Basement rigid insulation, vapor barrier, rim joist/header insulation
Hey! I really have appreciated this forum for how helpful it’s been.
I’m wanting to insulate the basement of my house built in 1950. It currently has uninsulated concrete block walls and no insulation against the rim joists/headers. I was able to get inexpensive sheets of 1.625″ thick polyiso that are faced on both sides with a combination of fiberglass and paper (typically used in roof applications). I would really like to use these, since it’s not in my budget to buy all new polyiso. How would I go about making use of what I have?
My current thinking is that I could put 10 mil polyethylene directly to interior of block wall, then two layers of 1.625 inch polyiso (faced with fiberglass/paper), then 1x furring strips, then drywall. I’m wondering if the fiberglass/paper facing will cause issues with mold? I know that drywall has paper facing, so I presume that the interior surface of the polyiso may not present mold issues, but that possibly the fiberglass/paper facing up against the poyethylene may collect condensation that will lead to mold/mildew? Or is the relative vapor impermeance of polyiso enough of an air/water barrier that this will be not present any practical issue?
In the other half of the basement, I plan to not do finished walls. The rim joists/headers are not insulated and I would like to add insulation. Will polyiso trap moisture against the sill plate as it rises up through the block walls? (Especially if I end up putting 10 mil sheeting up against the interior of the block wall.) I was thinking of going with unfaced fiberglass so as to allow the sill plate to dry. Also–since this unfinished part of the basement hosts the furnace, storage space, and exposed floor joists, I’m wondering how I would make polyiso up to fire code in this application? What would I add as a thermal fire barrier? Again, this is against the header/rim joist, not the wall proper.
Thanks so much!
(Edited to add: I’m in zone 4A.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Do NOT use fiberglass batts against the rim joist. That does allowing drying, but it also allows moisture to go right through and condense on the cold rim joist. That's going to cause problems. You're safe doing cut'n'cobble here with cut pieces of the polyiso you have. Fiber faced polyiso is somewhat vapor permeable, so it allows for some limited drying. You would cut pieces about 1" smaller in both dimensions than each section of rim joist you're insulating, then seal those pieces tight against the rim joist using canned foam. This is exactly what I did in my own home, except that I used EPS in this area instead of polyiso (because EPS was easier for me to get at the time).
The polyiso should be fine against the walls. I would use the layer of poly between the block and the polyiso, but you can save some money going down to 8 or 6 mil here -- you don't need 10 mil, since it's pretty well protected here. Stagger the seams of the polyiso sheets as you put them up. I like the Plastigrip anchors for this application, since they go in quickly and easily.
If you're worried about mold on the facer (which is probably not really an issue), you can paint the polyiso with a mold killing primer before (or after) installing it. In my crawlspace, I painted all the polyiso panels that were on the interior side with a fire retardant primer before installation. It is often easier to paint and then install, especially in a crawlspace, so that you have more room to work.
Bill
Thanks Bill, this is very helpful! Especially to hear that mold/mildew may not be a real issue when having fiber-faced polyiso up against block wall (with sheet of poly between). And thanks for the idea about painting the facer. I hadn't considered that. Is there a fire-rating issue with installing polyiso in the exposed joist bays when that part of the basement is also used for general storage and hosts the furnace? This seems like a common scenario and I'm curious how people manage fire risk in this situation?