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Community and Q&A

Spray foam or fiber glass batts for crawlspace?

Dan White | Posted in Green Products and Materials on

I have a circa 1900 house on the Pacific NW coast built with old growth Douglas fir, post and beam construction, fir subfloor, (new) oak strip tong and groove flooring with penetrating wax oil finish.

The 1st level floor was very cold last winter! I have a forced air furnance in the basement floor vents on the 1st floor and attic vents on the 2nd floor.

About 70% of the house footprint is underlain by an on-grade, vented crawlspace over the ground. The crawl space has 10inch joists with 24inch centers; the walls of the crawl space are wood and go to the ground. Beams are set in concrete. The remaining 30% of the footprint has a concrete basement. There are existing fiberglass batts in the crawlspace and black plastic vapor barrier on the ground, but both are poorly installed, old, torn and smelly.

I have evidence of rats in the basement and crawl space. I plan to remove all the old insulation and vapor barrier, inspect the crawl space for damages, make the repairs and re-install new insulation.

My question: spray foam or more fiberglasss? I want no more nesting places for rats, high quality insulation, and I don’t want to re-do this again. If the foam is actually as good as advertised, it will be cost effective and provide a better air barrier and better insulation.

My concerns: off-gassing from the sprayed foam, inability to remove foam later for any un-expected repairs, locking in moisture in either the flooring or in the crawlspace, risk of problems from wood-boring insects…

Please advise.

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  1. Stuart Fearn | | #1

    If you want the same result use the same product. If you want better results use the best product. The choice really could not be simpler than this.

  2. Dan White | | #2

    Thanks for your response. I assume you are implying that the foam is the best product. Can you address my concerns about the off-gassing, moisture, etc?

  3. GBA Editor
    Martin Holladay | | #3

    Whatever you do, don't install fiberglass batts.

    The best approach would be to seal your crawl space vents and to create a conditioned crawl space.

    If you are worried about spray polyurethane foam, you can insulate the walls of your sealed crawl space with a rigid foam product like polyisocyanurate.

    For more information, see Building an Unvented Crawl Space.

  4. Dan White | | #4

    Thank you. If I understand correctly, to creat a conditioned crawl space the thermal barrier should go on the ground and walls of the crawl space - not on the crawl space ceiling. The crawl space them becomes part of the conditioned air space of the house.

    I understand new constructions are beginning to be done this way. How would that work with my old house? The walls of the crawl space are wood. There are no cement wall to tie vapor barriers to. Wouldn't these old wood walls suffer from additional moisture?

  5. GBA Editor
    Martin Holladay | | #5

    I hope that the wood walls of your crawl space are sitting on a concrete foundation. If not, they are probably leaky and prone to rot. If you don't have any concrete foundation at the perimeter of your house, you may not be able to create a conditioned crawl space.

    Your worry about a vapor barrier is misplaced. There is no need for a vapor barrier on your crawl space walls -- just an air barrier and insulation.

  6. Dan White | | #6

    Your concern is correct. I do not have any concrete foundation at the perimeter. There are wooden walls that go down to the ground. The current insulation which I will remove has fiberglass batts between the joists of the crawl space (torn, dirty and rat invested) and a black plastic vapor barrier on the ground . The vapor barrier was run up the sides of the wooden perimeter walls with fiberglass batting between the perimeter walls and the black plastic.

    There is eveidence of moisture damage in the walls. Clearly all that fiberglass is a nesting area for rats as well as a safe passage for them.

    I suspect that I will want to seal off the crawl space from the subfloor, install a ground vapor barrier (only on the ground!) and insure that the vents are open into the crawl space.

    Is my logic correct? Will closed-cell, dense foam be the correct way to go?

  7. GBA Editor
    Martin Holladay | | #7

    What's holding up your house? Concrete posts? Wood posts? Your failing crawl-space walls?

  8. Dan White | | #8

    The crawl space makes up about 70% of the footprint. For that portion, the house sets on wooden beams buried in concrete. The perimeter wood is the original fir siding and does not provide any support. The remaining 30% of the house footprint has a concrete basement.

  9. GBA Editor
    Martin Holladay | | #9

    Do you mean beams or posts? I'll assume you mean posts. (Beams are horizontal, like joists. Posts are vertical.)

    So your foundation consists of wooden posts that either extend into the soil (encased in concrete below grade) or above-grade posts that are sitting on concrete pads -- is that what you are trying to say?

  10. Dan White | | #10

    Yes, thanks for correcting my terminology. I have wood posts that extend into concrete bases. The concrete bases are generally at grade level.

  11. GBA Editor
    Martin Holladay | | #11

    OK -- now I understand what you are describing.

    Q. "There is evidence of moisture damage in the walls. Clearly all that fiberglass is a nesting area for rats as well as a safe passage for them. ...Will closed-cell, dense foam be the correct way to go?"

    A. No. The correct way to go is to install a continuous concrete stemwall around the perimeter of your crawl space. However, this is so expensive that you probably won't be able to do it. It will be much less expensive to insulate the floor above your crawl space.

    Remove and discard all the fiberglass batts. If you are worried about spray-foam off-gassing, your best method of insulation is to install at least 2 layers of 2-inch polyisocyanurate insulation under the floor joists. Pay attention to air sealing; use caulk, spray foam, or housewrap tape to make the seams airtight. Protect the underside of the rigid foam with a layer of OSB or plywood.

  12. Charles Fu | | #12


    Investigate other options before you spend your money on the "conditioned crawlspace". or "encapsulation" option in PNW. I did it and now regretted it. Not that my contractor did a poor job or anything. It does not solve the problem we have and actually create more problems. With the crawlspace vents completely sealed off, the outgassing from the plastic cover actually makes the basement smell worse than before. We had to remove those vent covers and install some crawlspace vent fans to drive out the excessive smell.

    Those encapsulation vapor barrier can be bought oneline for $0.50/sqft. But the contractors typically will charge you $3-4/sqft to install them. If you do, negotiate hard since there is so much profit margin in their initial quote

  13. David Argilla | | #13

    I would also encourage you to investigate further before you make a decision. We live in PNW also, in a 106 year old house, that was post and pier construction over a crawl space with wooden walls much like you describe. Had fiberglass insulation between joists (rat infested as well), and thin foam sheathing on crawlspace walls. The posts were in very bad shape, some rotted so badly that one corner of the house was only supported by interior posts (corner cantilever basically). The extent of the damage was not visible until we removed the wall insulation. Other posts had been repaired poorly (See attached pics) . The work needed may exceed just insulation and sealing. If you are planning on living in the house for a sufficient length of time then adding a concrete stemwall as Martin suggests might make more sense structurally, seismically, and comfort/energy wise.

  14. John Mattson | | #14

    You live in the "ring of fire", earthquakes WILL happen eventually, and your compromised foundation may well not survive the next one. My 1903 Long Beach house survived the 1938 quake with its brick foundation, but I replaced it with a new one in 2002 for about 30K. I tried the unvented cripple wall, but even here in the semi-desert, it became too moist. With a good foundation, solid cripple wall, and metal screened vents, I have no dampness, no rats/mice, even no bugs, and no worries. Oh, I used radiant barrier foil-bubble-foil on the cripple wall because solar heat is my nemesis here. Works great.

  15. Rob Krebs | | #15

    Dan White, Can’t really add much more professional advice then the spray polyurethane folks have already written. We are an industry not-for-profit and believe continuous insulation (meaning other than air convecting batt insulation: SPF, polyiso, XPS, EPS all are insect resistant and can help if you decide to do the underneath or work separately on the sides.

    Rob Krebs
    American Chemistry Council

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