveness is proportional to amount of surface it covers, a 95 percent vapor retarder is about as good as a 98 percent retarder. It can tolerate more imperfection than an air barrier can.

An air barrier needs to be able to resist air pressure differences. Several parts of the building envelope already do that -- such as the gypsum-board walls and ceilings and the plywood floor. To make an air barrier out of these different structural components, the trick is to get them to work together. By systematically connecting the joints where the components meet, builders transform the individual components into a continuous air barrier that functions almost as if it were a single piece of material.

The drywall and the plywood subfloor serve well as air barrier surfaces, but one surface has to be connected to the other. The ADA solution is a compressible gasket between the components. One gasket is placed beneath the wall plate before the wall is tipped up. Another gasket is applied to the face of the bottom plate before the drywall goes up. These two gaskets effectively carry the air barrier surface from subfloor to bottom plate to drywall.

But, even with ADA, there have been problems and requests for “a simpler way.” It is still necessary for the framing crew to apply gaskets between structural members during construction. And many of the air sealing tasks must still be completed before the building shell is weather-tight.

Problems with Earlier Approaches

The problems with the other air barrier approaches are partially a result of the techniques themselves and partially a result of the difficulty in making them work in the real world of construction.

• They require detail-oriented work at an inappropriate stage of construction.
• The require an interruption in the normal flow of work on a building site.
• They tend to emphasize techniques for sealing the floor, wall and ceiling assemblies that are already covered by large sheets, while underrating the multitude of joints and penetrations.
• They fail to recognize the law of diminishing returns, resulting in greater effort and increased costs.

Air sealing a building is not an all-or-nothing proposition. The other approaches are too comprehensive, too complicated and too untrusting of simpler solutions. The next chapter offers options for air sealing that address these drawbacks, while offering flexibility for the builder.