Foam too thin – can “rethinking the rules” apply to these walls?
Restoring a many-times-remodeled 1950s ranch in zone 5A. The exterior air barrier will be blower door tested before insulation and drywall. The interior air barrier will be tested after the gasketed air-tight drywall is installed.
Existing wall #1 cross section: (detailed cross section attached) 4″ brick, 1″ ventilation gap, 1″ EPS with perforated aluminum facer, 15# felt, 1×6 plank sheathing, fluid applied air barrier (STO Gold Coat w/ fabric over gaps), polyether sealant “ringing the studs”, R-15 unfaced fiberglass batts, 1/2″ airtight gypsum with gaskets, 0.6 perm vapor retarder primer.
Existing wall #2 cross section: (detailed cross section attached) same as wall #1 except delete: the 15# felt, 1×6 plank sheathing, and the Sto fluid applied air barrier. Replace the polyether sealant with Contega HF sealant for “ringing the studs”and sealing them to the EPS to make the air barrier.
I prefer to stay with unfaced batts because they (1) enable better 6-sided cavity contact than faced batts and (2) leave the stud faces available for drywall adhesive as recommended by FHB’s drywall moderator, Myron Ferguson.
I’ve followed the recommendations in Lstiburek, 2006, Understanding Vapor Barriers, (relevant sections attached). Based on the perm calculation (attached) the vapor retarder paint (specification with perm rating attached) seems to meet the requirements in the reference.
- Is the perm calculation a reasonable approximation?
- Will these walls “work” with vapor retarder paint in this zone?
- Is vapor retarder paint considered risky in this zone?
- Is an alternative vapor retarder and giving up the 6-sided cavity contact and/or drywall adhesive recommended?
Thank you in advance for any and all comments and criticism.
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