Vapor barrier inside double stud wall — is this OK?
I’m building an energy efficient house this spring 25 miles Northwest of Minneapolis in the upper portion of zone 6. My wall design plan since the ’80s has been a double 2×4 stud with the vapor/air barrier in the gap between the two walls. This is similar to Joe Lstiburek’s Ideal Double-Stud Wall Design with 2 taped layers of 1.5” Foil Faced Polyisocyanurate foam board replacing the ½” OSB in Joe’s design. I believe this design would keep the inside polyiso vapor barrier well above dew point under the most demanding winter conditions. In the summer the outside surface of the polyiso would also be above dew point.
I have been getting pushback from insulation contractors that are scared of the vapor barrier being inside the wall. I have read much of the information written about moisture in walls by GBA and others. I have done wall temperature analysis that shows that this wall should work but it is such an important issue that I still need some reassurance. I would like your opinion.
This is the breakdown of my wall construction.
Double 2×4, 16” OC, 11 3â„4 in. deep, 2 layers of 1.5” Foil Faced Polyisocyanurate, staggered with seams taped in the gap between walls. Both stud walls insulated (exterior Mineral wool? Interior Fiberglass) R 45
Description Thickness R Value Perm Rating
1 LP Smart side
2 Furring strip (rain screen)
3 Exterior Air Film 0.17
4 7/16″ Zip sheathing 7/16″ 0.62 12-16
5 2×4 16 OC Mineral wool 3 5/8″ 15 30
6 Polyiso Board 2 layers of 1.5″ 18.6 0.05
Total R of Interior Vapor layer to outside 34.39
7 2×4 16″ OC Fiberglass 3 5/8″ 9.2 High
8 1/2 ” Gypsum 1/2″ 0.45 50
9 Interior Air Film 0.68
Total R of Interior Vapor layer to inside 10.33
By the way the other wall options I’m considering are 6” or 8” SIPS or a 2×6 wall with vertical I joists filled with dense pack cellulose.
Thanks in advance for all you help
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