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I'm in Minnesota. We have a unfinished/partially finished basement with daylight windows that we plan to work on. I'm trying to understand if it's OK on the kneewall where the Polyisocyanurate foam is attached to the concrete to use a narrower wall made of 2x2 and 2x4 studs sideways instead of the normal 2x4 wall framing in order to conserve some space? I understand I already need to leave a gap between the XPS foam and the framing, but with narrower walls I won't necessarily be able to put insulation in them. I also still need to be able to run electrical and low voltage wiring for Ethernet and Speaker wiring.Image of unfinished basement. Is it going to be a big hassle instead of just going to with standard 2x4 framing and losing 2 inches around the perimeter? Should there be more insulation on the bottom half besides the 3/4-1" foil backed Polyisocyanurate?

Thanks for your help

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    You usually want to be 4+ feet below ground before reducing the need for insulation. Above 4 feet/frost line you should have full insulation levels, R-15 or better.
    – crip659
    Commented May 17 at 19:03
  • More insulation equals less heat loss equals less heating costs.
    – crip659
    Commented May 17 at 21:11

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tl/dr, frame it out with 2x4s and insulation - that's likely what the builders planned, meets what is required for where you live, and allows you to run wiring and electrical with space to spare.

  1. You need an adequate amount of insulation to meet code. If you have 1" of PolyIso that has an R-Value of 6. According to the IEC Table Minnesota should have an R value of 15 (for continuous sealed insulation, such as foam), or R-19 (when in cavities). Check your local code, this may vary.

  2. You want to have access space to run wires, electrical, etc, and to install deeper electrical boxes (especially if you will be installing any extended outlets). While there are shallow electrical boxes, they won't fit things like GFCI or USB outlets - so you want to stick to the standard depth (at least 2.8", minus the drywall)

  3. Because your walls are not load bearing (the foundation/concrete holds up the house), you certainly can use 1x2s (furring strips) or even metal studs

  4. Because you're dealing with a basement, moisture and water infiltration may be an issue. This means you need to use solutions that can withstand water such as metal studs or a PT footer board.

  5. There's no requirement for an air gap between what you have and the stud bays; the purpose of a gap is to provide a "thermal break" and prevent condensation between the warm and cold surfaces. You already have this with your existing polyiso - it would go behind the foam that already exists.

Based on the above items, and needing R-15 to R-19, your options are:

  • Add 1.5" of PolyISO or XPS foam, for a total of R-15, then frame with 1x42s (furring strips) or 2x4s on their side - this is how my house was finished.
  • Frame with 2x4s, and fill the bays with Fiberglass, Rockwool, Cellulose, or similar insulation (3.5" of batts at R value of 3.8" per inch = 13.3 which equals 19.3 with your existing 1" of Foam) - This is probably what the builders originally planned for. I'd recommend Rockwool and avoid Fiberglass or Cellulose, due to moisture concerns.
  • Frame with 1x2s or similar and then fill with spray foam. This is a common option if you are retrofitting an existing build, but I would avoid this with any new construction as it makes any wires or electrical in the future very difficult.
  • Use finished wall panels that provide both the insulation and the studs all in one unit, such as the Dricore Smart Wall Panels or the InSoFast Panels. These are much easier to use and more straightforward, but may cost more.

Other crazy options could include insulating from the outside :) While you could absolutely go with thinner walls or boards - I mean, you could probably even glue the sheetrock directly to the foam if you wanted to (since the walls aren't load bearing), that likely doesn't meet code.

A great resource I'll drop here the Fine Homebuilding Article on insulating basements.

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  • I got the impression that OP intended to apply the "framing" as an additional layer to the insulation. IOW, that the foam was a continuous sheet of insulation, and the framing was being applied "around" the foam, instead of "inside" or "in between" the foam.
    – aghast
    Commented May 17 at 22:45
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    Thanks for all of the comments and helpful tips. It makes sense to go with the deeper walls with regards to the outlets @Stacks The comment regarding not needing the space between the polyiso and the framed and insulated 2x4 walls is helpful. I wasn't sure on that one and several pictures I saw online showed a gap in between.
    – medelman
    Commented May 18 at 0:33
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    Just a quick mention that there are products which are basically insulating foam with built in stud. The one I used seemed to work out pretty well, modulo the patchy stuff at the top of my 7' wall; it'd work more smoothly with an even multiple of 2.
    – keshlam
    Commented May 18 at 1:42
  • Great comment! I'll add those to my answer as well. @medelman hope my answer was helpful, if so plz upvote :)
    – Stacks
    Commented May 18 at 23:35
  • @Stacks It was helpful and I would love to but I don't have enough reputation yet!
    – medelman
    Commented May 20 at 1:32

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