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Community and Q&A

Crawlspace sealing and insulation

rf_engineer_5 | Posted in GBA Pro Help on

I have read through several of the threads here and while bits and pieces do apply to what I want to know, there were some things unaddressed that I wanted to cover here.

I have a crawlspace that needs to have a vapor barrier installed on the floor and possibly the walls insulated. In the spring/summer times, I notice that mildewy, moldy smell from the basement. While there may be a need for a dehumidifier at some point, until I get the vapor barrier down, the remaining humidity issues will be dealt with later on. I need to do the vapor barrier fix first then measure humidity after this to see what else is needed. Babysteps here folks.

Location is Western NY area. Crawlspace is 270sq ft, appx 4ft high block walls, is already sealed and the rim joist already has spray foam applied (I did a tiger foam kit last year). I have a handle on doing the vapor barrier. I am planning on getting a product called DrySpace. Seems ok. There are several methods to vapor barriers that I have researched including: Tu-Tuf, Tenoarm, VaporBlock (from, DrySpace and Stego. I did not want this to devolve into a barrier product competition discussion. My crawlspace has what is called a mudslab floor. It is not dirt and not concrete but a mixture. The floor is hard but not like concrete. The floor is also not level by any means with a grade drop of roughly 6-8″ over the floor length.

What I do want to ask if as follows: Most of the crawlspace is below grade. There may be maybe 1-2 coarses of brick above grade.

1) Would I benefit buy insulating the walls with XPS? Given that it is mainly below grade, and the temp remains somewhat constant (don’t have hard numbers to back up that claim), does insulating make sense? I figured that I would have to spend about $175 to put in 2″ XPS. While not a lot of money, I would rather spend money where it is better used.

2) If insulating does make sense, and using XPS requires a ignition barrier (drywall), what should I do about that? I had no intentions of putting drywall up because putting in a normal wall would require a real concrete floor so that the bottom plate would have something to attach to. It would also require putting in a concrete floor. No intention of doing that. Or I could put in studs that fasten to the walls (2″ thick), glue in the XPS, then install drywall. This all seems a lot of work. The important question to answer first is if I truly need to insulate?

3) Is there any instance where it is allowed to leave XPS uncovered inside of a residential home? Is this one of them and are there any caveats?

I do not plan on making the crawlspace into a true conditioned space with heat/AC ductwork branch. Perhaps I will have to reconsider this idea but I thought I would ask and see what others suggest here or have done in the past.

For illustrative reference, check out this link for the insulation scheme:



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  1. GBA Editor
    Martin Holladay | | #1

    1. Yes, you should insulate your crawl space walls.

    2. Concerning the need for a thermal barrier (drywall) on top of the rigid foam: Talk to your local building inspector, because that's the person you have to satisfy.

    3. Most building inspectors accept the fact that Thermax (a brand of polyisocyanurate), unlike XPS, does not require a thermal barrier for protection.

  2. rf_engineer_5 | | #2

    Hi Martin

    Agree on the item of Thermax but I have looked into its availability. The best I could do is get a shipment with a minimum order of 22 panels (something like that). I could not just buy 4-5 4x8 sheets.

  3. GBA Editor
    Martin Holladay | | #3

    I'm really surprised. I think you should ask around at a few more lumber yards.

    Thermax is fairly common in most areas of the country.

  4. rf_engineer_5 | | #4


    It is not so much that I cannot get it, it is that I have to buy a minimum. Nobody (and I have checked) has Thermax just sitting around. A lot of the places I called either never heard of it or what a MOQ of 22 pcs. It was going to be pricey (about $1k). That makes it a hard justification. If anybody out there that lives in or near the Rochester, NY area know of a source for Thermax, let me know. Otherwise, it has to be something else.


  5. Riversong | | #5


    The soil temps at above frost depth will vary considerably with season.

    If the floor above the crawl space is well insulated and there's no plumbing in the crawl, then you don't have to insulate it unless the ground vapor barrier doesn't take care of the moisture problem.

    Are the block walls waterproofed on the outside? Is any moisture coming through the walls? If not, the VB might be sufficient. But, if you do decide to insulate, XPS should not require an ignition barrier if there are no combustion appliances or other ignition sources. But do not use the picture in the link you provided as a guide, since the most important part of the wall to insulate is the above-grade portion. And the VB only needs to rise a few inches up the walls for secure attachment behind the insulation board.

