Importance of continuous air-sealing primary air barrier from Zip to Intello via top plates in double-stud wall
I have a new home under construction in Ontario – climate zone 6. Two levels, 1400 sq. ft. each. Lower level is walkout basement with ICF walls on 3 sides. Framed walls are (from outside to in): engineered wood lap siding, 1×4 vertical strapping, ½” Zip sheathing, structural 2×6 wall, 1” gap, 2×4 interior wall, Pro Clima Intello Plus vapor retarder, ½” drywall. Roof is mostly vented attic over flat ceilings with 18” energy-heel trusses. Ceiling is (inside to out): ½” drywall, 1×3 strapping, Intello Plus, trusses. Ceiling and stud wall insulation is blown-in fibreglass. The exterior Zip sheathing in the walls and Intello Plus in the ceilings form the primary air barrier. My question is: How important is it to air-seal the top plates of the double stud walls to connect the outside air barrier (Zip) to the Intello in the ceiling? I think this is very important, but I’m not sure my builder is convinced. Otherwise, if there are any air leaks in the Intello in the walls – not unlikely given the numerous penetrations for wiring, etc., and the Intello location immediately behind the drywall where it is susceptible to damage from nails, screws, and the like — that air has a path through the insulation into the unconditioned attic space via the 1” gap between the top plates of the stud walls. This seems like a recipe for moisture condensation problems as well as energy loss during the heating season. Most of the high-efficiency building documentation I have seen emphasizes the need to carefully air seal the top-plate area. It could be argued that well-sealed Intello in the walls could be the primary air barrier, but I’m reluctant to rely on it completely. One of the reasons we paid a premium price for the Zip was to make it the primary air barrier. Thoughts? Am I being totally nit picky?
Thanks, Jim Hawkings
GBA Detail Library
A collection of one thousand construction details organized by climate and house part