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Community and Q&A

Insulating and air sealing crawl space under existing addition that sits on piers

asahopkins | Posted in General Questions on

We are in greater Boston (zone 5A). The previous owners of our home expanded the kitchen back about 8 feet.

The main house has a basement with poured foundation, but they supported the addition on a set of sonotubes (or equivalent), with the tops of the tubes below grade level and a PT wood band connecting the joists above to the tops of the concrete piers. Reaching into the space from the basement, I can stick a trowel under the PT wood band between the piers, but this is entirely underground. (The floor is about a foot above grade, so after joists and then the wood band, the bottom of the wood/top of the piers is several inches underground.)

This shallow crawl space was originally insulated basically by stuffing it full of fiberglass, but the original state has not survived as it has become a great place for critters. I can also stand at the opening between this space and the basement in the winter and feel the cold breeze coming into the house. There is no vapor barrier, so we get moisture and smells above.

Running across the top of the crawlspace (between the joists) are a heating duct, a water pipe to an outside spigot, the refrigerant pipes to the AC, gas to the stove in the kitchen above, and a dryer vent.

The crawlspace is too shallow to get real access to, so we are planning to have it dug down to be 2 ft or so deep. This would allow access to seal/insulate. Once it is dug out, the “walls” will look like this, coming down from floor level: 1) rim joist, 2) wood band, 3) dirt. The floor will be dirt. (We can’t access from the outside even if we wanted to, at least without great expanse, because that would mean ripping out the deck that runs right up to the house.)

So, some questions:
1) Can we air seal and insulate in such a way as to enclose this crawl space? That seems preferable if possible, so we would have access to the services in the joists.
2) If so, what is the best way to seal/insulate the “walls” of this space? The floor clearly needs a vapor barrier, and presumably that should connect to something that goes up the whole wall.
3) If we can’t set the building envelope barrier to the at they ouside of the addition, and need to retreat to the floor of the addition, what do we need to know about preparing the services running through that joist space?
4) What other information/sketches/photos would help you figure out what to do in this kind of situation?

Thank you in advance for your advice.

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  1. Expert Member
  2. asahopkins | | #2

    Thanks, Malcolm. I am hoping that once we dig out the crawlspace so that we can actually get in there, that we won’t have to resort to insulating and sealing at the floor plane.

    I wonder if we can have them grade the floor of the crawlspace so that it slopes up to the wood, rather than trying to support a “wall” of dirt. Or maybe we should plan to build a wall out of some more permanent material?

  3. Andrew C | | #3

    I have had this company do some crawlspace work: ayersbasementsystems dot com.
    They have an informative website with a lot of pictures in their Crawlspace Repair section. They don't work in your area, but I think they are a franchisee of sorts for a larger company. Regardless, their encapsulation method is pretty good, IMO. Whether or not you need wall insulation, a sump pump, and/or a dehumidifier is situation dependent.

  4. GBA Editor
    Martin Holladay | | #4

    The contractor who built this addition made several mistakes. You don't have an adequate foundation.

    The best approach would be to dig down to below the frost line around the entire perimeter of this addition, pour new concrete footings, and then install frostwalls from poured concrete or CMUs (concrete masonry units, or concrete blocks). This work is best done from the exterior, but it's not impossible to do from the interior.

    My guess is that you don't want to fix this the right way. To save money, you'll probably compromise the details. If that's what you're thinking, you may want to read this article: "Crawl Spaces vs. Skirts."

  5. asahopkins | | #5


    My belief is that the foundation under this addition is pretty much a "simple pier foundation" as described by JLC here: or GBA here: The piers extend down below the frost line. Yes, it's not a complete frostwall, but the addition shouldn't move (and there's no sign of movement over 20+ years of having been this way). Note that this construction passed a building inspection in 1996. Is there a structural or safety issue associated with this type of foundation in our climate that would change that assessment in the intervening years?

    So, my inclination is to do roughly what the "crawl spaces & skirts" article suggests -- insulate and air seal the inside of the wood components with a combination of caulk, rigid foam, and/or spray foam, and cover the ground with a vapor barrier. That is, treat like a funny crawlspace that we are bringing fully inside the building envelope.

  6. GBA Editor
    Martin Holladay | | #6

    Yes, it's a pier foundation. What makes it undesirable is that the crawl space is too tight for human access, and the joists are close to the dirt.

    Excavating to lower the crawlspace floor, as you plan to do, is a good idea. Once you've done that, attempting to air seal the perimeter (using the advice I gave in the "Crawl Spaces vs. Skirts" article) is as good an approach as any (short of retrofitting a real concrete frostwall foundation, of course) -- which is why I provided the link.

  7. asahopkins | | #7

    Thanks, Martin! Glad to know that it's at least possible (and preferable, since it is possible) to think about setting the thermal barrier at the perimeter. Access is so tight that I only recently figured out that it wasn't actually a real wall, and had a crisis of confidence that we'd have to take a totally different tack.

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