GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Open-cell spray foam in joist areas with polyiso on exterior walls?

user-985898 | Posted in Energy Efficiency and Durability on

From all the reading I have done, am I correct that I don’t want to use closed-cell and need to use open-cell spray foam in the band areas of floor joists of the basement to 1st and 1st to 2nd floor areas when I have used polyiso sheathing on the outside of the walls?

I believe that is correct because of the perm is so low for the polyiso and you don’t want to use another very low-perm product like closed-cell spray foam sandwiching the Zip wall sheathing. I’m just confirming if I read everything correctly.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You are correct, although I don't believe that those who used closed-cell spray foam are ever likely to have problems. There shouldn't be much (if any) water intrusion in this area, and the advantages provided by the spray foam (whether open-cell or closed-cell) greatly outweigh any risks.

  2. Expert Member
    Dana Dorsett | | #2

    It also depends on how MUCH closed cell foam. Up to about 2"/R12 it's vapor permeable enough for seasonal drying. For thicker closed cell apps Icynene has a water-blown 2lb foam product that's still about 1-perm @ 5"/R25, but SFAIK it's the only non-HFC 2lb foam available in the US (and you have to seek it out and be specific- Icynene also sells an HFC-blown closed cell product that's much lower-perm and somewhat higher-R.) At 1 perm you would still have PLENTY of drying capacity for the wood.

    From a green/greener/greenest point of view HFC blown foam is a real dog, with a higher lifecycle greenhouse footprint than the energy use it's offsetting when going high-R. Open cell foam and water-blown closed-cell is much more benign.

    If going open-cell you still have to pay attention to the dew-point factors & exterior/interior R-ratios relevant to your climate.

  3. easyrider470 | | #3

    This is an OLD OLD thread but I am in this exact situation and trying to make a good decision. I have 1.5" of polyiso covering the exterior of my rim board area and need to decide on an insulation for the rim joist void between 14" TGI joists on top of a 10" poured concrete wall. I have 3" of XPS on the exterior of my poured walls that the polyiso meets up with perfectly at the sill plate level. I know i don't want to use closed cell in the rim board voids but how do I know the correct depth to spray the open cell foam in this area? I am concerned...... I am in the 47460 area code.

  4. Expert Member
    Dana Dorsett | | #4

    At 1-2" closed cell foam would not be a problem.

    In zip 47460 you could just do 4-6" of open cell foam, but a flash-inch of closed cell + R15-R25 (or more) of dense fiber such as damp sprayed cellulose or rock wool batts would would be more protective of the rim joists due to the ~1 perm vapor retardency of the closed cell foam.

  5. easyrider470 | | #5

    You say it would be better to use the closed cell even at 1" thickness? I was under the impression that the POLYISO on the outside and any closed cell on the inside was a sandwich with no perm. Bad idea for the band board. Anyhow, purely from a cost perspective the open cell is a better idea....I have 10" thick poured walls so I could do the 4-6 inches of open cell and still have room in that area for a batt or something. OR could I just have them spray the entire cavity full of open cell? I think it would end up being around 8 inches or so. I held my sill plates back 3/4 of an inch to allow my 1.5 inch polyiso over my sheathing to line up with the 3" XPS on the poured walls below grade.

  6. Expert Member
    Dana Dorsett | | #6

    Your impression about the vapor tightness the closed cell polyurethane appears to be a bit off.

    While the foil facers of polyiso are tru vapor barriers, most 2lb polyurethane spray comes in at about 0.8-1.5 perms@ 1". At 2" is still no more vapor tight than a kraft facer on a batt.

    The foil facers on polyiso run about 0.05 perms (or less), that's a full order of magnitude tighter than 2" of 2lb spray polyurethane. So while 1- 2" of closed cell foam is tight enough to protect the band joist from wintertime moisture loading, it's sufficiently permeable for the band joist to dry toward the interior the other 8-9 months out of the year.

