Basement insulation strategy
Hello, I recently decided to insulate my basement. After reading several blogs, I have decided to go with the continuous rigid foam strategy. I have a couple of options as far as used or factory seconds near me. I have Paper/Felt faced polyiso 4×8 1 inch at 10 dollars a sheet and 2 inch are 20 dollars. EPS unfaced 4x8x2.25 for 300 dollars a bundle and there is 20 sheets in a bundle. Factory seconds XPS 4x8x1 for 10 dollars a sheet. I am concerned with inward vapor drive and am wanting to stop it as much as possible. I had previously asked about just using the polyiso but was concerned about vapor causing mold and was advised on poly against my block wall. I was also thinking of doing a fluid applied vapor barrier as well. Just trying to figure out the best strategy with what is available to me.
Thanks,
Corey
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
A fluid applied barrier is likely the most robust option, but probably also the most expensive.
I would go with the polyiso, for max R per inch, especially since it's the same cost as your other options here (usually polyiso has a cost premium relative to EPS). That 2.25" EPS is about R9.5, and may be rated R10. The 2" polyiso is going to be around R12-R13 (for the 2" sheet). The XPS will be R5 for a 1" sheet.
Your code is likely R15 now, but it depends on your climate zone. R10 used to be what most of the Northern climate zones needed, but the more recent codes have increased the minimum. If you are trying to meet code, then you will probably need to target R15, which is most easily achieved with a combination of materials totalling about 3" here. I personally would use a layer of 1" polyiso followed by a layer of 2", with the seams staggered. My own crawlspace is insulated like that, just using a layer of 1/2" polyiso first, then a layer of 2.5" polyiso after that (the thicker material is on the side facing the interior of the space, which helps to keep the thinner material pressed tightly against the wall).
I was probably the one who advised you to use poly against the block, which is what I did here to avoid moisture issues with the polyiso. You could use a layer of XPS against the block, then a layer of 2" polyiso over that to use the materials you have. The XPS in that thickness is a bit over 1 perm, so semi-permeable, but pretty low. If your primary concern is to block moisture ingress, then the layer of poly gives you a true vapor barrier in the stackup, which will work better here.
You are likely to need some type of thermal/ignition barrier on the interior too. That can be drywall, 1/4" hardboard, or a few other materials. Sometimes you can use intumescent paint or other paint-on fire retardants, but it depends on the requirements of your local municpality for the type of space the materials will be used in.
Bill
Bill,
Not much point weighing in after that comprehensive answer!
Deleted
Thanks Bill! Yes, you were the one who advised the poly. So do you think the fluid applied membrane would be overkill vs the poly? Or is it considered a superior product?
I would use the poly for two reasons: 1, it's probably cheaper, and 2, the poly goes up quick, with no need to wait for it to dry -- and not possibility of stinking up the house, either. You can get "whack in" plastic anchors for the poly sheet, or you can use a bead of polyurethane to stick it to the wall (a small handheld roller will help to make a good application if you do that, in which case put the bead of sealant on the block near the top edge, then tack up the poly sheet, then use the roller to smooosh the poly into the sealant, making a good and wide layer of the adhesive sealant). I don't think the fluid applied membrane really buys you anything here.
Vapor drive concerns depend on your situation. If you run high humidity levels, then it's a concern. If it's dry all year, then not so much a concern. I live in an area with very high summer humidity levels, and there are springs, ponds, and marshes all around my area, so moisture drive is a big concern here. I've done some things to deal with bulk water issues too (gutters, dimple mat), but that's because I had to deal with some issues. If you don't have those issues, then moisture is less of a concern. If you're in doubt, the poly won't hurt anything, and it's cheap insurance. Just use 6 mil or thicker -- but 6 mil is probably fine (and is all I used on my walls, I used 20 mil on the crawlspace floor).
Bill
Or should I be concerned about vapor drive in a conditioned basement?