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I'm trying to figure out the best way to insulate the ceiling corners in my basement. The wood is flat against the foundation of my house, and I can feel cold air coming off of it. There are joists running perpendicular to the crevices and some piping and wires running in it.

I was looking into a spray-on insulation since they are tight spots, but I'm not sure if it would be easier to buy some type of insulation foam board and cut it to fit the crevices.

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Cutting rigid foam really won’t be much better than what you have unless you seal the boards with foam.

I would spray foam around the penetrations first. I use a closed-cell foam that sticks well and can be sprayed on then it expands to ~1-1/2” but that is a small space and the stuff I use really doesn’t store well. It covers ~100sf per can or so.

I might try with the big box stores foam in small cans around the pipes and wires then cut some rigid board fit in place and seal the seams. This would be the most cost effective way. If the rigid foam was cut and ready to put in it could be done with 1 can.

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An alternative to Ed Beal's excellent answer might be just sealing the penetrations with flexible tape designed to ensure air-tightness. It is amazing what a difference eliminating draughts does to the warmth of a room (both because it means the air temperature is higher, and because still air feels much warming than moving air).

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I'd do a combination of sealing the holes with tape/silicone and then tuck fiberglass or foam board insulation into the cavities between the joists. I've done this in a couple of houses I've lived in and the difference was remarkable.

The tape (or silicone) will take care of the drafts while the insulation will prevent heat transfer through the wood surface. You can then tape or use spray foam to seal up the gaps around the insulation if you want, but I'm thinking you may feel that you don't need to seal further.

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