Thursday, November 19, 2009

Do not spray foam your crawlspace foundation walls

One of the fastest growing crawlspace industries is spraying open cell or closed cell foam on crawlspace foundation walls. While I agree that foam is the most efficient type of insulation, it is rarely applicable in below-grade crawlspace environments. WHY, you might ask?. In the Midwest, crawlspaces are usually a below-grade foundation type. The foundation of a below-grade crawlspace is almost always susceptible to water and moisture intrusion from groundwater. So what, you might ask? Well, Spray foam insulation traps water and moisture in the block wall or between the wall and the foam. This moisture has nowhere to go but UP, due to the capillary action (wicking) of moisture on a foundation. Up above the foundation is a sill plate, floor joist, bandboard, and subfloor. These are all wood components that are susceptible to wood rot and mold growth when the moisture content in the wood rises.

We are in homes on a daily basis that had their foundation sprayed years earlier and now are paying thousands of dollars in structural wood repair. Most wood rot in a crawlspace is noticeable early and most of the outside wood structure is saved by some type of moisture control system. When the walls and wood are covered with spray foam, no one can detect the problem early. Homeowners do not find out until the house starts sinking and and all of the plates and joists must be replaced. The spray foam will speed up the wood rot process.

The picture above and below shows wood that has rotted behind the spray foam.

Just to be clear, every crawlspace has different amounts of moisture intrusion and some with lower levels may never have wood rot occur. However, moisture intrusion can increase over a period of years as new development pushes water into older areas. So if you still choose to foam your walls, just know, you are always susceptible to wood rot.

In below grade crawlspaces, the proper way to insulate is to encapsulate the crawlspace and seal off outside air from entering the crawlspace. Next, condition the crawlspace. Rigid Foam board can also be installed to the walls for added insulation. For more information, see my post "How to insulate a crawl space - Encapsulate and Condition"

While the title to this post seems to suggest that you should never spray foam your crawlspace foundation walls, that is not always the case. Many solid, poured foundation walls are not as susceptible to water intrusion; therefore, they could be insulated with spray foam. See my post "Insulating above-grade crawlspaces - spray foam the walls" if you have a BONE -DRY (above-grade or solid poured foundation) crawlspace.

If you need professional advice on how to insulate your crawlspace, give us a call @ 1-877-409-2837 or SIGN UP ONLINE for a FREE CONSULTATION.

Thanks for reading the rambling thoughts of a crawlspace inspector,
Larry Ralph Jr

Crawlspace Insulating Series:
Cold Floors above a Crawl Space
Do not use fiberglass insulation in a crawl space
Foundation Vents are causing higher heating costs
How to insulate crawlspaces - Encapsulate and Condition
Insulating above-grade crawlspaces - spray foam the walls



  1. We just posted a video on spray foam insulation on our site and I decided to blog search and found this article. Nice post... You are 100% correct and people would be wise to heed what you are saying here. If you trap moisture between the block and the foam you've got a problem... especially if you've got a lot of moisture. This applies not only to spray foam though - rigid foam board installations can also cause this problem, as well as any other vapor barrier that doesn't completely seal water entry from the bottom.

  2. Thank you for the heads up. I was strongly considering to have my crawlspace insulated with spray foam. But after reading your post, I'm going to go convert my crawl into an unvented space with insulated foundation walls (XPS foam).

  3. Thank you Larry for giving me tips about crawlspace foundation walls. It seems that spraying foam in any crawlspace foundation wall is not good for the foundation. If problems occur in your crawlspace foundation walls after spraying foam, repair cracked foundation service is an option.

  4. I live in South Louisiana and I've been told that my crawl space is very moist. I do not think that we have standing water, but mostly just very humid and also very cool (we may have a leak in an AC duct). The house had a brick skirt placed and has a few vents it in, but our exterminator (we have a flea problem in the crawlspace) claims that we don't have enough vents and need a better vapor barrier (80% coverage). They want to do the work to dry it up with these two things, but I seem to find contradictory things about crawlspaces. I just want to block out the mold, reduce the humidity (and hopefully kill the fleas) , and improve air quality in the house and improve efficiency. Is encapsulation the answer?


    1. Did you find a contractor? I am also in S. Louisiana and finding on to do encapsulation has been very hard.

  5. Chris,

    Yes, Encapsulation will seal out moisture from the ground, walls, and outside humidity levels. Adding more vents will likely make your problem worse.

  6. If you have a interior perimeter drain with sump pump installed, can you not apply spray foam insulation to the concrete walls from the crawlspace floor up to floor and apply a thick vapor barrier on the floor and seal it to the bottom of the spray foam on the concrete wall? Wouldn't this allow for the water to enter the perimeter drain and eliminate the "stack effect"?

