How much insulation in rim joist?
Danw7118
| Posted in Energy Efficiency and Durability on
Hi. I plan on insulating my rim joists with 2″ ridgid foam board, sealed with spray insulation Question is, should I double it up to 4″ / r20 or leave it alone with 2″? I’m not sure of my zone, but I’m in a suburb outside of Pittsburgh PA.
Basement is completely unfinished, someday might get it done.
Thanks,
Dan.
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Replies
Dan,
The goal would be to insulate the rim joists to the same R-value as your framed walls. I believe you are in Zone 5A, so you should probably double the rigid foam. Here's a link to some information about climate zones and an article on insulating a rim joist.
https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/4_3a_ba_innov_buildingscienceclimatemaps_011713.pdf
https://www.greenbuildingadvisor.com/article/the-best-way-to-insulate-a-rim-joist
Thank you! To follow up, when doing the double foam board insulation should I place the first peice, seal it with spray foam then immediately do the 2nd layer? Or would you let the first layer dry then follow with and do the 2nd layer the exact same way?
I would put in the first piece, loosely foam it in around the edges with only enough foam to hold it in place (you don’t want it expanding out from around the edge), then place the second piece and foam that one completely. If you use too much foam on the first piece, it’ll expand too much for you to get the second piece to fit tightly against the first. Been there, done that :-)
Cut the pieces for a loose fit too. It’s easier to let the canned foam finish off the fit than to try to get a perfect fit with the rigid foam. Leave maybe a 3/8-1/2” gap all the way around so that you can get the straw for the canned foam in easily. It sounds counterintuitive, but tight gaps are actually more difficult to seal with canned foam than large gaps.
Bill
Allegheny county / Pittsburgh is zone 5, where IRC code minimum for basements is R15 continuous insulation.
XPS labeled R10 is only warranteed for R9, and will over the lifecycle of a house drop toward R8.4. The HFC blowing agents used for XPS have an extremely high environmental impact compared to the hydrocarbons used for blowing polyiso or EPS. Using polyiso (even reclaimed, used roofing polyiso) would be R15 or higher at 3". Used EPS is good for R16 or better at 4". Reclaimed foam is dirt-cheap. Damaged reclaimed foam is even cheaper, and perfect for cut'n'cobble applications like this, since any damaged sections can be discarded without tossing the whole board.
Going with 1" polyiso would be sufficient dew point control for R15 rock wool or 3" rock wool sound abatement batts, which would also significantly reduce the fire hazard compared to exposed foam.
Keep running this search every week or so- you can probably find used foam closer to you:
https://pittsburgh.craigslist.org/search/sss?query=rigid+insulation