A Simple Solution for Winter’s Wettest Walls : Airtight-Drywall Outperforms Polyethylene Insulation in Study by Two Canadians

Two Canadian building engineers are touring North America with a message for the construction industry: Forget polyethylene, they say. There’s a much simpler, longer-lasting, easily inspected, readily repairable and, yes, even cheaper way to avoid moisture buildup in super-insulated walls during winter.

Tight interior drywalling (gypsum board) applied without interruption around the perimeter wall and ceiling will do the job. What they call the airtight-drywall approach, combined with a controlled ventilation system, will ensure that wall and ceiling cavities remain dry.

James K. Lischkoff and Joseph Lstiburek, graduates of the Center for Building Sciences at the University of Toronto--and builders themselves--recommend the drywalling approach over polyethylene for two principal reasons:

--Moisture movement through materials (vapor diffusion) has proved negligible.

--Polyethylene is essentially a useless air barrier.

Cause of Moisture

Investigation into condensation within walls in hundreds of homes across Canada has never shown moisture diffusion to be the cause of the problems. In every case, warm interior air moving through cracks and poorly sealed joints brought moisture into the walls.

Calculations show that air movement will conduct at least a hundred times more moisture through a given aperture than will pass through the same hole by diffusion. In other words, moisture can be kept out of a wall by a good air barrier alone.

What Lstiburek and Lischkoff see as the “perils of poly” are these:

--Installing polyethylene is complicated, labor intensive and requires a level of supervision not available to the production builder.

--During strong winds (air pressure can build up even in fiberglass-filled walls), polyethylene can separate at the seams.

--Some evidence exists that over the years polyethylene will become brittle and crack.

--Polyethylene, once installed, is out of sight. If a crack or tear does occur, it will probably go undetected, and, if the damage does become known, repairing it will be costly. James Lischkoff sums it up this way: “If you’re going to rely on an air barrier that is out of sight, you had better be sure that it lasts.”

Simple to Install

Drywall, in contrast, is a durable air barrier that, with two coats of oil or latex paint, becomes an effective moisture barrier as well. It is simple to install, using long-established conventional construction practices, and needs no special level of supervision. It is also inexpensive. If a problem does occur (a hole or a crack appears), it is easy to spot and repair.

To be fully effective, however, the drywall approach requires two minor modifications in standard building practices: The drywall must be continuous around the circumference of the house and over the ceiling (in other words, the gypsum board cannot be interrupted even where an interior wall meets the perimeter wall); and continuous compressible gaskets must be used to seal wood framing members at the tops and bottoms of walls.

Airtight-drywall homes, say Lstiburek and Lischkoff, easily achieve half an air change at 50 Pascals of pressure--or, less technically, the exchange of less than one-twentieth of the home’s natural air exchange each hour. The added cost to these homes works out to between $200 and $500 for the air barrier and anywhere from $100 to $2,000 for the mechanical ventilation system, depending on the sophistication of the heat-recovery unit used.

Heat exchangers, coupled with the ventilation system, are currently not worth the additional cost, according to the Canadian researchers. So far, 32 airtight-drywall-approach houses have been built in Canada, and some 30 are under construction in the United States. Repeated inspections of the walls of many of these homes have shown no moisture buildup within the insulation.