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Live in Zone 5b, going to shoot for 2" of XPS DOW foam board. Planning on gluing it to the concrete wall with foam board glue and bracing it while it cures. It will run to either the top edge of concrete or to the I-Joist and then have spray foam behind it to fill the void between P.T. sill plate and the back side of the XPS foam. Taping the vertical seams. Then going to run 1x4 (since 1x3 isn't available locally) horizontally and use ramset w/ washer or tapcon screws or a mix of both to secure the 1x4 on 24" centers. I don't like how short the ramset nails come in. Then going to run 1x4's vertically on 16 or 24" centers that screw into the horizontal 1x4's. Then going to run wire, and drywall in those steps. Considering cutting puzzle pieces of 1.5" XPS to place in the grid I create but that's not for sure.

What am I missing, if anything?

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Seems like a lot of work but. Why not just do standard 2x4 walls and insulate with fiberglass? I don't know conventional methods for your area but visualizing your description, it seems your wiring may be susceptible to puncture by screws. Make sure you use nail plates.

  • Joe, In my zone we have all 4 seasons and I would have to leave a gap between the 2x4 and concrete. The concrete behind the wall would get condensation since there is no vapor barrier. The foam acts as a vapor barrier and seals indoor air from touching the concrete that is a condensing point. – Nic Mar 28 at 15:07
  • Here in Toronto many people have gone to spray foam. But with batt insulation we go framing, mechanical insulation and then a vapour barrier such as 6 mil poly. The vapour barrier is on the warm side of the house. I've seen a lot of these polystyrene insulation methods on DIYSE but it's something that is not suitable for our climate. I can see how it would help with water infiltration. We use air gap membrane (also known as dimple board) embedded below the slab. Your method seems sound. It also uncouples the lumber from transmission of heat to or from the foundation. – Joe Fala Mar 28 at 15:18
  • We just frame off the wall by an inch or so and spray foam the wall. Nowadays the entire house is sprayed. Even duct in unconditioned space is sprayed. I don't think that would be worth the extra expense given your climate. – Joe Fala Mar 28 at 15:21
  • Spray foam is just too expensive here, but I know what you mean. If I were to use 6 mil poly here before drywall, it would still allow outside humidity to come inside and condense on that poly during the summer months where we run AC and chill the indoor air. The warm humid air from earth / outside would touch that chilled 6 mil plastic and be a recipe for disaster, which is what I also fear would be the case with kraft faced insulation in a 2x4 stud wall. – Nic Mar 28 at 17:03
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How old is the house - sounds new as you mention I-Joists.

Concrete and construction water can take a long time to be fully removed from concrete - I think I remember reading 5 years. Is the concrete foundation exposed on the outside to air?

I like the continuous insulation approach as a thermal barrier. Why not use 2x4 on flat instead of your 1x4 grid? I would use 2x4 on flat (assuming the point is to save space otherwise a standard 2x4 wall) with a bottom and top plate on 24 oc and frame that wall plumb. The foam glue should hold the insulation to the wall. Once wiring was finished I'd add more rigid board between the 2x4 on flat. Typically you can run the wires behind the studs on flat as the foundation walls are almost never perfectly plumb. If the wires go through the studs on flat then you'd need to add nailing plates.

I am assuming this is a poured concrete wall. Concrete is a great air barrier so you don't really have to worry about a moisture barrier here though I'd probably go with a vapor barrier paint over the drywall.

Also why not go for the higher R-value of poly iso board (~R6 per inch)?

  • (1/2) It's not quite 5 years old. I understand the concern but the outside is some exposed and the dirt below helps wick or bring moisture to/from the concrete. The issue with the 2x4 on flat is that the wall will follow the foam and the foam follows a non-perfect flat/plumb wall. I was going to use shims between the first layer and second layer of 1x4 but I guess shims behind the 2x4 on flat could do the same. And yes, after typing it out I realize the wire will run in a channel behind the wood that I dig/melt into the foam. It would be much cheaper to do Joe's method in rooms space isn't – Nic Mar 28 at 16:55
  • (2/2) needed like larger rooms, but I fear it would be a much lesser quality of insulation job. Maybe not? I've just always dismissed fiberglass. – Nic Mar 28 at 16:56
  • It is normal that framed walls have gaps between them and concrete. You frame the 2x4 wall plumb and can use shim if you need to attach the 2x4s to the concrete wall through the insulation. I don't attach the 2x4 wall to the concrete. The top and bottom plate hold the wall in place. If you frame the wall plumb you'll have a space for the wire without melting a channel. I don't use fiberglass. If you want batt insulation go with roxul or a rock wool. It doesn't loose r-value when wet and is fire/vermin proof. – Fresh Codemonger Mar 28 at 18:49
  • The continous layer of insulation is way better for thermal performance - this is the approach I take on concrete basement walls. – Fresh Codemonger Mar 28 at 18:49
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    Fresh, I would not be able to use 2" XPS (or Poly ISO) and then traditional frame a 2x4 wall. It would eat up too much space in the smaller rooms. So it sounds like overall you are in favor of the XPS foam and 2x4 on flat face anchored to the concrete to also hold the foam tight, shimming them to stay plumb. – Nic Mar 28 at 19:08

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