With the new energy codes comes the infamous 1" XPS under siding systems. Questions are continually asked "Where does it go?" Under the weather barrier? On top of the weather barrier? Both are code compliant and acceptable to the weather barrier manufacturer's. I'm curious what you builder's out there have to say...What are some of your experiences (good & bad), and suggestions for either scenario?
Most of the time around here the air barrier is put on first.
The thing that bothers me is faced polyiso and thick XPS acts as a vapour barrier. In my area there is already a code mandated vapour barrier on the interior side of the wall. A lot of places online seem to 'solve' this problem by using thicker (ie 2+") insulation so the sheathing doesn't get cold enough to allow condensation (although leaks still pose a problem). Without foam the sheathing always gets cold enough to condense, so you'd think that using a vapour permeable insulation (EPS, glass faced polyiso) would allow moisture to escape whether it get in there via condensation or leaks.
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