Insulating interior AND exterior basement walls OK?
Hi,
We have an old field stone foundation here in Connecticut. Generally speaking the foundation is holding up well except that it does leak after multiple days of heavy rain – maybe once a year. I will be repairing the concrete both on the exterior and interior. After I repair the concrete on the exterior I was going to add a few inches of XPS, followed by multiple strategies to divert water from the foundation, in the future. I am fairly confident that I can keep water out of the basement considering the house is actually on a slight hill with the whole yard sloping away from the house on all 4 sides.
After I do that I wanted to finish the basement and so was going to repair the wall, throw up metal studs, spray foam, followed by XPS over the studs.
I thought I saw an article years ago somewhere that said not to insulate both sides of a basement wall – Is this true?
Thank you in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It's fine to insulate both sides of the foundation wall (that's the standard insulated concrete form approach, after all). Fieldstone foundations wick considerably LESS moisture than poured concrete, so it doesn't need as much drying capacity to keep the foundation sill happy. (If you jack up the house and slip in an EPDM membrane as a capillary break it wouldn't matter even if it were poured concrete sitting in a puddle- it would still be happy.)
Given it's a field stone foundation, I'm not sure what you mean by "...repairing the concrete both on the exterior and interior...", or how you intend to attach & finish rigid sheet foam to it.
Spray foam between metal studs is a waste of expensive foam due to the high thermal bridging of the studs, even if you added rigid foam over the interior side of the studs. For a fieldstone foundation in CT it's reasonable to spray 2" of closed cell foam to the uneven surface, then install a wood 2x4 studwall to the interior side, with 1" EPS under the bottom plate of the studwall as a capillary & thermal break against the slab.
There is little to no advantage to using XPS instead of EPS in this application. The XPS has a somewhat lower vapor permeance, but with even 1" of closed cell spray foam against the stone or concrete it's already moderately vapor-tight at about 1-perm. If the interior wall is all smooth concrete instead of field stone, doing the interior side with 2" of EPS or polyiso trapped against the foundation with a rock-wool batt-insulated studwall on the interior would be higher performance, cheaper, and perform about the same as 2" of closed cell foam on the exterior + 2" of closed cell foam on the interior.
This would be my 1st choice 2" of EPS or polyiso trapped against the foundation with a rock-wool batt-insulated studwall on the interior would be higher performance, cheaper and I also live in CT. You could also splurge if your wallet allows it to smooth the stone interior wall using shotcrete.
http://www.foundationsupportworks.com/foundation-repair/wall-stabilization/wall-anchor/shotcrete.html
and then install 2" of EPS or polyiso trapped against the foundation with a rock-wool batt-insulated studwall on the interior.
Nikola,
If you are exposing both sides of an old field stone foundation that "leaks after a heavy rain," you have a great opportunity to make it waterproof. After pointing up the stonework with mortar, install a footing drain that leads to daylight or a dry well far from your house. Consider installing dimple mat against the outside of the foundation wall. Backfill with coarse material that drains well -- bank-run gravel or crushed stone -- capped by soil that slopes away from your house.
The main reason that people don't do this kind of retrofit work is the expense of excavation. But if you are planning to expose the wall on the exterior, don't waste the opportunity to fix the foundation, once and for all.