I am insulating my cottage
I am insulating my cottage. I need to know if I need to seal all the cracks completely in the outside wall? It is wood Dutch Lap siding.
I want to use 2″ blue extruded (XPS) board, and 4″ fiberglass insulation with vapor barrier, for a 6″ wall. And also, what about the seams in the 2″ foam boards? The interior wall will be 1/4″ OSB with knotty pine 1/4″ paneling. Is the vapor barrier on the fiberglass enough?
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Every wall needs at least one, and sometimes two, air barriers. For more information on this topic, see Questions and Answers About Air Barriers.
It's unclear from your question whether the rigid foam will be installed on the exterior side of your wall or the interior side of your wall.
It's also unclear whether you are talking about new construction or retrofit work. If it is retrofit work, are you opening up the wall from the interior or the exterior?
If your wall has OSB or plywood sheathing, and if you have access to the sheathing seams, you can create an air barrier by sealing the seams with high-quality (usually European) tape.
The kraft facing on fiberglass batts is not an air barrier.
A true vapor barrier on the interior side of a wall with 4" of cavity fiberglass and 2" of exterior XPS is risky due to the fairly low vapor permeance of XPS at that thickness (about 0.5-0.7 perms.)
In most climates the ~2 perm vapor retardency of the interior OSB layer (if made air-tight) would be plenty. If you used half-inch OSB instead of 1/4" you'd be at about 1 perm, a bit less when you add in the vapor retardency of the 1/4" pine paneling, which would be fine.
In most US climates R10 on the exterior of R15(?) fiberglass would be sufficient for dew point control at the sheathing even with 5-perm paint on sheet-rock as the interior vapor retarder.
Kraft facers on batts are variable permeance, about 0.4 perms when dry, 5+ perms when wet, but it's probably better to do without them. Air tightness is going to be the most critical thing here, but try to leave yourself about 1 perm, not a lot less, or the very slow drying rates can impact moisture resilience.
Also, if the true depth is 4", as with rough-cut full dimension 2x4s, and not the 1.5'x 3.5" dimensions of a milled 2x4, fitting batts correctly is going to be a problem. The widths may need to be sculpted & trimmed to keep them from buckling, forming depressions & voids, and you'll have to use a thicker batt than an R13 or R15, since the manufactured loft is less than 4". Using low density R19s or R22s designed for 2x6 framing can be made to work, but it's a bit of a pain. Blown insulation is a better solution when using full-dimension framing lumber. An R19 compressed to 4.0" will run about R15-R16: