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Community and Q&A

Insulating a crawl space

user-1101025 | Posted in Green Products and Materials on

We have a single level ranch style home with crawl space that connects with the basement living space. The crawl space is located at the end of the ranch home. This crawl space has a vapor barrier over the rocky dirt floor, originally installed as part of radon remediation. The concrete walls in the crawl space contribute to cold air and possibly some moisture. At this point, though, I think that it is safe to say that no water seeps through into the crawl space.

Rodents have come through what was “sealed” 12″x6″ vents within basement wells. The radon contractor…27 yrs later…is coming back to place a second plastic vapor seal over the floor and he will use plywood, instead of rigid foam (now there), to seal the vents. I can only hope that rodents were coming through a hole in the vent and not tunneling up through the dirt/rock filled crawl space. Despite this issue, I would not say that I see signs of an infestation.

Additionally, we are thinking of wrapping the walls within the crawl space with some type of insulation. Two questions come to mind.

One, what type of material…IF ANY…would you recommend? My wife is concerned about the toxic qualities of rigid foam and the threat of mold growing between the concrete walls and insulation material. Plus, there is the apparent problem of moisture migrating up to wooden material at the top of the concrete foundation. Which points to another issue: Should the concrete wall be coated with something that water(moisture)proofs it OR does it need to breathe?

Secondly, which walls should be wrapped: exterior, interior(the walls that interface between the crawl space and the living space of the basement), or all walls?

William Phillips
Louisville, KY 40242

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  1. GBA Editor
    Martin Holladay | | #1

    William Phillips,
    The best approach is to seal all crawl space vents, and to install insulation on the interior of your crawl space walls and your basement walls. The best types of insulation are closed-cell spray foam or rigid foam insulation (XPS, EPS, or polyisocyanurate). Don't use fiberglass.

    For more information and a list of valuable resources, see Building an Unvented Crawl Space.

  2. user-1101025 | | #2

    Which insulation is more eco friendly...does not out gas? I have heard that Thermax or Rmax are good products; are they suitable for meeting my wife's concerns about toxicity? I have not checked into pricing to see if this is a consideration, as well. Can you suggest a preference for something that is foremost eco friendly, and secondly....affordable?

    After the walls are insulated, would it be advisable to take down existing barriers to the crawl space that connects to the live-in part of the basement? Would this help with ventilation to compensate for closing off the exterior vents.


  3. GBA Editor
    Martin Holladay | | #3

    Very rarely, spray foam insulation has been associated with odor complaints, so if you are nervous about those issues, you should probably avoid spray foam.

    None of the rigid foams -- XPS, EPS, or polyiso -- has been associated with health problems when installed in homes, to the best of my knowledge, but some sensitive individuals can't tolerate many ordinary building materials. All three of these foam insulation products are nontoxic.

    Of the three insulation types, polyiso is considered the most environmentally friendly.

  4. Foamer | | #4

    You are right to be concerned about mold between rigid foam and the foundation. Remember that air sealing the rim joist and insulating it is an important part of the job. For that you really can't beat spray foam. It will provide both thermal and moisture control and can integrate very nicely with the vapor barrier on the floor. If you wrap the poly an inch or two up the foundation, the foam will tie everything together.on Put mesh in your access hatch so that the heat from the basement can help condition it. If you have ductwork in the crawl, they will warm it and your floors will be nice and toasty.

  5. user-1101025 | | #5

    Re: spray foam:
    I love watching the application when seen on some quality tv productions...BUT how toxic is it, particularly when applied? My wife will be fairly strict on the ecological health issue.

    Re: You write, "Put mesh in your access hatch so that the heat from the basement can help condition it." I'm not sure that I grasp. Can you elaborate?

    Thanks for your response.

  6. GBA Editor
    Martin Holladay | | #6

    You're being a little casual with the use of the word "toxic." Cured foam, assuming it was installed properly, is not poisonous to humans. Of course some people -- perhaps including your wife -- are sensitive to building materials that most people find unobjectionable. For that reason, you may decide not to install spray foam.

  7. user-1101025 | | #7

    Yes, "toxic" needs to be better defined. Outgassing metics, if they exist, comes to mind. Paint companies use VOC's. If the people that spray foam use a mask and suit to protect themselves, then is there not a problem for those living in the enclosed space? After the product cures, toxicity may not be an issue. This can become another discussion and I don't want to sound obsessed.

    I looked for Thermax and RMax products at the websites of lowes and home depot; not there. So I assume that these are special order products. Both are polyiso materials, which you recommended. Do you have a preference of a polyiso product? Any suggestions about application which may be overlooked by the typical handyman?

  8. GBA Editor
  9. user-1101025 | | #9

    Martin, searches failed...not sure why "insulation" alone didn't work.
    Anyway, I assume that this is your choice for a polysio and it gives me a sense of expense.
    Moves me along.

    Re: installation....sorry if I 'm harping on this, it's just that I'm not sure the radon guy who is putting down the poly on the floor does more than consider what gets him at the crawl space asap. Over the years I have come to appreciate the value of proper installation

    On line research: attach panels to concrete(adhesive or screws w/ large washers); tape panel to panel edges; foam in a can to seal top/bottom edges. ??????

    The more I read the more I question. Err. I've overlooked the cavity above the floor joist. Roxul is mentioned as a good product to stuff in this area...after cutting in tuff-r insulation.

    But, the ultimate problem that I skirt is mold. What if I'm wrong in assuming that the concrete is dry and that no more water is running over the it once did but seems to have stopped after I got my gutters fixed and re=graded the ground in the trouble spots? After a heavy rain, I'm thinking that I need to get into the crawl space and do an inspection. Is there a moisture meter that would help clarify just how safe I am from future moisture?

