1.5″ Mineral Wool Exterior Insulation in Climate Zone 6A
Hi GBA’ers,
Hoping to get some feedback on this wall and roof assembly. In particular I’m concerned about condensation forming as I don’t think i quite hit my percentages correctly and Ive already done my install.
Climate Zone 6a
From Interior to Exterior
Drywall
2×6 framing
4.5″ Clean shaved Open Cell Spray Foam
1″ Closed Cell Spray Foam
1/2″ Plywood Sheathing
Adhero Vapor Permeable WRB
1.5″ Rockwool
1″x3″ Purlins – Rainscreen Gap
Siding.
So roughly I have R32 Interior cavity insulation and only R6.3 Exterior insulation. Should I be worried?
The roof assembly is the same but the cavities have much more insulation:
2″ closed cell and 8″ of open cell.
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Replies
R10 to R20 would be safer for zone6a (exterior) however you do have a very vapor permeable rock wool layer/WRB/plywood sheathing along with strapping which should allow excellent drying to the outside if the dew point migrates into your exterior insulation layer.
The other factor in your favor is that you have a good air seal via the closed cell foam. If siding is not on then you have a choice of adding more exterior insulation, or just making sure your strapping/siding layer is well provisioned for venting top and bottom. Air is not going to be moving super easily in your inner foam sandwich either, so again this would be in your favor vs using fiberglass inside where moist air leaks can introduce moisture to the wall system.
If moist air cannot migrate within the wall’s inner cavity (when winter brings the dewpoint inwards) then again, not a huge risk there.
the closed cell inner surface is the condensing surface due to its vapor retarder action.
That closed cell R value is already exterior R - to a point at least. So this assembly is not that bad, although more stonewool would not hurt..