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Jmoc
Air coming in through bump out
Jmoc

My first floor has a bump out over the walk out basement. There is a vinyl soffit built around the joists where they extend out over the house foundation. There is insulation in the bay's between the joists but no framing that prevents air from coming in between the joists bays and going under the first floor ( i.e., looking in from the outside, if I remove the insulation, I can look all the way down the joist bays into the space between the basement ceiling and first floor subfloor. Shouldn't there be some type of blocking between the joists at the foundation that prevents outside air from coming into this space, or is insulation all that is needed? The floor above this does get cold in the winter, and on really cold days the kitchen pipes freeze. Is it standard building practice just to use insulation in this space, or do I need to add some other barrier to air flow?

ed21
Re: Air coming in through bump out
ed21

Yes there should be some kind of blocking to seal off the joists of the cantilevered joists. At this point adding wood blocking would be difficult. Rolling up insulation tightly and packing it in the joist/wall intersection should provide an air seal to the house. Then insulate the rest of the cantilever properly. Since all the soffit may be off I would staple some building wrap to the joists before reattaching the soffit. I can't speak to the water pipes since I don't know how they are arranged.

hollasboy
Re: Air coming in through bump out
hollasboy

For situations like these, I prefer to use fire-block expandable foam in a can to seal off the space at the top and bottom of the stud bays. Use any other type of filler insulation for inside the bay before sealing it off. Fiberglass insulation does not completely block air flow (even when tightly rolled/wadded up in the bay), and blow-in cellulose is flammable and needs a solid surface at the bottom of the bay to keep from falling downward. Normal spray foam cans have flammable foam and could contribute to a fire in the joist bay, making the situation much worse during a conflagration. Fire-block rated foam is ideal for blocking air flow, while simultaneously providing fire protection.

ed21
Re: Air coming in through bump out
ed21

I like the foam idea to hold wood blocking between the joists at the location of the exterior basement wall. Cut the blocking to fit and it doesn't need to be exact, then foam it in place at the edges. Fire rated foam isn't really required to just hold the blocking.
For the cantilevered joists just fill them with fiberglass insulation and reinstall the soffit.

Mastercarpentry
Re: Air coming in through bump out
Mastercarpentry

I always add blocking between cantilevered joists at the wall support point to prevent joists from leaning or flexing, but very few others do this. Solves both problems at once with little wood or time involved if it's done when it should be, but retrofitting may not be so easy.

Phil

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