  10. GBA Editor
    Martin Holladay | | #10

    There are lots of resources listed in the "More Information" sidebar included with my article (Building an Unvented Crawl Space).

    But if this kind of work makes you nervous, you can always hire a crawlspace insulation expert.

  11. user-1101025 | | #11

    I'm afraid that so called experts, here, are just spray foam guys that charge a fortune with lower level guys doing the work, and no one really "green" knowledgeable. I've called and spoken to them. My best bet is to read and collaborate with my handyman.

    I very much like your linked article, and read the sidebar articles when available.I will pass this on to my handyman.He is increasingly being asked to resolve leaks in basements.

    Any chance I could read:(free trials that require prior use of credit card info, not something that I am comfortable with)
    The Stay-Dry, No-Mold Finished Basement
    Insulated Crawl Space. Concrete Block with 1-in. Exterior Rigid Foam

    Can we discuss the air conditioning aspect of the re mediated crawl space? I assume that air movement is important in order to avoid mold....see quote that I extracted from one on the side bar articles. And, yep, we have a lot of sinus many of us in the ohio valley do.

    Here are some options that come to mind. Can you briefly comment? (my comments in () )
    But certainly feel free to elaborate on your own suggestions from the linked article:
    Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below. (sounds expensive)
    Install an exhaust fan or a forced-air register to meet code requirements for conditioning the crawl space. Be sure that the fan does not exceed air flow requirements for the size of the crawl space, since exhaust fans carry an energy penalty. (does fan fit into the exterior wall of the crawl space?)
    (My suggested options)
    1. Cut a small hole in the forced air return duct already within the crawl space.(not good for the balance of the system)
    2. Do the same in the cold air return vent. (ditto, above)
    3. Remove access barrier to crawl space and place a circulating fan in the crawl space.(may just blow dust and mold around into the house)
    4. Keep access barrier intact and place a circulating fan in crawl space.
    5. Purchase an exhaust fan designed to be inserted into crawl space vent. (could pull energy expensive air from within house to outside)
    6. Explore if radon expert can splice a second tube onto corrugated tube now UNDER poly vapor. Second tube will lie on TOP of poly vapor.

    "According to a study by the Mayo Clinic, nearly all chronic sinus infections are a result of mold. Since up to 40% of the air we breathe in the home can come from the crawl space, mold in the crawl space means mold in the home. The EPA recommends to keep humidity levels in the crawl space to 40%–50% to reduce the likelihood of mold formation."

  12. GBA Editor
    Martin Holladay | | #12

    It's up to you to decide whether you want to take advantage of GBA's free trial offer.

  13. user-1101025 | | #13


    Can you address the how-to conditioned air issue? I am trying to do my share of reading.
    Or, Am I at the end of the line here? Please, just one more elaboration ;)

  14. GBA Editor
    Martin Holladay | | #14

    I stand by the recommendations in my article. I also recommend that you follow the building code. Most of your suggestions ignore the recommendations in my article, so I'm not sure I can help you.

    You write that my recommendations "sound expensive." If you can't afford to cut a hole in the floor of the living space above to install a floor grille, I strongly recommend that you do nothing. Doing the job halfway in order to save $50 is not a good idea.

    As I wrote in the article, the code provides two methods for conditioning a sealed crawlspace. Both methods require the installation of the floor grille which "sounds expensive."

    Once the grille is installed, choose Option 1 or Option 2.

    Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.) This option means that you need to install an exhaust fan in your rim-joist area or a wall connecting with the outdoors.

    Option 2 requires that the crawl space have a forced-air register delivering 1 cfm for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.) That is a register delivering supply air from your furnace, not a return-air grille.

  15. user-1101025 | | #15

    I've scanned a lot of postings and your linked article is the most far. I apologize if I my words came across as resistant. I do have a habit of over doing "what about this way" and not being aware of how my ideas may be counter to the suggestions that I am asking another to share.

    What I really mean to do is to probe, however clumsy I may be, for further clarification. I feel that I am tremendously better informed, though I'm slow to grasp some details. I can only hope that other lay folks are the same way..and, hence, are learning from this thread. Whomever does the work, I will be a much more informed consumer.

    Having conditioned air is the crawl space makes a lot of sense. Having crawled down under many times, it really connects. I think there is just one more question.

    For Option 2, can I use an existing forced air duct in the crawl space as long as the air coming out can be made to fit the specifications of 1cfm for each 50 sq ft of crawl space area?
    p.s....ok to remove plywood barriers that currently block access to crawl space and interface with basement area that houses furnace?

  16. GBA Editor
    Martin Holladay | | #16

    If you remove the barriers between your crawl space and basement, you may not need to condition the air in your crawl space. It all depends on whether you have heating equipment in your basement, and how dry the air is in your basement.

    If you decide to keep the barriers between your crawl space and your basement, I think you should follow one of the options listed in the code for conditioning the air in a sealed crawl space.

    Yes, you can install a register in an existing supply duct in your crawl space -- as long as installing that register doesn't rob the duct of air flow needed in one of the rooms serviced by that duct. Consult an HVAC contractor if you are uncertain of the air flows required to each of your rooms.

  17. user-1101025 | | #17


    Got it.
    I'm calling "an expert"...

    for an evaluation. Having said that, thanks to your patience with me I can interact from a much more informed perspective, and have the option of going with the radon specialist or my local handyman...who has increasingly done more basement work in this city where the water table has been exceptionally high this past year.

    Thanks again.

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