Home>Discussions>INSULATION & HVAC>Spray foam vs fiberglass for rim joists to seal out air/moisture???
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WINT142000
Spray foam vs fiberglass for rim joists to seal out air/moisture???

Okay, I searched and cannot really find my answer. I have an unfinished basement in 2 story with approximately a 1000 sq ft footprint. We had an energy audit a while back and one of the recommendations was to use spray foam in the rim joists to seal air and moisture that come in along the sill plate and the joist cavity. About 5 years back I had insulated the joists with fiberglass, but the energy auditor claimed that moisture and air still come in, especially in the summer when the A/C is running. He claimed this is one of the reasons that basements smell damp in the summer is that as the hot air rises warm moist air is sucked in from outside, but this would not be as much as an issue in the winter which is why basements don't smell as much in the winter.

Anyone able to verify/comment on this? Obviously the auditor's company wanted to help me by doing this, but I can't find much info out there on it.

Thanks!

keith3267
Re: Spray foam vs fiberglass for rim joists to seal out air/moisture???

The auditor is correct, but there are alternatives. You can use caulk to reduce the infiltration. Run a bead along the seam where the subfloor meets the rim joist, where the rim joist meets the sill plate and where the sill plate meets the basement walls. Then put fiberglass against the rim joist so that the rim joist is fully insulated.

Be sure to do this when the wood is good and dry. You could also paint the wood with a vapor barrier primer before caulking or treat the wood with a sodium borate solution before caulking, or both.

Rekonn
Re: Spray foam vs fiberglass for rim joists to seal out air/moisture???

You could also try spray foam, there are DIY sources like sprayfoamdirect dot com.

Gizmo
Re: Spray foam vs fiberglass for rim joists to seal out air/moisture???
Timothy Miller
Re: Spray foam vs fiberglass for rim joists to seal out air/moisture???

Howdy consider using acoustic caulk at the rim joist to the foundation it expands and contracts with temp changes and works great. The fiberglass does not restrict air flow- its first a filter secondly an insulator. Might add 2" of pink foam board into the ends of the joist bays and caulk or spray foam , use thee cans a little bead goes along way, The 4 by 8' sheets of foam board are easy to cut an install and a high R value per sw inch. Advantage is reducing the heat sink at the rim and the foam does not allow air movement.

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