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Community and Q&A

Should I use closed-cell foam to replace rodent-ruined insulation?

Moya Mim | Posted in Energy Efficiency and Durability on

20-year old house has uninsulated basement (both a crawl space and a concrete part) and attic.

There is insulation in the ceiling of the basement of the type that you roll out, as well as the floor of the attic.

The insulation has been ruined by rodents.

It seemed that replacing the ruined insulation with 3″ of closed cell foam that would be clean and rodent-resistant would be an easy choice. But I want to make sure there are no serious drawbacks.

-Besides price, are there any drawbacks to properly installed closed cell foam?
-How much should I expect to pay for 3″ of closed cell foam?
-What questions should I ask to make the installer is qualified (I’ve read that improper installation would be bad).
-Instead of insulating the ceiling of the basement and crawl space, should I instead insulate the concrete walls down there? Similarly for attic, should I instead insulate the roof of the house?

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  1. Expert Member
    Dana Dorsett | | #1

    It's probably better bang/buck to use 2" of cc foam and put it on the foundation walls & band joist than to put it on the underside of the subfloor, since it's much easier to achieve a good air tightness that way, and puts anything that's down there (like boiler/furnace/water-heater and stored goods) inside the thermal and pressure envelope of the building.

    If you want/need even higher-R you can add to that with an interior studwall snugged right up against the cc foam (and you might want to frame that wall before you spay, using the 2" space between the studs & concrete as the depth gauge for the foam installer), using unfaced rock wool or unfaced fiberglass batts. Put an inch of XPS or EPS between the bottom plate of the studwall and the slab as a capillary & thermal break.

    In an unvented crawlspace put down a 10-mil poly or an EPDM (membrane roofing type) vapor barrier on the floor lapped up and sealed to the foundation 8-12" before installing the wall-foam.

    In most situations any more than 2" of closed cell foam carries a bigger lifecycle greenhouse gas burden (due to the HFC245fa blowing agent) than the energy use it offsets. SFAIK there is but one cc foam product available in N.America that doesn't have that issue (Icynene MD-R-200,which is ~R5/inch and is water-blown, with a very low carbon footprint.)

    Prices vary by region, market condition and chemical feedstock costs, but about $1-1.25 per square foot per inch of depth is roughly the range.

    In the attic 1-2" of cc foam at the roof deck with the rest of the R coming from fiber insulation works for most US climate zones. See:

  2. GBA Editor
    Martin Holladay | | #2

    It would have been helpful if you had told us your geographic location or climate zone.

    In general, it's better to insulate basement walls (or crawl space walls) rather than to insulate between the floor joists. In your case, tightening up the walls will have another benefit (assuming that the contractors do a conscientious job): it will help keep out the rodents.

    Here's an article about creating a sealed crawl space with insulated walls: Building an Unvented Crawl Space.

    Here's another article you may find useful: How to Insulate a Basement Wall.

    It sounds like the type of insulation that was installed in your basement (and was ruined by rodents) was fiberglass batts. Fiberglass batts should never be installed in a basement or crawl space. Either closed-cell spray foam or rigid foam insulation is a better choice.

    Before choosing a spray-foam contractor, ask for references and the names of former customers. Do a little research before accepting a bid.

    Here are some articles on attic insulation:

    GBA Encyclopedia: Insulating Roofs, Walls, and Floors

    Creating a Conditioned Attic

  3. Moya Mim | | #3

    Thank you both for your thoughtful responses. The more I know the less sure I am about how to proceed.

    Here are a couple of more facts. Please let me know if there is any other relevant info.

    (1) The climate zone seems to be 6A. It's in midstate NY.
    (2) The crawl space is nearly 6 feet tall. I believe the walls are concrete block of some type. I believe there are a couple of vents to the outside. There is a big hole between the crawl space and the basement for access to the crawl space.
    (3) The mechanicals (furnace, water heater and water tank) are in the crawl space.
    (4) The basement is a walk-out. I believe the walls are concrete. It will eventually be finished and made into a living room.
    (5) The floor of the crawl space is rock and dirt.

