Thermax over XPS rim joist
humm9er
| Posted in Energy Efficiency and Durability on
I believe the answer is no, but no problem layering foil faced polyiso over XPS in the rim joist, correct?
The XPS was can spray-foamed in, no issue if there is a slight air gap between the layers, correct?
Lastly, should I airseal the thermax perimeter with additional spray foam?
I have leftover 3″ thermax and only 2″ of XPS in that area so I wager it’s a worthwhile application.
Thank you.
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Replies
Justin,
Q. "No problem layering foil-faced polyiso over XPS in the rim joist, correct?"
A. Correct.
Q. "The XPS was can spray-foamed in; no issue if there is a slight air gap between the layers, correct?"
A. More or less correct, especially if the second layer has an air seal at the perimeter. But the installation is better without the air space. So knock off those blobs of cured spray foam with a putty knife before you put on the second layer of rigid foam, to minimize the air space.
Q. "Should I airseal the Thermax perimeter with additional spray foam?"
A. Yes, that would be a good idea -- especially if you have an air space between the two layer of rigid foam.
Justin, are you planning to leave the Thermax exposed, or will you be covering it? There is not a moisture issue with what you propose, but there may be a code issue--Thermax is rated to remain exposed, but not for protecting another layer of foam. If you're concerned about meeting code, you'll probably need to finish the interior with a thermal barrier. (There are some exceptions.)