Insulation basement along foundation wall
Hi everyone,
I just bought a house and am making some initial plans for finishing my basement and I figured a good place to start would be with adding some insulation. Along the top of the concrete foundation walls there is a gap where some plumbing pipes (which I need to secure) run parallel to the joists, and on the walls perpendicular with the joists, are pretty much empty gaps. Can/should I add insulation into these areas before finishing? What options do I have as far as insulation choices?
Also, I’ve been doing some reading about adding insulation to concrete walls. Most articles I have read have said to first check for any moisture first. The concrete walls are painted white but I’m not sure if they are a waterproofing product or not. Either way, there is not any signs of moisture, so I think I am OK to insulate. What is the best approach for insulating along my concrete wall? Foam insulation, and frame on top of the foam? Frame directly on the concrete and add insulation between studs?
I am attaching some pictures of the space. I appreciate any guidance you all can offer.
Thanks,
Brian
Replies
There are many 'details' for these very issues at https://www.greenbuildingadvisor.com/detail-library
Yes, Insulate the Sill Joist gaps. Remember they are 'walls' so batt insulation should be installed vertically just like stud bays. Rigid needs to have the edges spray foamed with similar closed-cell insulation as the tight-fitting boards you put in.
"Look for moisture" that would be cracks or overspilling at window sills and sill plate. Also, if there is any 'white powdery substance' on the face of the concrete. The first two are bulk water issues, the last is vapor pushing through and bringing calcium out of the walls with it. Having painted walls, look for peeling or bubbling paint.
Next, outside dig about 6 inches down the wall and see if they applied any asphaltic looking sealant to the outer edge and/or if there is any other kind of drainage along the face of the wall like rocks or plastic 'cardboard' that would drain down to a footing drain.
For interior furred walls, I like the detail where an inch of XPF is pushed against the wall by the new stud wall. This means only the wall's base plate needs to be treated and sill seal would be recommended. You can creatively tie the top plate to the concrete wall's sill plate. I would suggest you use shims along the studs to force the XPF boards against the concrete wall after you straighten and plumb the new framing. If the XPF is not tight against the concrete there is room for condensation and mold. Or, use rockwool. This will dry out and you don't have to place vertical fire breaks every ten feet along the wall (pressure-treated stud/furring strip).
And by the time you have the R15(R13) batts in the stud bays, you will be wishing you had just trenched out the exterior walls and put R10 of rigid on the outer surface.