Basement Insulation retrofit in finished room
Hello, I am attempting to retrofit insulation into a basement exterior wall in a finished basement office, before the drywall goes back up – approx 15’x7′ wall. We took out the drywall to cut an egress window and are now down to the studs.
Climate zone 4A, very near 5A. The house was constructed in the 50s w/ no exterior basement waterproofing. This room was finished before we bought the house, and no insulation was used in the construction. From outside-to-inside the walls are currently: concrete wall -> approx. 1/8″ to 3/4″ air gap (walls not uniform) -> studs -> drywall
From research I’ve done, I have 3 sub-optimal options (not ready to re-frame the room):
– Spray-foaming: Too expensive for such a small space, unless that is truly the only good route to go. This would only leave 1/8″ to 3/4″ of foam between studs and concrete, could go w/ 2″ in stud bays.
– Rigid foam board: Could cut 7’x14″ sections of 2″ rigid board and slide into the stud cavities, can-foaming behind the studs as I went to seal the 1/8″-3/4″ gaps between concrete and studs.
– Slide a continuous sheet of poly behind studs against the concrete, and use unfaced fiberglass batts or mineral wool between the studs. A lot of info on this site says to avoid poly behind rigid/spray foam and behind drywall. That makes sense, but is that no-poly rule consistent if I go this route?
Any recommendations on the best option for this? Thank you.
Replies
https://www.fairfaxcounty.gov/landdevelopment/sites/landdevelopment/files/assets/documents/pdf/publications/basement-details.pdf
Close to your zone. they call for vapor barrier in the heated side. You have not stated that you see water or air leakage problems with the concrete basement wall, or if the wall is all underground on the other side.
You can use a smart vapor control film if you like. Example:
https://www.certainteed.com/building-insulation/products/membrain/
I would consider unfaced rock wool in the stud cavity covered with the smart film and drywall.
Even if you are not re-framing the wall, pay attention to the fire blocking requirements while you have access to the studs.
Thank you for your response. The basement is below-grade. We had some previous minor water issues, but seem to have resolved with grading and drainage. There is interior drain tile below the slab along that wall.
Do you recommend anything to isolate the insulation from the concrete? Theoretically there would be a continuous air gap, but some of the insulation may press a little too deep and come in contact with the concrete.
Thanks!
Rock wool is better than fiberglass in dealing with a little water.
https://p-cdn.rockwool.com/siteassets/rw-group/media/factsheets/factsheet_moisture.pdf?f=20201215113119
It also does a pretty good job of staying where you put it. Cut the bats to fit instead of folding it up if some of your wall studs are spaced closer together. (or to fit around electric boxes or pipes) You can use a yardstick or similar object to check for clearance if you like, but it should be fine.
I asked some questions about insulating a basement wall prior to framing studs and what I learned is that you want to eliminate anywhere that condensation could occur along the basement wall. What I decided on for my application is foil faced poly-iso with stud wall frames in front of that and cavities filled with rockwool (r-21 total).
My concern with leaving an air gap would be that it leaves an area for vapor to condensate on the surface of a cold masonry wall in winter. Granted I am located in climate zone 5 (almost 6).
On the other hand, you have no insulation so you basically have a 3.5”+ air gap to masonry wall and didn’t seem to see evidence of prior condensation issues?
With the 1/8” to 3/4” gap between the walls and studs I don’t think trying to retrofit rigid panel insulation between the stud wall and masonry wall would work because you don’t have at least 1/2” gap for the entire assembly to slip foam board in.
If it were me, I would probably spray foam so you can eliminate the possibilities of a condensing surface and insulate your wall at the same time. Might as well spray the depth to at least the minimum r value if not more if space allows. Certainly not the cheapest approach, but at least you won’t have to worry about it. I think I heard an anecdote once that doing it right the first time costs less than doing it half assed and then having to do it again. Something like that..
Thank you both! The small gap preventing sliding rigid sheets behind was the main design challenge I was trying to work around.
Regarding condensation on the concrete: I guess I wasn’t worried about that, but should I be? Every rigid-foam-against-concrete installation out there will have moisture coming through the concrete into air gaps with impermeable foam on one side and a cold basement wall on the other where it will condense. In that setup, moisture can generally re-absorb back into the concrete and there is no organic material to decay.
Wouldn’t poly separating the concrete and the studs/insulation perform a similar function as rigid foam board against the concrete? Agreed w/ rock wool as the superior insulation between studs - thank you.
With no vapor barrier behind the drywall, the walls can dry to the inside (large dehumidifier in basement).
Thanks everyone!
Foam boards used inside basement walls should be selected which allows some moisture to pass. The biggest difference between foam board and a sheet of poly is that the poly would have no insulating value. Condensation on the outside would , of course not bother your concrete walls, but the inside would be an ideal place for moisture to collect (from inside the house as air contacts the surface at temperatures below the dew point). Mold and rot are not your friend
https://www.finehomebuilding.com/?s=foam+board+for+basement
Plenty of good info has been put together.
One thing to consider (while you have the wall down to studs) is to open up a foot or so of ceiling and seal and insulate the rim joist and top of that basement wall.
Mike - thank you. I was so focused on the moisture coming into the house from the concrete that I didn’t think about the moisture already in the house condensing against that cold poly. Even though both poly and foam board would keep the moisture from the concrete away from the insulation/studs, the main difference between is that the interior surface of the rigid shouldn’t be cold enough to condense.
You’re probably right on the spray foam (and the rim joists). I’m a huge fan of one and done, I’ll price that out. But if it’s too much, I’ll just put mineral wool in the bays, leaving the air gap. May use some batt hangers as extra precaution to keep it from pushing too deep and touching the concrete.
Appreciate all the insights!