    It's pretty common to use 1-2" of closed cell foam on band joists with unfaced batts to fatten out the R on the interior.

  7. easyrider470 | | #7

    Does it matter that my POYISO is NOT foil faced? I have Fiberglass Faced Polyiso board installed on the exterior.
    So the 2" closed cell route is better than the thicker open cell route

  8. easyrider470 | | #8

    I was wondering if the fiberglass faced polyiso has different characteristics than the foil faced for PERM levels?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "I was wondering if the fiberglass-faced polyiso has different characteristics than the foil-faced for [permeance] levels?"

    A. Yes. Foil-faced polyisocyanurate is a true vapor barrier. As Dana noted, the aluminum foil layer has a permeance rating of 0.05 perm or less. Fiberglass-faced polyiso will be significantly more vapor-permeable than foil-faced polyiso. Contact the polyiso manufacturer if you want more information on the permeance rating of any particular product.

  10. easyrider470 | | #10

    So, since there is such a higher vapor permeance with the fiberglass facing I should be fine with the 2" closed cell against the band board because I have a better capability of drying in either direction? Now I understand why Dana said that.

  11. Expert Member
    Dana Dorsett | | #11

    Unfaced polyiso runs anywhere from 1.5-3 perms @ 1", depending on density & manufacturer. Facers of lower permeance are usually added to lower the total vapor permeance, and limit the moisture uptake of the foam itself. (Polyiso is hygroscopic- it will take on moisture and drop in performance if chronically wetted by bulk moisture such as roof leaks.) Most fiberglass faced goods are less then 1 perm, but more than 0.5 perms typically characterized in the specifications as "less than 1 perm". Some manufacturers of "breathable" polyiso specify that the permeance will be greater than 1 perm, independent of thickness, but that would limit the thickness to 2-3", due to the inherent vapor retardency of the polyiso itself.

    But even 0.5 perm fiber-faced polyiso offers 10x the drying rate of a single foil facer, and something like 20x the drying rate of 2 foil facers. With a half-perm or more of drying capacity to the exterior and (with 2" of ccSPF) half perm of drying capacity toward the interior moisture won't be permanently trapped in the band joist, and it will dry at a rate fast enough to avoid rot issues (even if it got rained on prior to insulating.)

  12. easyrider470 | | #12

    Thanks Dana.
    My house got rained on quite a bit before the siding was installed but it dried out nicely before we put the siding on, I don't expect I have any issues with the foam being saturated, I hope it didn't soak up to much from the rain it endured when it was exposed, i assume this happens quite often on homes with polysio on the exterior

  13. easyrider470 | | #13

    Forgot to mention, the cost of the closed cell isn't a crazy upcharge over the open cell in these band board/rim joist areas and you mentioned that the closed cell would actually be a safer route to go than the 5-6 inches of open cell foam with my that still true since I am using fiberglass faced polyiso?

  14. Expert Member
    Dana Dorsett | | #14

    There is far less danger to the band joist (with either method) due to the fact that the polyiso is likely to be at least an order of magnitude higher permeance than foil faced goods.

    In zip code 47460 with 1.5" of 0.5-1 perm polyiso on the exterior you'll be fine using 5-6" of open cell foam on the interior side, but it's worth spraying the interior surface of the open cell foam with "vapor barrier latex". That will reduce the vapor permeance of the foam layer to about 5 perms (instead of 10+), making it comparable to a standard latex paint on gypsum. (Vapor barrier latex on paper-faced gypsum runs about 0.5 perms, but is far more permeable when applied to open cell foam.)

    An open cell solution would be preferable to 2-3" of closed cell foam for a couple of reasons. The deeper foam depth means the thermal bridging of the I-joists is cut by half or more, the drying capacity for the band joist is higher, and the substantial environmental damage from the HFC blowing agents used for close cell foam is avoided completely.

  15. easyrider470 | | #15

    Outstanding! Open cell it is. Thanks a ton.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |