GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Spraying closed cell spray foam directly to fiberglass batts?

Brian Knight | Posted in Energy Efficiency and Durability on

Working on a house in Chattanooga TN (zone 4) that needs weatherization badly. Already decided on cc SPF to seal some “crawlspace walls” and exposed floor system from below that are outside the “crawlspace”. There are many angles, transitions and irregularities that would make rigid foam or air barrier protecting batts very difficult hence the reason for going with SPF.

The plan is to remove the batts and spray 2-2.5″ to the subfloor and cover the joists (including bottom edge) with around 1″. Iam tempted to skip the batt removal as much of it is installed decently and there would be plenty of framing for the foam to bond with. Is it a terrible idea to directly spray to the bottom of the batts and joists?

I realize the correct way is to remove them and reinstall them under the spf but detailing an air barrier to protect them would be extremely difficult, expensive and labor intensive. I dont think the homeowner would want to add the appropriate amount of insulation (under the spf) given the mild climate, and long payback (and I sure dont want to work with FG batts). Why not maintain all that R, protect it with the cc spf and it would even make it easier to remove the foam in the future if needed.

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

    Q. "Is it a terrible idea to directly spray to the bottom of the batts and joists?"

    A. Yes.

  2. Brian Knight | | #2

    Just to be clear, all pictures are from COMPLETELY outside the building envelope. I was taking them from outside. Not in a basement and not in a vented crawl.

    Martin, is this because of the incomplete bond to the framing or other reasons?

  3. GBA Editor
    Martin Holladay | | #3

    I can think of several reasons, but the most important one is that you need a firm surface to spay against so that the insulation isn't subject to cracking or being broken if bumped.

  4. wjrobinson | | #4

    Be interesting to try it and see what is the good bad and ugly of the idea. Try it and post back. Go to a spray job and spray a mock up.

  5. GBA Editor
    Martin Holladay | | #5

    Just because the installation looks good a week later doesn't mean it will look good in 5 years.

  6. Brian Knight | | #6

    Thanks guys. It would certainly be a disaster should the foam ever pull free. The areas that would cover the FG batts are at virtually no risk of being bumped into. I would think that there could be more forgiveness in cracking compared to the "proper" install..

  7. wjrobinson | | #7

    I would rather it be tried. The glass might just aid structurally to the foams durability or not. Martin, with your attitude there would be no fire, wheel, flying, you name it, gotta try things sometimes instead of armchair negativity. I say mock it up.

  8. GBA Editor
    Martin Holladay | | #8

    I try all kinds of crazy things on my own house. That doesn't mean I'm going to recommend on a national website that a builder undertake an experiment at a client's house.

    Of course, the decision is entirely Brian's.

  9. wjrobinson | | #9

    Reading way too fast Martin. I said mock it up. Neither of us are saying just start spraying the home.

    Most of the time we agree and we sort of do now. And even about the life of a bear on a nutless ridge given a full enough keg of Lake Placid Ubu. (Adirodacker attempt at some inside or as it may "outside" humor)

  10. wjrobinson | | #10

    And knowing your house and your posted history, I'm glad you just posted to this thread that there is a side of you that personally would go for experimenting on your own home.

  11. Brian Knight | | #11

    Full disclosure: clients are my folks. They seem open to experimentation as long as the possums, bats, rats, cats and coons are kept out of the living and semi-living space.

    In the interest of "green": less wasted resources (existing FG batts, energy, money) and less material headed to the land fill.

    Surely someone has done or seen this and lived to tell the tale?

  12. bdrfab | | #12

    I fully respect your option Martin, but I would wonder your feelings on retrofoam? Obviously there is structure on both sides so the worries about being bumped are ot there, but what are the rest of reasons? Would open cell work better?

  13. wjrobinson | | #13

    Brian, one phone call, 20 minutes to make your test mock up, drive to the on going spray job, spray, done. Get home and post results and pics here.

    Much better than someone here posting anything bro. Get err done my man.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |