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Rim joist insulation

I have several questions about insulating the rim of the 1st floor platform above the basement of my 2001-built house. I have recently found condensation on the rim joist after cold nights under the squares of the open-faced FG bat that had been shoved between the I-joists. I removed the FG and no longer get condensation, but would like to reinsulate effectively.

I plan on installing rigid foam between the I-joists, and need to determine what R value is appropriate for Northern Colorado.

Secondly, can the foam board just be fitted push tight to the rim joist and sealed around the edges, or does it need to be bonded, and if so, with what?

I have been told by the building inspector that I will need a thermal barrier over the foam, or have approval to use Dow Thermax. I wondered if I were to place more than one layer of foam if I could place XPS next to the rim joist and use the Thermax as a thermal barrier?

The ends of the platform parallel to the I-joists were finished with an I-joist at the outer edge of the foundation, and the OSB sheathing was nailed to the outer side of the I-joist leaving a 1-in open gap which is inaccessible except through the I-joist wiring knockouts. An insulation contractor suggested I try to put foam into these narrow gaps through the knockout holes, and not insulate the inside of the I-joist until i did this. I am concerned about getting good fill in such a restricted situation and about problems resulting from expansion of the foam pressuring the interior (I-joist and sheathing). Would this foam fill be necessary and advisable? Would it work to just insulate the interior side of the I-joist?

Finally, at one end, due to spacing, another I-joist is located only 4-in from the outer edge I-joist, making access to the open area very restricted to an 1 1/2-in slot above the concrete basement wall edge. I could box the narrow space between the I-joist in with foam board and insulate the side of the inner I-joist, or try to foam the space between the I-joists. Which approach would be safest with respect to avoiding condensation in cold weather?

Asked by William Fryer
Posted Dec 21, 2012 11:52 AM ET
Edited Dec 21, 2012 12:37 PM ET


1 Answer

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In your climate zone (climate zone 5 or 6), the code calls for at least R-10 insulation on basement walls and R-20 on above-grade walls. That's the range you should consider. You also need to satisfy your local building inspector, whose opinion is more important than mine.

Yes, you can install two different types of rigid foam if you want, in two layers. Only the last layer (the exposed layer) needs to be Thermax.

You don't need to glue the rigid foam to the rim joist, as long as it is pressed tightly in place, and as long as the perimeter of each piece of rigid foam is sealed with caulk or canned spray foam.

Concerning the proposal to use spray foam to insulate the rim joist that is parallel to the other joists: we can't determine access from your description, so that judgment has to be made by someone who is on site.

Concerning the joist that has 4 inches of access: use spray foam. I suggest you buy a two-component kit like a FrothPak kit.

Answered by Martin Holladay
Posted Dec 21, 2012 12:36 PM ET

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