Air sealing the building envelope while avoiding moisture issues
First off, thank you GBA community for all the support so far in getting my project planning done for my net zero build. I’ve been doing a good amount of research on the best way to provide a good air barrier for my future home. I have significant concerns on a cost effective way to get the proper air sealing in order to hit the goal of sub 1.5 ACH. Below is a quick overview of what the build envelope looks like so far:
Exterior Walls – Stucco, 2.5″ or 3″ inches of exterior EPS rigid foam, rain gap, Tyvek house wrap, plywood sheathing with Roxul or fiberglass batts in the wall cavities, vapor barrier and gypsum panels. The rigid EPS foam will be either InSoFast EX 2.5 Panels, 3″ EPS Type II foam sheathing with taped seams or Insulfoam 3” EPS CI Panels (these come with the OSB sheathing glued to the exterior of the foam) with taped seams.
Roof – Composite shingles, synthetic underlayment, cantilever hip roof trusses, R-60 blown cellulose and gypsum panels.
Floors – 4″ concrete slab with 3″ Type 2 EPS sheathing underneath and stem walls with with 2″ Type 2 EPS using this detail.
Options I have researched so far for achieving high levels of an air tightness include the following list below (please don’t limit suggestions to my list of course):
1) Taping the exterior plywood sheathing seams.
2) Gluing the exterior plywood sheathing to the studs, top plate and bottom plate during installation.
3) Caulk Tyvek house wrap at top and sides of exterior wall, tape at seams (leave bottom un-caulked).
4) Use bottled spray foam to seal the seams between the exterior wall cavities and the studs on the interior side of plywood sheathing (poor mans spray foam).
5) Use an Airtight Drywall Approach – Caulk all interior seams in framing, seal any intrusions to drywall with foam/caulk, foam top and bottom plate seams, seal all electrical boxes/fixtures or use airtight boxes, glue all drywall to studs during installation.
Part of me thinks doing them all would be overkill and may prevent my wall from breathing enough to dry out in the event any moisture accumulates in the wall cavity (since air may be quite stagnant when caught between air barriers like sheathing, WRB wrap, rigid foam and drywall).
Really hoping to hear some advice from anyone who has achieved 1.5 ACH successfully given the build envelope constraints we are working within. None of the options I have listed provide a continuous air barrier except for the Airtight Drywall Approach, the rest seem to be covering for holes/gaps created throughout the construction process.
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