I have a 1910 bungalow with an unfinished basement. The issue I'm encountering is that the main floor gets extremely cold in the winter and requires a lot of heat ($$$) to keep it warm. The unfinished basement has no insulation whatsoever, and I haven't done anything to keep the cold out other than spraying foam to plug up the gaps around pipes entering the house from the outside.

My thought process is that by insulating the unfinished basement, it won't get quite so cold, and less heat will escape downstairs/less cold air will creep upstairs. According to energy.gov, insulating the walls in a basement is preferable to insulating the ceiling between the basement and the main floor, so that's what I'm looking to do.

This is a photo of one of the walls in the basement:

Side wall

I've seen lots of videos on how to insulate when you have a solid concrete wall in a basement, or how to insulate new construction, but I'm at a loss for how to approach these walls, as the top half appears to be reverse side of the bare siding and the bottom half the foundation.

How do I insulate this whole mess without risking moisture issues? Do I build a stud wall in front of the existing wall from floor to ceiling, leave an air gap between the bare siding and add a foam insulation board, then put fiberglass insulation against the foam board up to the face of the new stud wall, then attach the vapor barrier? Do I put foam insulation board directly against the concrete at the bottom or do I need an air gap there as well?

Any help (especially a diagram if possible) would be very much appreciated!

  • Where do you live? As in what's the climate and frost line? How much of the concrete is below grade?
    – Joe Fala
    Commented Mar 7, 2019 at 2:59
  • I'm in the Seattle area, so no frost line as far as I know. Though I found this on Quora: ### "For building purposes for those few years when it does stay below freezing for a length of time the Seattle building Dept put our air freezing index at 1,500 F days or less and a minimum footing dept of 12 inches is required.“ ### All but ~9" of the concrete is below grade. The ground comes up to right around that first indentation in the concrete from the top. Commented Mar 7, 2019 at 3:34
  • I'm not familiar with the climate or the code in your area but I can put together something in an answer to maybe point you in the right direction.
    – Joe Fala
    Commented Mar 7, 2019 at 3:41
  • okay answer is done, I hope it can at least guide you through your diy project.
    – Joe Fala
    Commented Mar 7, 2019 at 4:20

1 Answer 1



Waterproofing the siding should be done from the outside. As in pull the siding off if necessary and replace/repair damage. If you are concerned about waterproofing below grade this is something you should look into. It's called dimple board (forgot it's proper, name air gap something or other), but it would require you breaking concrete around the interior perimeter and installing a sump pump and weepers for it to be fully effectiveenter image description here



This is what I would call minimalist insulation enter image description herehttps://www.homeconstructionimprovement.com/basement-wall-insulation-detail/

This is similar to the method I would use enter image description herehttps://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall

Your situation is a little different as you have some siding up above. I would continue a plumbline straight-up creating a large void in behind the wall where the siding is. I would then fill that cavity with insulation. I would then apply vapour barrier with acousti seal in the joist cavity's and at the bottom plate. If your plan is to finish the basement make sure to waterproof, if necessary and run all your mechanical and electrical before you insulate. If your plan is just to insulate to make the upstairs more comfortable then you can stop here.

Side Note the diagrams show a pressure treated bottom plate. We don't do that here in Toronto because it's toxic. We wrap the bottom with a water resistant plastic which is then carried up the stud wall on both sides and becomes part of the vapour barrier.

  • Thanks a million, Joe! One follow up question, if you don't mind. You mention running a plumb line straight up in front of the siding portion, and filling the resulting cavity with insulation. Would you adhere rigid insulation board to the siding portion before stuffing fiberglass insulation in front of it? Or is it okay for the fiberglass insulation to be directly against the siding because it's above ground? In either scenario (rigid insulation board or fiberglass insulation), do you need an air gap between siding and insulation? Commented Mar 7, 2019 at 4:22
  • Again I'm not familiar with your climate or building codes and I've never seen siding directly attached to framing. So with that in mind I'd say it's probably a good idea to tightly attach rigid foam board and caulk any gaps.
    – Joe Fala
    Commented Mar 7, 2019 at 4:30

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