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Importance of continuous air-sealing primary air barrier from Zip to Intello via top plates in double-stud wall

jim_hawkings | Posted in Energy Efficiency and Durability on

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

Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Jim,

    The simple answer is you need a continuous air-barrier. If it's the sheathing on the walls and the Intello on the ceiling, then the two must be connected.

    I suspect your builder is detailing the Intello on the walls as the primary air-barrier, much as they are used to doing with poly, and that's where the reticence comes from.

    Do your drawings specifically call out the Zip as the primary air-barrier, and ask for it to be detailed that way? There are a number of other intersections (sills, openings) that need to be dealt with differently if that's the case.

    1. jim_hawkings | | #2

      Malcolm,

      Thanks so much for the prompt reply. Extremely helpful! Unfortunately the construction drawings did not provide intimate details so there has been some uncertainty. It really shows the importance of ongoing and open communication between owner, builder, and any energy consultants involved in the project.

      Jim

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