Installing an Interior Air Barrier – GWC Mag

I’m from Minnesota, one of the few areas in the lower 48 that still regularly uses polyethylene sheeting as a vapor retarder. We’ve become very good at detailing these interior membranes for use as an air barrier. The average blower door test I conduct for new homes in my market is below 2 ACH50, most achieving these scores using only an interior air control strategy. In most cases, I don’t recommend using poly. Instead I suggest moving to a class II vapor retarder, (smart, variable, and responsive vapor retarders). These products can also be used for air control. Installation is similar in either application but there are some tricks to getting an effective interior air barrier. That’s what I’ll talk about here.

No matter which product is used, or whether it’s located outside, inside, or in the middle of the wall assembly, continuity is the key to an effective air barrier. You may remember the old song by Dem Bones…“The foot bone’s connected to the ankle bone, the ankle bone’s connected to the leg bone”… Well, it is exactly how an effective air barrier works—by connecting the foundation to the wall and the wall to the ceiling.

Think through the process

The most difficult part of interior air-sealing is the interior partition walls. Interior walls create lots of breaks in continuity both at the ceiling and where any interior wall connects to the exterior wall. Do all those walls need to be constructed before the roof framing is placed? The homes I test that have the lowest blower door numbers are designed so the roof is supported only (or mostly) by the exterior walls. This leaves one big…