While installing a vapour barrier is smart, it’s also a requirement of the building code – and for good reason! Without air-tightness, all that insulation is for nothing, at least in terms of energy conservation. A drafty wall assembly is also an invitation for moisture buildup from condensation, which leads to a drastically reduced efficacy of insulation, mold growth and rot.
In a Passive House, the vapour barrier has the same job, and is installed in the same place (on the warm side of the wall assembly), but with three subtle, yet important differences. The first is detail; we’re going for a very tight building envelope, and achieving 0.6 ACH @50 Pa (or better!) requires a very high level of detail in terms of planning and installation. The second and third are permeability and strength, hence the title of this post, but with a slight change in punctuation: “A vapour barrier that’s smart“.
As the picture shows, we have specified an engineered truss design with a rather substantial 32″ raised heel at the wall. A “regular” truss would allow for about 6-8 inches of insulation at this location. We’re looking for full-depth insulation everywhere. That amount of insulation (we’re blowing in cellulose) introduces a couple challenges, namely mass and reduced airflow. This is where smart vapour barriers outperform the standard 6-mil poly. We’re using a product which has 2 layers; the first is a fabric which greatly improves its strength, and the second is a membrane. Limiting airflow through insulation is what makes it effective. With this much insulation, it also means a reduced ability to dry out, should moisture be present. It’s not that we’re planning for a leaky roof – moisture will always be a factor; take a look in your attic after a snow-storm with winds. Vents will allow snow into the attic and when it melts, evaporation is how it dries. With 32″ of insulation there’s very little airflow, so we’re relying on a smart membrane to assist. What makes the membrane smart, is its ability to become vapour-open in the presence of moisture.
The fabric layer is required to resist stretching under the weight of all that insulation. We were also concerned about the possibility of staples being ripped out over time, creating holes in the vapour barrier. To that end, we decided to use a super-sticky reinforced double-sided tape in place of staples. Should the weight of the insulation force the tape to let go, we won’t have to worry about a thousand holes in the membrane.
At this point, we’d normally install metal resilient-channel to the underside of the trusses. This gives us something to attach drywall to at 16″ O.C. (trusses are typically 24″ O.C.) and helps protect the drywall joints from truss-lift in the dry season. We chose to do things a little differently; in place of resilient-channel, we installed 2×3 on its edge. This is much stronger (better support for the vapour barrier and insulation) and provides a 2.5″ deep service cavity for all the electrical work. Our electricians did not poke a single hole in the vapour barrier. The plumber is another story, but that’s not his fault; in Ontario we’re not allowed to use air-admittance-valves (aka “cheater-vents”) in new construction.
We went a little O.C.D. on the 2×3… to ensure the long screws didn’t result in leakage through the vapour barrier, we chalked lines and placed foam gasket tape at all intersections of the 2×3 and trusses. The scaffold took on a new “look” with all those gaskets stuck to it. With one person pushing the scaffold and two people placing, it was done in no time.
Installing the 2×3 also went well with teams of two. We chose to pre-drill the 2×3 so the wood screws had more truss to bite into, drawing the 2×3 in and compressing the gaskets – no leakage there!
We were also careful to eliminate leakage potential around the perimeter. This being a SIP house, the inside OSB sheathing is our vapour barrier for outside walls. Where the walls and ceiling V-B meet we applied a generous bead of polyurethane caulk and then nailed strapping to the wall (through the calking bead) to support the weight of insulation.
This method is strong and air-tight, but means we’ll have to be careful when screwing drywall to the 2×3… keeping screws to a minimum where the ceiling meets internal walls. 18 inches should do the trick – allowing the ceiling to flex a bit at those points.