what to do with rim joist that has 1/2″ foam board exterior
I am on Long Island.
I have a basement that we use. It is finished and heated/cooled (dehumidified).
However the rim joists are behind a drop ceiling, and only have glass fiber stuffed in there.
I was planning to do the “foam board+spray can foam”-vapor barrier, with rockwool (r-value + fire barrier) on the inside of that.
However, I just noticed that we have a 1/2″ (or 5/8″, hard to say) of some foam board on the outside under my siding. Presumably XPS, but I’m not sure. This does cover the rim joist.
Therefore, assuming 1/2″ exterior is not enough to keep the joists warm enough to avoid condensation in winter, I may need to add r-value on the inside.
A spot check does show there is no current evidence of past condensation on the rim joists.
However, if the outside board is a vapor barrier, I likely should not add one on the inside (foam board+spray can foam); having a rim joist sandwiched between two vapor barriers should be avoided.
So, given my situation, what should I do on the inside?
Just r-value? Or semipermeable foam board? Or nothing at all?
Thank you for any expert insight you can provide.
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Replies
1/2" foam board is, at most, somewhere around R3, so just enough for your climate zone (CZ 4A). I don't like going with the minimum though. I would foam in some 1/2" low density EPS on the interior, which is the most vapor open of the rigid foams, and allows for a little bit of inward drying ability. You could put some of the Safe'n'Sound 3" mineral wool over that for fire resistance, and a bit shy of an additional R13 worth of insulation. That would give you about R15 or so overall on the interior (1/2" EPS + mineral wool), and another R2-R3 on the exterior, which ends up being pretty good.
Bill
Okay, thank you.