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Basement Insulation-Spray Foam

Aldrichian's picture

I'm taking advantage of a rebate offer provided by my local utility company that allows me to insulate my drafty old house at a reduced rate. The auditor who analyzed our house listed, as part of his recommendations, that we spray foam in the basement. This would include the rim joists and three feet down the cement block walls. I'm new to this stuff and while I want maximum efficiency I worry that the spray foam may hide any issues I may be having with the sills or other areas the foam is covering. I'd love some opinion on this. I'd also love to know if in fact insulating in the basement (we'll be insulating elsewhere, too) is indeed worth doing in an older house (c. 1915) like mine. Thanks for any advice.

It would help to know where (post #206716, reply #1 of 4)

It would help to know where you live.  Also, you say "issues I may be having..." -- does that mean that there are known problems, or you simply don't know?

Insulating that portion of the basement is wise, even when the basement is mostly "unoccupied", since it will help keep the floors warmer and will increase comfort upstairs.  You don't heat to maintain a given temperature, you heat to maintain a level of comfort, and warmer floors lets you actually lower the thermostat upstairs.

As to whether foaming the area is a good idea, that's a mixed bag.  First, any obvious areas of rot or deterioration should be taken care of, and anywhere where water tends to leak in (near the rim joists) should be fixed.  Soil on the outside should be at least 4-6 inches below the top of the foundation, and anything like a porch or sidewalk that is even with the rim should be sloped away, and the joint with the house appropriately flashed or caulked.  Once you've done this work, while any possibility of rot is not eliminated, it's greatly reduced.

Leakage through the walls well below the rim is not a serious problem from the standpoint of the foam causing rot, even if the foam covers parts of the leaky area (though that should probably be avoided).  Concrete doesn't rot.  The only problem is that the foam will prevent the concrete from drying as quickly, and that will cause the top of the foundation to be a little damper.

The main problem with the foam, of course, is that it will hold in moisture that somehow gets into the rim joist area.  If the area is always dry this is no problem, but if the area is damp then the rate of rot will be increased.  A modest increase in rot rate in a 100-year-old house is probably acceptable, but it depends a lot on how many more years you think the house will be around (especially given the cost of heating the thing).


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Thanks for your response, (post #206716, reply #2 of 4)

Thanks for your response, Dan. I really appreciate it. I live in southern NH, so, you know, we get a decent winter and some hot summers. The structure of the building seems to be in good shape. No issues yet. I was just worried that by foaming over the rim joists I'd be preventing easy access to the sills should something ever come up. I have a friend who is a home inspector and recommends not foaming over the rim joists but just going with regular fiberglass. However, having spent the last week pulling out a lot of mice infested fiberglass in my basement, I'm inclined not to go in that direction. 

Yeah, there is no perfect (post #206716, reply #3 of 4)

Yeah, there is no perfect answer here.  Every approach has some drawbacks.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Basement insulation (post #206716, reply #4 of 4)

is a topic well-covered at greenbuildingadvisor.com. If you are thinking of going this route I would do some serious reading.