What’s the best way to insulate concrete basement walls? Does it matter whether the insulation goes on the inside or the outside?
There are several approaches to insulating the walls of a foundation or crawlspace, but a good place to start is with the International Residential Code (IRC). The most recent version, published in 2021, requires foundation insulation in climate zone 3 and up, although it’s always a good idea to check with local building officials for specifics in your area.
Insulation can be applied on either side of the foundation wall, and the code permits both continuous insulation (like rigid foam) and cavity insulation (such as fiberglass placed in a stud wall on the inside of the foundation). Code requirements are summarized in Table R402.1.3, but here are the basics:
- Climate zone 3: R-5 of continuous insulation or cavity insulation of at least R-13.
- Climate zone 4, except Marine: R-10 of continuous insulation or R-13 in cavity insulation.
- All other zones: R-15 of continuous insulation, R-19 of cavity insulation, or a combination of R-13 in cavity insulation plus R-5 of continuous insulation.
These are minimums. If you’re building to Passive House standards or you like the sound of the Pretty Good House approach, you’ll have to adjust accordingly. Insulation will make the basement more comfortable. It also will lower heating bills, although estimates of exactly how much seem to vary from study to study.
Foundations can be insulated when the house is under construction, before the foundation has been backfilled, or as a retrofit down the road. Two of the most common questions are what type of insulation to use, and whether an interior or exterior application makes the most sense.
Before diving in, it pays to remember that insulating basement walls differs from insulating above-grade walls or the…
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