Thursday, November 19, 2009

Insulating above-grade crawlspaces - spray foam the walls

For those who do not know, a crawlspace above grade refers to a crawlspace with a ground floor that is level with the outside ground level. In many instances, these crawlspaces can be considerably dry. Since many crawlspace above grade are somewhat dry, the best method of insulating is to encapsulate with plastic and apply spray foam to the walls and exterior wood framing. I am a large advocate of the foam method of insulating; however, only when the conditions are optimal (a dry crawlspace). The biggest mistake a homeowner can make when insulating a crawlspace is to spray foam the foundation wall when it is wet or damp. The foam will trap the moisture in the foundation allowing it to rot the structural components such as the sill plates, floor joist ends, and bandboard. For more information on whether you should spray foam your crawlspace walls, see my post "Do not spray foam your crawlspace foundation walls".

The proper method to insulating a DRY crawlspace is to first encapsulate the ground moisture and seal off outside air. This is achieved by installing 20 mil plastic over the ground floor and up the walls by at least 12". (Warning: Encapsulation liners should normally be installed within 3" of the top of the foundation wall, but since the crawlspace foundation is dry and we will be applying spray foam to the walls, there will be no need to.) All the seams of the liner should be taped, and the wall plastic should be fastened and adhered to the foundation wall. Once the encapsulation liner is installed, all of the foundation vents should be insulated and sealed. Sealing the foundation vents prevents cold, freezing air from entering the crawlspace in the winter (cold air still drafts in even when the vents are closed); and hot, humid air from entering in the summer. This is by far the most important step in insulating a crawlspace, and if you didn't do anything else, you could look at 10% - 15% energy savings in a fully vented crawlspace by just encapsulating. Once encapsulation is finished, the crawlspace can be conditioned.

The next step would be to hire a contractor to spray the interior crawlspace foundation walls and wood framing with closed cell foam. They would spray from the bottom of the foundation wall covering some of the plastic up to the subfloor and between the floor joists.

This is by far the best method to insulating a dry, above-grade crawlspace; however, this requires hiring 2 separate contractors that will probably try to talk a homeowner out of using the other contractor. The encapsulation company will want to just encapsulate the crawlspace and add foam board to the walls. This system will work just fine, but it isn't the most effecient. The foam contractor will want to just insulate the walls and throw down a vapor barrier, which still leaves the vents open to bring in cold air in the winter, and does not prevent all moisture from evaporating into the structure. A homeowner will have to be insistent on this exact process when speaking with both contractors.

As I said before, this process will only work in an already dry crawlspace which anymore seems like a rarity. If there are ANY signs of moisture at all, the insulating process will be completely different. Please refer to crawlspace insulating series for more information.

Check out this video on insulating a basement wall with spray foam.

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
Do not spray foam your crawlspace foundation walls
How to insulate crawlspaces - Encapsulate and Condition



  1. Very informative! Perfect timing for me, as I have recently realized that the reason my recently-purchased 1976 split-level home is so friggin cold all the time, hard to regulate, and EXPENSIVE to heat is that the crawl space has NEVER been insulated-!!?? NO WONDER the tile floors in the kitchen are so miserably cold-!

    I was googling to find out what grade fiberglass insulation to use, and found your articles. SO glad that I read them!
    The internet is a modern marvel of objective, up-to-date professional advice at your fingertips! The rambling thoughts of a crawlspace inspector are VALUABLE!!!

    Thanks for helping a first-time homeowner all the way down in Atlanta-! : )


  2. Can this same procedure be used in below grade crawlspaces? I have moisture coming in up through the floor not through the foundation walls. I have installed an interior perimeter drain with sump and now there are just damp spots seen on the crawlspace floor, there is no more standing water. A spray foam installer is wanting to spray foam the exterior walls from the floor to the block ends (spraying overtop the crawlspace vents) and then suggested installing a 20 mil vapour barrier on the floor and sealing it to the bottom of the spray foam applied to the walls. I asked him about encapsulating first and then applying spray foam overtop the encapsulation, however he said that spray foam won't adhere very well to the poly and if the poly is pulled from the wall the spray foam may break off. Would his recommendation be adequate?

    What would you suggest in my situation, I've had the cleanspace people come out and tell me one thing and the spray foam guys tell me another, and general contractors tell me something different.

  3. my concern with just encapsulation is that I have 10 inch block ends and over 3 feet foundation walls, The crawlspace walls are about 2/3 below grade and 1/3 above grade on the back of the house (where most of the moisture penetrated. On the front of the house the foundation walls are 1.5 feet and the block ends/bandboard are 3 feet, therefore most of the the crawlspace in the front is above grade. I need advice on the best solution is permanently dry and insulate my application.

  4. It is possible for this same procedure to be applied in a below grade crawlspace; however, you need to be confident that the foundation wall above the "liner" is never going to be saturated with moisture.

    The spray foam does not adhere very well to poly, but in this scenario the poly is only going to go up the foundation wall about 6"-12". If the poly is fastened and sealed properly to the wall prior to the spray foam, then it should never come off.

  5. Thanks for the articles - they are not too technical but aren't dumbed down either !
    I have a slightly different situation I would like to explain and hopefully get some advice for. I live in Montana. My foundation walls rise about 2'9" above ground level. If you look at the illustration at the beginning of your article, imagine that the internal crawlspace shown is partially filled with dirt - enough to prevent me from getting in and applying any insulation to the inside. I have instead been advised to spray insulation against the outside of my concrete foundation walls and go to a height just above the bandboard. I guess my main concern is that, yes I would be creating an insulating barrier around the perimeter, but not along the "4th" side - the floor. Would I therefore be wasting my money and my floors would stay cold? Do I need to remove my floor and somehow install insulation under it too or lay a barrier on the then exposed earth of the crawlspace? I hope I have set up a clear description of my situation and that y'all can make suggestions that could help ~ thanks again !


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