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Rigid foam on basement walls – is a stud wall still required?

Kevin_in_PA | Posted in General Questions on


Starting a basement renovation on a mid 70s ranch in zone 5A. House 1000 sf, with poured basement & walkout facing south & 66″ below grade on north side. I will use 2″ rigid foam on the walls and then 1.5″ on the floor. I’d like to do the walls first and have a removable lower wall section to allow for inspection and pest treatment.

Is a stud wall needed over the rigid foam on the walls or can sheet material be applied directly over foam (like the floor?) I was thinking of having electrical drops with EMT exposed over the drywall or OSB. Also, any concern with doing the walls first and having a removable lower section? It seems like most of what I’ve read so far, the floor insulation is first with wall insulation on top of floor.


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  1. Expert Member
    Akos | | #1

    You would want to strap out the wall to hang the drywall. This would also let you run wiring behind the drywall instead of all surface runs. If you use surface mount boxes but mount them so they are flush with the drywall and cut a small pocket the foam around them, it will be a much cleaner look.

    You will need a fire stop at the top of the wall, with 2" insulation and 3/4 strapping, you can rip a 2x4 to 2 3/4" and screw it to the bottom of your floor joists for this (with 1/2" strapping you can use standard 2x3). The foam can then start bellow this.

  2. Expert Member
    NICK KEENAN | | #2

    If the wall is smooth you can glue the foam to the wall and the drywall to the foam.

    Here is what I am thinking for a basement workshop. Cover the wall with 2.5" polyiso. At the top nail a piece of 2x3 to the floor joists flush with the polyiso, and at the bottom put a piece of 2x3 pressure treated attached with tapcons to the floor. So the insulation is 3" less than the floor to joist dimension and rests on the PT at the bottom and sits under the top plate. Then a piece of 3/4" tongue and groove floor underlayment is screwed top and bottom. The corners are joined with a piece of 2x2. I'm thinking the stiffness of the underlayment, the tongue and groove, and the corner pieces will keep it from bowing. If not I'll put a few tapcons in the middle.

  3. Expert Member
    BILL WICHERS | | #3

    If you use 2x3 or 2x4 lumber on the flat as your strapping, you can fit 1.5” deep electrical boxes in the spaces. Use 4” square boxes and mud rings and you’ll have a normal electrical install when you get to the drywalling stage even though you’ve built a compact type wall. Unless you really need to minimize the overall depth of the wall assembly, this is the way I’d build it.


  4. Kevin_in_PA | | #4

    Thank you everyone for the helpful replies. I like the idea outlined in reply #2, but after further inspection of the nonuniform walls from the concrete forms, I am now likely go the stud wall approach presented by Zephry7 in reply #3. I also think I will end up with (2) layers of rigid foam to achieve a minimum of R15 for zone 5. Any concerns for layering ISO over XPS? I've already got half the wall space covered in 2" xps, but really want future purchases to be ISO for the fire & environment concerns.

    1. Expert Member
      BILL WICHERS | | #6

      There is no problem putting a layer of polyiso over a layer of XPS. I recommend you stagger the seams between panels for a little bit better air seal.


  5. Expert Member

    I dunno if it's worth it. Strap this, tap-con that, glue the other, try and route the electrical to meet code for cover - all to save a couple of inches. I'd frame stud-walls and fill them with batts. You are done, and the next guy when he renovates is so happy he goes to the cathedral and lights a candle in thanks.

  6. TNCave | | #7

    I have found this to be a very helpful guide.

    Good luck with it.


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