How to increase the R-value on walls with closed-cell insulation?
user-5311948
| Posted in Energy Efficiency and Durability on
I want to increase the R-value of exterior walls that have closed-cell insulation. Rigid foam on the exterior wall will stop the wall drying to the exterior. Is there a way to apply exterior insulation on a wall with closed cell insulation while still allowing the wall to dry?
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Replies
Kristen,
Yes: Use mineral wool. For more information, see these articles:
Installing Mineral Wool Insulation Over Exterior Wall Sheathing
Installing Roxul Mineral Wool on Exterior Walls
Wrapping an Older House with Rock Wool Insulation
How much closed cell insulation is there?
What climate zone/location?
I have 2x4 studs, with an R21. I am in Alaska, climate zone 7. It can get fairly cold and windy here, often down to -20F in the winter for a few weeks at a time.
At ~3- 3.5" most closed cell foam runs about 0.3 perms, which is on the tight side of class-II vapor retardency, but not a true vapor barrier.
With unfaced 1.5lb density "Type-II" EPS on the exterior you'd still have 1 perm of drying capacity toward the exterior at about 2.5" /R10, which is the very high end of class-II vapor retardency, and definitely NOT a moisture trap. Drying rates would be pretty slow compared to 30 perm rock wool, but if the flashing is lapped correctly it's not particularly risky to install R10 EPS, as long as there are no facers.
XPS is significantly more vapor tight, but you can still add as much as an inch (R5) without risking a moisture trap.