Basement wall insulation
deb
| Posted in GBA Pro Help on
I recently glued 2′ 4×8 (XPS) rigid foam to my basement walls. I cut pieces to fit between the joists and along the rim joist. I put pieces on top of the cement wall, and then foamed around all those pieces, and the bottom of the 4×8 sheet. I then installed a 2×4 wall in front of and touching the rigid foam.
Do I need to put unfaced batts in the bays ?
Thanks
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Replies
Deb,
If your XPS is 2 inches thick, it has an R-value of R-10. That meets the minimum code requirement (according to the 2012 IRC) for basement walls in Climate Zone 3 and Climate Zone 4 (except Marine Zone 4).
If you live in Marine Zone 4 or Climate Zone 5, 6, 7, or 9, you are supposed to install at least R-15 insulation on your basement wall.
I think it is best to avoid the use of fiberglass batts to insulate a basement wall, so I would quit if I were you. However, if you live in a cold climate zone, you could try to move the studs away from the wall so that you can install more insulation. Alternatively, you could install rectangles of R-5 rigid foam between each pair of studs. The perimeter of each piece of rigid foam should be sealed with canned spray foam or caulk.
For more information on this topic, see How to Insulate a Basement Wall.
Thanks for the reply. I live in Rhode Island, which I think is zone 5. I had been told that if I didn't put batts, I would get condensation.
Deb,
Condensation problems are much more likely in walls in walls with fiberglass batts than walls with rigid foam. Considering the fact that you have apparently installed R-10 foam, you won't have a condensation risk.
R-15 would have been better (because R-15 would have reduced the rate of heat loss compared to R-10), but condensation won't be a problem.
Putting in unfaced batts will not increase or decrease the condensation or rot risk in the stud bays in your climate, and would cut your heat loss out of the basement walls by half. If there is a history of bulk-water intrusions, keep the bottom of the batt above the high-tide mark. Rock wool batts would be a better choice than fiberglass (the blue box store carries R15s), but the long-roll R13 fiberglass would be fine too.
If you take the cut'n'cobble approach with rigid foam between the studs recommended by Martin you can use reclaimed roofing foam (available from several MA/RI vendors- search craigslist materials for "rigid insulation", or if you feel like driving to Framingham MA with a truck there's always insulationdepot.com) which could cost even less than virgin-stock R13 batts, but are more work to trim, seal & install. (I did my basement in central MA with reclaimed 3" roofing iso held in place by furring through-screwed to the foundation on which to hang the gypsum, no studwalls or batts.)