Where is the “warm side” ?
Seems simple. Before the advent of exterior insulation or double stud-walls, if a building code required a class I or II vapor retarder on the “warm side of the insulation”, that would obviously mean covering the interior studs with poly prior to drywall or using an airtight drywall approach. But many people including Martin H. seem to frown on polyethylene under drywall, even in climate zones 6 or 7. How to interpret “warm side” when there is insulation on both sides of WRB+sheathing layers ?
Besides … if there is sheathing, WRB, and R20+ insulation outside a stud wall, that WRB is already on the “warm side” of a lot of insulation, so should it be a class I/II ? Does an interior class I/II vapor retarder under the drywall still make sense ? Is this where that Canadian rule of thumb about 2/3rds of R-value being outside the VB is still good ?
If building a double stud wall with the interior wall being structural and requiring structural sheathing, it would seem natural to place a vapor retarder there before building the non-structural exterior wall to hold more insulation and cladding. Then separately insulate that interior wall cavity from the exterior cavity including the gap between walls. So, once again, there would be a vapor retarder on the “warm side” of a lot of insulation. This seems to be what happens when people retrofit by adding Larssen trusses to hold more insulation — lots of new insulation outside a new VB attached to the existing sheathing.
Building Science’ Joe L. “Perfect Wall” claims a vapor retarder on the exterior sheathing then exterior insulation works in any climate, so no interior vapor barrier should be used. Keeping the whole house interior, including the studs and sheathing, inside the VB and insulation layer makes so much sense, but this whole “warm side” verbiage is bugging me when we now have insulation on both sides of what would have been exterior sheathing in the olden timey days.
Thanks for any comments,
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