Unavoidable thermal bridge on foundation, exterior vs interior insulation – or a mix of both?
Hi everyone,
I am in the situation of having a poured concrete retaining wall connected to the foundation, causing an unavoidable thermal bridge through the concrete into the conditioned space. The design intent was to insulate the exterior of the foundation for all the known beneficial reasons, but this unavoidable thermal bridge has me questioning – is it better to do 100% interior insulation, or have a mix of exterior and interior insulation, with an overlap on the interior of 10′ to achive an approximate r10 w/ thermal bridge?
In a similar Q&A, Holladay says “Even better would be to put 100% of your basement insulation on the interior, or 100% of your basement insulation on the exterior.”
Is this really true from a thermal perspective, or is this a more practical answer given the additional materials and labor?
Thanks!
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Replies
With foundations that include crawlspaces or basements, I always locate the insulation on the interior. The supposed benefits of exterior insulation aren't worth the risks, including damage from backfilling, insect infestation, finishes that either look weird or don't hold up, and becoming saturated with water. The "thermal mass" effect is minor at best. Concrete is an excellent exterior material.
Haddende,
I agree with Michael. Exterior insulation can make all sorts of sense on wood framed walls, but below grade is too vulnerable to the things he mentions, and is extremely disruptive to fix if it is damaged. Unless there is some practical reason to have it outside, or to go with the hybrid solution, I would put it all on the interior.
You may find this useful: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
Another vote for putting it inside. The biggest issues outside are water saturation and insect damage from what I've seen. The "thermal mass" effect of that basement wall is likely to actually be a net negative anyway here, since it tends to slow down the ability of the HVAC system to condition the space. I don't see any downside to putting the insulation on the interior aside from the need for at least code minimum levels of fire protection over the foam, but if you plan to finish the basement anyway, that too is a non-issue.
Bill
I've done both exterior and interior, and whole agree with the previous posters. Exterior is a never-ending maintenance issue; there is no better maintenance free material for dealing with dirt, than concrete! But I note you're just asking about walls - ALWAYS insulate the floors prior to pouring concrete! Use at least 2" of EPS; we've used up to 6" but 2" works; cover it with heavy poly, and pour on top of that. DO NOT forget the edges!! Install vertical strips around the floor, and extend it about 2" above the finished concrete. Your concrete floor should never contact exterior concrete walls. Using foil faced Polyiso on the walls, EPS and poly on the floors gives you a pretty dry and very comfortable basement.