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I am finishing my basement and have an insulation question. I have roughly a 30 ft exterior wall that I plan on building a wall in front of to attach the drywall. Only a small portion of the exterior wall has concrete exposed. Maybe 10 feet of the wall has a 1 foot tall concrete section and another 5 foot section has maybe a 3 foot tall concrete wall.

Is it worth the time to cover the concrete with foam board and then maybe insulate in front with fiberglass or just leave it exposed and frame/drywall around it? I live in Zone 3 and the house was built last year. enter image description here

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  • If they are not below the frost level of your area, insulating them is a good idea.
    – crip659
    Jan 20 at 1:32
  • is the concrete above or below grade on the outside? Jan 20 at 4:53
  • @FreshCodemonger I haven't had a chance to look but my gut feeling is that the top inch or two of the concrete is at grade level and the rest under ground. I think the frost level here is 4 or 5 inches.
    – Rick S
    Jan 21 at 17:51
  • I’ve seen a complete basement mold the primed drywall in a couple months during construction. They didn’t insulate the walls, and left windows open during the summer. If walls are below indoor dewpoint there will be problems. Jan 22 at 3:09

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If you're insulating the rest of the wall, then yes you should insulate the concrete.

Assuming R-15 insulation, a 20'x7' total wall area, and 6" thick concrete, the R-15 wall cavities have an area of (20')(7') - (10')(1') - (3')(5') = 115 ft2. The concrete has an area of (10')(1') + (3')(5') = 25 ft2 and an R-rating of about (0.2/in)(6") = 1.2. The whole assembly, then, would have an effective R-rating of

(115ft2 + 25ft2) / [(115ft2)/15 + (25ft2)/1.2] = 5.

See Can R values be averaged? for the reasoning behind the math. And that 1.2 R-rating for the concrete was charitable.

You can take the limit as the non-concrete's R factor approaches infinity to get an upper bound on the assembly's effective R factor. It's impossible to achieve better than R-7 without insulating the concrete.

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  • Now that I think about it, I don't really have room to add foam board as there will only be about a 1/2" gap between the wall I build and the concrete. This is because the wall i'm attaching to is coming off the stairs and that is what I have to work with. So any ideas? Also as I commented, I believe the majority of the concrete is below grade.
    – Rick S
    Jan 21 at 17:54
  • @RickS, if you look at that link, you'll notice that I canceled a bunch of ΔT terms in the algebra. Each term had the same ΔT. If your concrete wall has a smaller ΔT because it's warmer below ground, then obviously the effective R-rating increases.
    – popham
    Jan 21 at 18:12
  • @RickS, if you're building another wall, then just insulate it uniformly. If you're not building another wall, then consider not insulating at all. You can straddle the two by exposing the odd shape in the face of your finished wall, where that configuration can be uniformly insulated too.
    – popham
    Jan 21 at 18:17
  • My plan was to build a wall all the way along the existing exterior wall you see in the pic. I guess I could use rockwool for the spots where there is concrete. I believe it would be safe to do so?
    – Rick S
    Jan 21 at 18:27
  • @RickS, safe? Are you worried about moisture through the concrete? I have concrete sealer laying around, so I would use that on the concrete behind the wall. Vapor barrier against the concrete is a good idea, where people will often substitute closed cell foam insulation boards and tape the joints. Otherwise and in zone 3, your vapor barrier goes to the interior (see codes.iccsafe.org/content/IRC2021P2/…), although it's not required. Don't ask me about how to transition between the two orientations.
    – popham
    Jan 21 at 18:49
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Below grade concrete is considerable better as you are primarily insulating against the air and you get the R-value of the soil distance from the air to the exterior concrete surface if you look at the isothermos.

That said I'd probably use roxul batt insulation in the stud cavities and then get 1.5" isoboard for the concrete and 1.5" xps for all the rest of the areas and then tape the seams for a continuous 1" layer over your wall. You can then drywall right over that you just have to mark the stud locations on the floor and use longer drywall screws. Use 2x4 on flat against the concrete for your nailing edges and filling in the void with the 1.5" isoboard.

popham is right that concrete is very low resistance to flow of heat - .1R per inch vs 1R per inch for wood.

Not sure where your vapor barrier goes in your zone.

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  • Thanks for the answer. How about this, I just build the secondary wall along that entire exterior wall and fill in the stud cavities with rockwool and call it day? So i'll basically have a double insulated wall, the existing exterior insulated and my new wall insulated.
    – Rick S
    Jan 21 at 18:45
  • you can do that. if you do the continuous insulation then you avoid thermal bridging from the framing members. You'll only loose 10-15% effective insulation but gain by having greater thickness of the insulated areas. if you want to build another wall then you can go 24" oc to have less thermal bridging and mostly offset the thermal bridges. Jan 21 at 19:30

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