Good building science is important for every house in any climate—and homes with double stud walls and cathedral roofs in a cold climate are certainly no exception. And if you must do one thing, it’s this: Make sure the wall and roof assemblies have the ability to dry.
As a construction manager for a custom homebuilder in northern Vermont, I take responsibility for the implementation of energy-efficiency details, as well as the critical building-science details. A large custom home we’re currently wrapping up on the shore of Lake Champlain is a worthwhile example.
The double stud walls, sheathed with Zip System OSB panels, are 13 inches thick, and the unvented low-slope roof is framed with 18-inch-deep wood I-joists. Wall and roof cavities are both insulated with dense-blown cellulose. The roof also has another 1 to 4 inches of polyiso under the TPO membrane roofing.
In this example, where outward drying is constrained (especially for the roof), we’ve chosen to set up reliable drying to the interior. To do this, we installed Majrex directional vapor barrier membrane on the walls and ceilings, with the directional drying face pointed toward the interior.
The Directional Vapor Control Layer
For this application, we needed a variable vapor control material that permits good inward drying in summer, but slows the outward vapor drive in winter. Either Intello (from ProClima) or Majrex (from Siga) will perform in this way, as will MemBrain (from CertainTeed). For this project, we chose Majrex.
Carefully installed, Majrex controls vapor diffusion as well as air transport of moisture, but it’s only as good as the installation details. With any moisture-control strategy, it’s critical to control both air transport and diffusion of vapor, and to avoid discontinuities in the materials. The good vapor control achieved by this fabric could be compromised if we allowed air-driven moisture to bypass the control membrane. So we covered every inch of the interior, and we were careful to tape-seal around every outlet, every light fixture, and every penetration of any kind, as well as along the floor and the ceiling.
The Energy Challenge
This house is high performance, but only moderately so. The clients in this case didn’t place an especially high priority on energy efficiency or power bills; they were more concerned with comfort and appearance. Large expanses of glass, for example, make the most of this home’s exceptional water views (see “Installing Lift-and-Slide Doors,” Apr/18); but all that glazing is a challenge for the space conditioning system.
Even so, our advisors at Efficiency Vermont, the state’s energy-efficiency utility, are confident that the home’s mini-split heat pumps can do the job. However, the owners wanted to be sure their floors wouldn’t be cold underfoot, so we have also installed radiant-floor-heat tubing under the main floor’s concrete slab, as well as beneath the bedroom floors. Highly insulated wall and roof assemblies were key to helping this custom package perform effectively.
Collaborating With the Trades
In theory, you could use a membrane such as Majrex to contain your dense-blown cellulose insulation. In practice, that’s not a great idea. Too many times, we have tried to dense-blow against a membrane in this class, and then when we come back later to inspect, the insulation has settled. The membrane seals so well that the blowing machine’s air can’t get out of the cavity, so the insulation stays fluffed up and doesn’t pack densely enough. The manufacturers suggest cutting escape holes for air in the membrane, but that complicates the taping job. So instead, we used open netting to contain the insulation. That way, we would get a good install job and the ability to inspect easily. The added cost was negligible.
The insulation crew was fine with this, of course. But we had trouble later with the drywall crew. The first drywallers we hired had never worked on a house with this vapor-control detail, and when they arrived to hang drywall, they realized they couldn’t use their usual methods. We had carefully sealed all the outlets and other penetrations, and they wouldn’t be able to hang their sheets and then buzz out all the openings with a RotoZip—that would break all our well-crafted air seals. After a heated discussion at their van, that crew drove away without doing the job, and we had to find another contractor. Interestingly, the new drywallers we found were able to give us some practical tips about how to complete our air seals without interfering with their work. That’s the future of high-performance construction: collaboration among the trades.