Rim joist insulation with blocking
I am finishing my new construction basement addition and I’m a little unsure of what to do with the rim joists. The rim joists are 2×12 LVL with double rims over two groups of two windows. A cripple wall separates the rim bays from the foundation. Tyvek covers the sheathing with a rainscreen gap. There is TJI (I beam) blocking in between each floor joist, leaving an open space behind each block. I currently have R30 Roxul filling the rim joist bays but I’m wondering if I should pull it out, install rigid foam-sealing the edges with spray foam, then reinstall the Roxul. The space will be finished with drywall and most likely PVA primer.
Three main questions:
1-Is it necessary to air seal these rim joist bays since they should be sealed from the exterior Tyvek? Is there anything wrong with leaving the roxul and not installing foam?
2-If it’s deemed necessary to pull the roxul and install the rigid and canned spray foam to air seal, does the air space behind the TJI blocking present an issue, creating a place for condensation?
3-The ends with non bearing walls have a TJI too close to the rim joist to get any insulation in there so there is a gap of 2-3″. I wish I could have put rigid foam in there when they were framing but, alas, didn’t happen. Is there anything I can do about this besides drilling holes in the TJI’s and sinking 30 cans a spray foam in there?
I’m in Seattle, zone 4a. Thanks!
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Replies
Pat,
Exterior Tyvek does not provide a good air seal at a rim joists. There are multiple cracks in this area, and during the winter, the basement area is depressurized with respect to the outdoors due to the stack effect. This depressurization pulls outdoor air into the cracks. So don't assume the Tyvek helps in any way.
If your builder remembered to use gaskets, caulk, and high quality tape when assembling the elements of your frame, you might be fine. Otherwise, spray polyurethane foam is a good way to seal air leaks at the rim joist area (and to provide some R-value).
I don't know what kind of access you have between the blocking and the rim joist. But whatever plan you adopt, the steps are the same: air sealing and insulating. If there is enough room in this area to use the spray wand of a two-component spray foam kit, that might be your best approach.
Here's a photo of the blocking I'm referring to. Because it's a TJI, there is a space behind the osb portion and this is why I'm concerned condensation could occur. I don't want to go though the time and expense of spray foam if at all possible and would rather use the cut and cobble technique. Should this blocking be removed fit a tighter fit to the rim joist? Is it structural? Is the space behind the blocking a concern for condensation?
Martin, unfortunately, my builder didn't caulk much besides the mud sill and the siding sub didn't tape the tyvek at all. I think the siding guy was annoyed I insisted on rain screen furring as they didn't think it was necessary.
Pat,
Your main goal needs to be air sealing. While the current details aren't great, everything should be safe if you can get a good air seal on the interior.
There are several ways to proceed, and the decision is up to you. You can either (a) use a two-component spray foam kit, (b) use caulk, or (c) use cut-and-cobble rigid foam with careful perimeter sealing. Good luck.
I caulked mine, had 2" of exterior foam over taped tyvek and taped sheathing. Then I filled it with cellulose that was dense packed into the breathable fabric. It's dry, warm and sealed, but it took time to staple the fabric in every bay. I am thrilled with the performance of my dense packed house.