  7. The interior drain will allow water to drain, but will not eliminate the "stack effect". The "stack effect" is when warm air moves upwards in a house such as in winter or summer. I can't say what would be best in your scenario without seeing the amount of water intrusion that is occurring. As water enters a crawlspace, it is raising the moisture content of the foundation. The problem with just a perimeter drain is the amount of moisture that could still wick up the foundation in between the foundation and the foam or inside the block wall. This moisture that is wicking upwards can still cause the plates and joists to rot. I've seen homes with interior perimeter drains still have wood rot from trapping moisture against the wood with spray foam. Dry Rot begins at 20% MC and wet rot at 30% MC. If there is very little to no water intrusion (which I doubt since a interior perimeter drain was installed), then a spray foam application could be installed. I do recommend spray foam in crawl spaces, just not against damp crawlspace walls.

    The best option in your scenario would be to prevent all water intrusion from occurring. This might require better grading, an exterior perimeter drain, an exterior foundation moisture barrier (plastic, epoxies, sealants, or dimple core drainage board), and all the downspouts extended.

  8. We have found that the reason for our moisture intrusion, through the crawlspace concrete floor is because the skim coat was not poured above the footing (1.5 inches of the top of the footing is exposed) and is sitting lower than the exterior perimeter drain, and there is no vapor barrier applied under the skim coat.

    If you encapsulate wouldn't the wicking up the foundation walls still apply?

  9. If you encapsulate, the liner is sealed at the top of the block wall. This will prevent wicking because the moisture will not wick up above the sealant. As for the inside of the block wall, weep holes should be drilled to allow water to escape the foundation wall into the perimeter drain. There will still be some wicking inside the wall after this, but by spraying foam you will trap all of this moisture in the plate, where as by encapsulating, you will allow the plate and joist ends to breathe. If you are confident there is minimal water intrusion into the block foundation wall, then there should be no problem with the spray foam application.

  10. stack effect causing moisture to rise in a concrete wall? Stack effect has only to do with rising air. Moisture in concrete foundations can rise up due to capillary wicking not stack effect. A grade beam or basement wall that is not properly waterproofed or at least damp proofed may allow wicking but to make a blanket statement that SPF doesnt work is just plain silly.

    Grade beams should have a capillary break where they sit on the footing to prevent rising damp.
    there should be a capillary break between the top of the grade beam and the sill plate or floor joists.

    even if there is no foam on the inside the capillary draw may cause rot on the sills/joists

    the climate zone also determines best practices for crawlspace insulation.

    for real unbiased advice on crawlspaces visit and search for crawlspaces

  11. I was looking for info on spray foam because I thought it would be my solution but ... glad I found this site..

    I ended up sealing vents in my wooded crawlspace and replacing fibergrass with rigid, footing to floor joist and fiberglass between the floor joists BUT what worries me is in the winter the fiberglass is stuck, iced to the outside wall between the floor joist and if I remove the rigid the inside walls look wet (treated plywood). Is this normal? The crawlspace is 1/2 above and 1/2 below ground.

    I have also noticed mold in the corner of a room and I went under to find that one corner of the house ... looking from the crawlspace .. the floor is wet. 6 inches or so in the corner. The only thing I can think of is .... was that one corner not insulated (box corners I believe), the condensation is building up and sliding down to the floor? Is this possible? I sure hope you help me out on this, it is scary seeing the most important part of your structure looking like this..thanks sheila

  12. Sheila, call our office and leave your contact info. I will call you this week. 877-409-2837.

  13. I tried calling but since I am in Canada the number does not work. I have checked out your site and see a number of issues that I need to address, insulate pipes, seal vents better, replace ripped barrier on the floor etc. I went under today and humidity is 75% down there 65% in the house. I think if I can totally seal the floor, vents, pipes .. the wall problem will be solved. I think. On your website you show the spray foam between the floor joists just above the sill plate, that was where I wanted to spray.. everything is dry under there right now..on the walls but with the humidity,, what should it be .. maybe get a dehumidifier for now? I have looked for that white vapour barrier in the past ... where do you find it? thanks for your time.

  14. I am having my crawl space floor concreted and
    was going to have the walls which are below grade sprayed with foam with a material that is attached to the concrete walls first before spraying which allows any water that may leak to run down the wall into the drain tile. Should I have the walls sprayed in foam which are also concrete?


  15. I just read your post and it got me a bit worried. I had my crawlspace sprayfoamed earlier this year. Only 2 (perimeter) walls got sprayfoamed --- one of the walls we had the outside of the wall waterproofed as it had moisture issues previously.

    My concern is with rotting...prior to sprayfoaming, I did not notice any rotting at the sills. i am concerned that this may occur. What are my options? Should I remove the spray foam applied at the sills and joists. After reading your post it seemed to me that I would need to allow the concrete foundation to breathe a bit...would it be better to maybe remove the spray foam off parts of the concrete foundation?

    Please advise.

  16. I have a client who through a energy efficency program had spray foam closed cell applied to her very wet crawlspace. She continued to ask if her crawlspace was wet and the contractor told her it was taken care of. She now is ill...has major wicking into the sub floors buckling (up to 25% moisture to date)...significant mold growth on the ceiling of the crawlspace and standing water in the crawlspace on top of the vapor barrier. What is her recourse with this company.


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