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Rim joist air-sealing options

We are using 2" of exterior polyiso foam with air-barrier established at the Zip sheathing. Our first choice in regards to the basement rim joist was to use closed-cell spray foam. Now I understand that we should avoid this "foam sandwich" but open-cell is an option due to its vapor permeability properties. Then I read more about foam and am a bit afraid of spray foam in general.

Since our exterior air barrier is the Zip sheathing, should I still be concerned about getting the air-sealing details perfect in this area? Any non-foam option seams like a ton of work for likely inferior results. Any thoughts or ideas appreciated.

Asked by Troy Stevenson
Posted May 4, 2013 10:34 PM ET


4 Answers

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There are at least two possible air leakage points: between the concrete and the mudsill (usually addressed with sill seal plus caulk), and between the rim joist and the Zip sheathing (which could be addressed by a bead of caulk at the bottom of the Zip sheathing, or even with European tape between the Zip sheathing and the concrete).

If you weren't taping the Zip sheathing, there would be other areas of concern (for example, between the rim joists and the subfloor -- usually handled with construction adhesive -- and between the subfloor and the bottom plate -- usually handled with caulk or a gasket).

Presumably, the 2 inches of polysio will cover the exterior of your rim joists. That will keep your rim joist warm (assuming that you are located in climate zone 6 or somewhere warmer -- you didn't mention your location), so you really don't have to worry about moisture accumulation in the rim joist.

If you are in climate zone 6 or anywhere warmer, you can insulate your rim joist on the interior with fiberglass batts or mineral wool -- as long as the exterior of your rim joist is insulated with 2 inches of polyiso.

If you live in climate zone 7 or 8, your exterior polyiso should have been thicker.

Answered by Martin Holladay
Posted May 5, 2013 4:15 AM ET


Martin - thank you for the response. Yes - we are taping the seams of the ZIP sheathing and the exterior foam will run down over the rim joist. We are in Zone 5 - Detroit, MI.

If we end up using spray foam in the rim joist and use a reputable contractor who applies the foam under the proper conditions and don't occupy for the house for a least 5 months until after it's application, how much should I lay awake at night worrying about off-gassing health related issues?

Answered by Troy Stevenson
Posted May 5, 2013 11:42 AM ET


Only you can decide whether the off-gassing risk of spray foam is worrisome. If you're worried, use rigid foam, fiberglass batts, or Roxul.

Remember, you don't need to worry about air leaks if you have done a good job with the sill seal, caulk, and Zip sheathing.

Answered by Martin Holladay
Posted May 6, 2013 6:18 AM ET


If you're using closed cell foam on the interior you have to limit it to ~2" or ~R12 to avoid the moisture trap. If going with open cell you can take it up to R20 or even a bit further, since it can still dry toward the interior due to it's much higher water vapor permeance.

But rock wool works fine there too, and probably has the fewest chemical offgassing & friable-fiber issues of any insulation suitable for that application. (It's not as if rigid foam is completely offgassing-free either, eh?). Rock wool fiber very inert stuff, with only the tiniest amount of binder added for making batts, and what's added mostly gets cooked off in the manufacturing process.

Answered by Dana Dorsett
Posted May 7, 2013 12:23 PM ET

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