Vapor barrier in middle of double stud wall will lead to condensation?
algoodwin
| Posted in Energy Efficiency and Durability on
Climate zone 7 (Montreal, Quebec, Canada)
Wall assy:
Cladding
1×4 strapping
R7.5 EPS with air barrier taped (+-1.2 Perms)
R12 Fiberglass (16 OC studs, load bearing wall)
6 mil Vapor barrier, taped seams
R12 Fiberglass (24OC studs, wiring/plumbing/hvac)
Drywall
will i have moisture issues in my wall?
if i increase the interior fiberglass to R20 will that cause moisture issues? i.e. condensation in the isulation?
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Replies
algoodwin,
The code says "the vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent condensation at design conditions."
So the assumption is it will be on the warm side of the wall, and if you move it in you have to model the dew point of whatever assembly you are using to see if it stays safe.
The very rough rule of thumb is that only one third of the insulation should be on the interior side of the VB, and I suspect it is too far towards the exterior - but the really problematic part of your wall assembly is including exterior foam. Double walls and exterior foam are an either/or choice for a couple of reasons. First because if you want more R-value, it makes more sense to simply make the space between the walls a bit thicker, not switch to another expensive and labour-intensive type on the exterior. But more importantly, the amount of permeable insulation in a double wall means the inside face of the foam will be cold enough to become a condensing surface, which is a big problem.
https://www.greenbuildingadvisor.com/article/exterior-rigid-foam-on-double-stud-walls-is-a-no-no