Can I install foam board on vapor barrier
Bluurocks
| Posted in General Questions on
I have a basement that I want to finish. We previously had water during hurricanes so we had waterproofing done. With this, they installed a vapor barrier halfway up the wall. See attached image. Since I want to insulate, prefer to use foam board, I am curious if there would be any issues with attaching the top half directly to the cinder blocks and the lower half to the vapor barrier. Or, would I be better to attach the foam board to the back of the framing (not sure if I will frame with wood or metal yet). Any help is appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Foam is a vapor barrier itself, so having another VB underneath it doesn't matter. You can install the foam right over that strip. Make sure to seal the foam to the block wall with adhesive and tape the seams with a tape that will adhere to the foam you select. The important part is for house air to not make it behind the foam.
Don't forget to insulate and air seal your rim joist while at it, that tends to be a huge air leak in most houses.
Thanks. The rim joist was sealed when the waterproofing was done.
Appreciate the info, I will look to glue it to the valor barrier air tight and tape the seams between all the panels. Unfortunately I will have to go above and below the drainage pipe because it is against the wall around half the house so can’t move it or get behind it. But I will also glue that to prevent air from getting between the foam and the vapor barrier.
"Foam is a vapor barrier" is not an accurate statement; it varies significantly depending on the type of foam and the thickness. Otherwise I agree with Akos.
Thanks. So would you suggest putting a vapor barrier at the top and tucking it behind the existing then gluing unfaced foam OR should I just do the entire wall with faced foam and seal air tight?
Michael,
Un-faced foams vary in permeance widely. An inch of XPS is about 1.1 perm, EPS 2 to 6 perms, and polyiso can be 26 perms. Given that, I'm a bit surprised by Martin's advice that "On the interior side of a basement wall, all three common types of rigid foam insulation—polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)—perform well".
I assume that's because he is primarily concerned with it serving as an effective air-barrier. What do you think is an appropriate perm rating for foam on the interior of stem-walls? Or isn't it particularly important.
Thanks. I am not concerned about the perm value, just want to get warm insulation without causing a moisture issue behind it. I don’t care which foam board type I use. Just hoping to find if I should use faced or unfaced. I see conflicting reports online about faced or unfaced on cinderblock walls.
My preference would be to either use a foam with a low permeance--my go-to is foil-faced polyiso--or to include a poly vapor retarder/barrier between the foam and the wall. I describe the reasons why in this blog post: https://www.greenbuildingadvisor.com/article/vapor-retarders-for-foundation-walls.
There is another camp that says it's fine, or beneficial, or important for the foundation wall to be able to dry to the interior. They aren't wrong, I just don't see why you would want moisture constantly trying to get into your basement.
Bluurocks,
The facer is what affects perm value. We are basically asking the same question.
Mike,
Thanks - that was the reference I was looking for, with Martin’s clarification of his comments on poly. I wish he would amend his original ones, as they cause a fair amount of confusion.
You say you had waterproofing done. Can you elaborate? What did the waterproofing entail? Did they excavate around the exterior of your house and install waterproofing? If so, has that been backfilled yet?
Hi. The waterproofing was just interior perimeter drainage under the concrete slab.
If you've had enough issues with bulk water to have had waterproofing done, be careful with your selection of rigid foam materials. You could probably still use polyiso (my preference for this type of project), but I'd recommend leaving a gap of an inch or so between the bottom of the panel and the floor to help avoid the foam panel ever sitting in water. Polyiso can wick water. I'd also be sure to use foil faced polyiso here.
EPS and XPS could both be used without concern of water absorption.
Bill
Thanks for the information. I am planning to use the Dricore subfloor or Insulation-Armor and putting the frame on top of that as instructed. That will bring the foam up off the concrete. I have been going back and forth and wasn’t sure which foam to use, was thinking EPS but will review the PolyISO since you suggested. I will use Faced whichever I go with. Thanks for the help.
Do you know if I should use faced or unfaced on cinder block walls? I thought faced since has the vapor barrier but I was just reading it should be unfaced to allow it to “breathe”. Thoughts?
Not worried for the bottom since that vapor barrier goes to the drainage.
The "allow it to breathe" stuff doesn't matter, especially here. You're not going to have any issues with a "double vapor barrier" here, because there is no air space for moisture to accumulate in. You could use any facer and be fine. If you had no vapor barrier in place, I'd use foil faced polyiso because I'd want the foil facer as a vapor barrier. Since you already have the poly in place, you could potentially save a bit of money and use reclaimed polyiso, which is often a fiberglass mat or kraft paper facer. Use whatever you can get at the best price and don't worry about it.
Note that you'll likely need some fire protection on the interior if you aren't planning on finishing things off with drywall.
Bill
Deleted