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How to air seal a board-sheathed house?

user-1075465 | Posted in Green Building Techniques on

I am in the process of renovating my 1950s New England ranch. I am currently working on the attic and once the weather warms up I will be placing 4″ of polyiso on the sheathing.

Because the house is board-sheathed I’m unsure about how to air seal it. One local architect, Bruce Coldham, is in the habit of wrapping the sheathing in Ice & Water. Is this a reasonable approach for me to take? The existing wall is 2×4 construction which will be dense packed with cellulose. Is this idea just nuts? Ice & Water isn’t cheap or easy to work with.

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  1. GBA Editor
    Martin Holladay | | #1

    Bruce Coldham is a smart guy. His method isn't cheap, but it works -- as long as the exterior rigid foam layer is thick enough. This is the traditional PERSIST approach. (For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.)

    However, Bruce's approach isn't the only way to proceed.

    First of all, dense-packing your walls with cellulose will go a long way to reducing air leakage. It won't eliminate it, but it will greatly reduce it.

    Second, your polyiso layer can be used as an air barrier. You should caulk the perimeter of each wall (between the polyiso and the sheathing) and tape the seams of the polyiso with a high-quality tape. If you insulate your walls with 4 inches of polyiso, you should install two layers of 2-inch polyiso, with staggered seams. Tape each layer. That should provide a very good air barrier -- even without the peel-and-stick.

  2. user-1075465 | | #2

    Thanks. I do plan on staggering and taping seams with foil tape. I'm just worried about the seal between the foam and the sheathing not holding up over time. I routinely tape polyiso to wood with foil tape and I am less than impressed with how it sticks to the lumber. Also I have seen caulk exposed to weather fail. We've all seen it. The bead of caulk keeps it's shape but the seasonal movement of the materials it's applied to frees it up and you are left with a loose strip of silicone. Is Tremco the ideal caulk for this application? I have never seen it used outside but I have seen it used inside and it retains it's malleability even after many years. Also I'm going with "innie" windows so I plan on keeping the WRB at the sheathing plane. The Ice & Water would fit that role nicely. Putting something like Tyvek between the foam and the sheathing would make air sealing with caulk much more difficult. I'd really like to get away from the Ice & Water approach if I think I can do so in a way that will be durable over the years.

  3. GBA Editor
    Martin Holladay | | #3

    Ultimately, this is a judgment call. The peel-and-stick approach is bulletproof, and is likely to result an less air leakage than alternative methods. Whether the very small amount of air leakage that you are likely to get with caulk-and-taped-polyiso is enough to bother you depends on your air sealing goal and your budget.

    Tremco does stay flexible, so it's an option.

    Whatever you do, conscientious workmanship is just as important as your material choices. Do a careful job of air sealing, and your house should perform well.

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