Basement Wall Insulation
Hi All,
I am finishing a basement in a new-to-us home in Minnesota, there was some previous work already completed that I am working around and I am seeking some advise on how to best handle wall insulation.
I have two different walls that I need to deal with –
Wall A is an 8′ tall poured concrete wall that is mostly below-grade, probably 80-90% of it, and has a 2×4 wall framed up about 1.5-2″ away from the exterior concrete wall
Wall B is also an 8′ tall pured concrete wall and it is completely below grade (borders an unexcavated area below the garage). This wall has a 2×4 wall framed up but it is much closer to the concrete – 1/4″-1/2″ away in some areas.
I’m not especially worried about additional insulation since the basement currently stays reasonably warm and has just a single vent supplying 1000+ sqft. Once we have the end-state HVAC in place it should be a non-issue. My concern is more around protecting against any condensation or mosture issues.
Do I:
A) Insulate the 2×4 walls with Roxul/Mineral Wool insulation, vapor barrier (poly) and then drywall over everything and call it a day.
B) Remove a stud or two along Wall A to slide in 1/2″ Rigid Foam sheets, glue them, tape seams, and make that as tight as possible and either just drywall over the studs or additional insulation + Drywall and then for Wall B do spray foam and try and encapsulate the areas of concrete behind the studs as much as is possible?
C) Something else?
Looking for some guidance, I’d really rather not tear all those walls down and start over from scratch. There is another wall that will need framing on the other side of the basement and I’ll proably line that poured concrete wall with rigid foam first, but trying to decide what to do about the already started area.
Appreciate everyone’s help.
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Replies
Picklz,
Martin's article answers your questions much better than I could: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
Not A.
I vote for B.
Option A won't work reliably, because you'd be sealing the batts between the foundation wall and the poly vapor barrier, which is a no-no. Remember that this isn't your usual exterior wall assembly, since you typically have significant moisture drive coming IN from OUTSIDE, instead of the other way around.
Option B is better, since you seal up the foundation, put up a little continuous insulation, and have less moisture risk. The downside here is the labor to take out some of the studs. If possible, the best option would be to use ALL rigid foam insulation, then reframe the wall wherever it needs to go to accomodate that rigid foam insulation.
A potential option C would be to spray foam the wall in place, since spray foam can get behind the studs while in place, and it forms a fully adhered insulating layer for low moisture risk if you're using closed cell spray foam (which you should here). Spray foam is less labor, but a lot more materials cost.
If I were doing this work myself, I'd be very tempted to reframe the wall to allow for rigid foam to go up at the proper thickness, but I'd have to see the assembly to make a final decision on that. I'd consider spray foam if the labor to reframe the wall seemed excessive. I would not consider your Option A at all.
Bill