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Polyiso foam in basement: a rim joist question

Hi all,
Love this site, great hints
Now Im in Rockland county a little north of NYC and have a 2007 2 story colonial
I have added to the R38 with another 8" of cellulose and now am looking at the basement.
It is unfinished but I has r19 batts in the ceiling (unfinished)
So I was looking at putting 2" Poly Iso board glued to the basement walls, the ceiling height is 8ft so perfect fit for the boards
My question is the rim joist I did a cheap job of putting blocks of 1" expanded polystyrene in and sealing with Great Stuff, now if I put up the Poly Iso do I need to add another 1" of say spray foam (no time for patience to cut all those board pieces again) or because the boards are flush with the ceiling and basement wall, there will be no more humid air touching the foam blocks I put in.
Im worried that I need to add more because before I put the blocks in the batt plugs the builder put in where wet.
So do I need to add anything else to the joist space or because the iso will be flush with the floor joists once I put up sheetrock on the framed walls and ceiling I should be ok
Id hate to make a mistake to find rotting sill plate in 10 years
Any advice would be appreciated

PS I will mention this again, I LOVE THIS SITE!!

Asked by Darren Finch
Posted Tue, 02/11/2014 - 13:23
Edited Tue, 02/11/2014 - 13:51


1 Answer

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In general, it's a good idea to insulate rim joists to the same minimum R-value as above-grade walls.

In your climate zone (Zone 5), the 2009 IRC requires walls to be insulated to at least R-20.

You have insulated your rim joist to about R-4. That's not much. If you add an additional 2 inches of polyiso to your rim joists -- and from your description, it's hard to tell whether or not you are planning to do so -- you would end up with a total of about R-16. That's better than R-4, but it's still not R-20.

If you did a good job of air sealing the perimeter of each piece of expanded polystyrene that you installed at your rim joist, then I don't think your rim joist is in danger of rotting. But it's certainly true that your rim joists are poorly insulated.

For more information, see Insulating Rim Joists.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/11/2014 - 14:02
Edited Tue, 02/11/2014 - 14:04.

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