Girder and interior basement insulation
Hi,
I’ve come to the conclusion that I am not going to be able to get a FPSF built in my area. I’m looking at a rigid foam insulated basement, with the foam on the inside of the walls. There’s going to be a wooden girder running across the basement that will be hung from the concrete at either end. I’ve got two related concerns.
1. If air seeps past the foam along the girder to the wall, it will condense, and leave the end of the girder moist and vulnerable to rot.
2. The whole thing is a thermal bridge. (But probably not a huge thermal bridge.)
Are either of these problems worth worrying about? Are there any good solutions?
Thanks,
John
Zone 5, Rochester NY
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Replies
I've designed a beam pocket on a 12" thick concrete wall and insulated the pocket on all sides with rigid foam. The pocket was 6"x6"x15 1/2" lined with 1" rigid foam to fit a 16" beam. (15 1/2" pocket + 1 1/2" sill plate - 1" foam = 16" beam)
John,
I don't know whether Armando's solution makes sense. I would be worried that the beam would crush the rigid foam at the bearing point, causing settling.
I think it makes more sense to stop the beam two inches short of the concrete wall, and support it with a lally column. You should talk to an engineer to make sure that the joist-to-beam connections and beam-to-post connections will work.
My detail has been engineered with high density foam at least a dozen times over 20+ years. I've never heard a complain yet.
Armando,
Thanks for the explanation about using high-density foam. I'm not going to argue with success.
It's too bad you can't get your FPSF built. I've had trouble getting concrete contractors in rural areas to build anything other than what they've done for the last 40 years. Frustrating to say the least.
I like Armando's detail, and wish I had thought of it for a recent project with the same situation. I'm not worried about it but if it was a Passivhaus or you want to be extra careful, I would quantify the load at the beam pocket, and use the appropriate density of this foam: https://foursevenfive.com/product-category/thermal-insulation/compacfoam/. Or I would use Martin's idea of extra lally columns. Or flush-frame the girder and buy a few joist hangers.