    I called a foam sprayer, who advise that open cell would be better than closed cell because (1) it's slighlty cheaper and (2) it's more fire resistant and therefore doesn't require painting, He said the rodents typically do not burrow into the open cell, and that 6" of open cell would provide a vapor barrier.

    I feel totally in over my head. Many options, and much conflicting advice.

    Besides the obvious goal of insulating the living space (1st and 2nd floors), my goals are:

    (1) Insulate the basement for future use as a living room.
    (2) Have a clean, accessible and rodent-proof crawl space that can be used for storage.
    (3) Help stop Radon and other gasses travelling up to the first floor.
    (4) Have a rodent-proof attic that can be used for storage.
    (5) Not release too many VOCs into the environment or, over time, into the house.

    Where would you advise that I begin? I am reading the articles you've sent, but I feel that I will never have enough command of the subject to weigh all of the advantages and disadvantages in time.

  4. GBA Editor
    Martin Holladay | | #4

    If all of the information available on this website is daunting, then you should find an architect, designer, engineer, or builder that you trust and put your project in the hands of the professional you hire.

    To find such a professional, start doing some research. Ask your friends and relatives about professionals they have worked with and trust.

  5. Moya Mim | | #5

    I am working with a contractor but was hoping to have some knowledge myself. I am not sure whether the contractor is an expert on insulation however.

  6. GBA Editor
    Martin Holladay | | #6

    I admire your desire to educate yourself on these issues. Don't be afraid -- start reading, and keep asking questions.

  7. Moya Mim | | #7

    Do you have any additional advice after the additional info I posted (climate zone etc.)?

  8. GBA Editor
    Martin Holladay | | #8

    You need to seal the vents in your crawl space wall.

    You need to clear out the rocks, level the dirt on the crawl space floor, and install a vapor barrier over the dirt. Ideally, you will pour a concrete slab on your crawl space floor.

    Either spray polyurethane foam (I think closed-cell makes the most sense for basements) or rigid foam can be used to insulate your walls. If you choose rigid foam, make sure to pay attention to air sealing details before insulating.

  9. Moya Mim | | #9


    I've read a bit about the pros and cons of sealing the vents in the crawl space. My main concern would be whether that might excacerbate the Radon.

    With air sealing, you mean sealing the walls, perhaps with spray foam out of a bottle?

    I love the idea of pouring concrete in the crawl space. My concerns there are (1) will they be able to pour a really good, hard slab in the limited space of the crawl space? (2) will this cause any moisture problems (water coming up from the water table below)? (3) will the initial drying phase of the concrete harm the house?

    Btw, how expensive is pouring concrete (it's less than 500 sq feet)?

  10. Moya Mim | | #10


    How about open cell spray foam? It seems to have some advantages for use in a basement (more fire resistant, slightly less expensive than closed cell), equally good at blocking air.

    I understand it absorbs moisture. I am not sure whether that makes it better or worse than the other options.

    I am told mice don't normaly chew through the open cell stuff.

    Climate zone is 6A, midstate NY.

  11. GBA Editor
    Martin Holladay | | #11

    Closed-cell spray foam is more resistant to air infiltration than open-cell foam. However, if open-cell foam is installed at a thickness of at least 4 inches, it can be an acceptable air barrier.

    Fire resistance is all over the map. I have never seen any evidence that open-cell foams (on average) are more resistant to fire or to ignition than closed-cell foams. However, some formulations of foam may have been tested in a way that the results of the tests convince local code officials to waive requirements for an ignition barrier or a thermal barrier. For more information on this question, contact your local code official.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Moya, I use Northeast Sprayfoam. They may also know contractors to aid you. There also are radon specialists that could put on a system while other work is performed.

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