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Best Practices

Insulating Concrete Foundation Walls

Start with a check of the codes, and take care of any water problems first

Insulating the exterior of a foundation or crawlspace wall with rigid foam is one of several insulation options. One disadvantage is that the foam must be protected from light and physical damage. FoundationPRO by ProgressiveFoam Technologies comes with a laminated facing that solves that problem. Photo courtesy Fernando Pages Ruiz.

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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  1. Stuart Smith | | #1

    There is another option, commonly used in tilt-wall and pre-cast construction, where a layer of insulation is integrated between two continuous wythes of concrete which are connected by structural ties through the insulation. Thermomass ( is an example of this type of system. I'm curious if anyone here has experience with something like that on a residential basement/foundation.

    1. hockipuck | | #2

      Hi Stuart, in residential (at least in my area) a similar approach is common with a concrete skin, insulation, and concrete "studs" in ready-made panels. Superior Walls is the most common:

    2. GBA Editor
      Kiley Jacques | | #3

      There’s a residential development in Newburyport, Massachusetts called
      Hillside Center for Sustainable Living that uses a tilt-turn panelized wall system that was custom-fabricated by Blueprint Robotics. I’m not sold on the idea because of all the concrete but it’s noteworthy.

  2. Peter Taggart | | #4

    Thanks for the informative article. But I have to wonder, How can building this close to a water body be considered "green building"? From the picture it appears to be a virgin site. Where is this project located? Is there at least a silt fence and some type of erosion control somewhere hidden from view?

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