  6. Chris | | #6

    You might try Henrietta Building Supplies in West Henrietta...worth a shot anyway and may not have been on your initial radar screen. Also Energy Control Insulation, in same area.

  7. rf_engineer_5 | | #7


    The floor above the crawlspace is not insulated. Everything that I have read says that this approach is not necessary for a closed crawlspace.

    I have no water leaks that I have ever seen. Everything looks dry and I have even had test sheets of poly on the walls to see if moisture accumulated underneath or on top of the poly. Nothing. Did it both in winter and summer cases.

    Not sure about "wateproofing" in the strictest sense. House was built in '64. I bought it a couple years ago. I do not have any water coming or seeping in. Bone dry. There is a sump pit but there is not much water in there at all. That is in the basement though, outside of the crawlspace. I guess by waterproofing I was thinking of a membrane on the outer walls. That probably does not exist. I don't really know what tehy did back in '64 for exterior basement walls.

    The crawlspace is next to the basement of the home. There is a block wall between the house and crawlspace area. Somebody put a hole in that wall to run the exhaust from the furnace and water heater. The hole is big enough to fit a typical person through but I don't know if I could get a 4x8 sheet of anything through it. Right outside of this hole is the furnace. My thoughts after I finished the vapor barrier project was to put in a custom door to make the crawlspace "separate" again. So in a sense, there will not be any direct combustion within the crawlspace. The only possible hit here would be the exhaust flue. To me, as long as that is sealed up with high temp mastic, I don't really see a problem.

    The items in the crawlspace are ductwork for parts of the house and plumbing for a powder room. I suppose for the plumbing the walls should be insulated.

    My understanding of the ignition barrier is that it did not matter if the foam was in an area with a combustion source or not. It was a matter that if a fire happened and the foam was there and exposed, it would ignite.

    Your last comment completely contradicts the guys at that crawlspaceinfo website. I asked them about running the vapor only 6" up the wall and they said no. Go up the full wall to the sill plate.

    I have the insulation book by Bruce Harley go here:

    and in that book, for the crawlspace section (pgs 46-48 something like that), he only has the poly going up 6" up the wall. But he also has the walls done with XPS. In that case he insulated the walls first then did the poly where he taped the poly to the XPS. That is pretty easy to do. I would prefer to do this.

    I guess the best answer I can get is to ask the building code guys here and see what they have to say. I swear, why does this home owning stuff has to be complicated? So many gray areas of possible gotchas.

    Back to you Martin,

    It is possible that Thermax is aka Celotex. But from what I can tell, Celotex is really Tuff R from Dow. Tuff R is a polyiso material but does require a fire barrier. Thermax is the only material with an exposed fire rating.

    I went back and called every Bldg matl distributor in my area today. Nothing. Nada. Zip. Very few knew what Thermax was, even if they carried Dow products. It just comes down to demand. Nobody here wants it therefore nobody carries it. I did talk to one of the dealers here a bit more familiar with the material and he told that Dow has a patent on Thermax, which is why nobody else makes anything similar. The only way to get it here would be to order the minimum amount (a couple pallets or twenty some sheets) material and get more than I need. It is not like I have a bunch of crawlspace buddies needing the material right now. If that were the case then I would do it.

  8. rf_engineer_5 | | #8


    I tried Henrietta Bldg supplies. They were one of those who req'd a min order qty. Between them, Hep Sales, Matthews and Fields, Conking and Calabrese, they would only order for a big batch. Norandex was a no as well. Outside of trying Buffalo and Syracuse, the game is over. And I am not driving 1.5 hrs for either of those. I would have to rent a truck to carry sheets of foam for 3hrs and mileage charges. Yep, the project is getting expensive.

  9. Riversong | | #9


    Either the floor is insulated, leaving the crawlspace unconditioned (whether open or closed) or the crawlspace walls are insulated making the crawlspace at least semi-conditioned. While the latter option increases the conditioned volume, it almost always results in a more energy efficient and dryer house.

    Typically, block and concrete foundations were damp-proofed with brush-on asphaltic emulsions, but a crawl space may have nothing to seal it. If you don't see some black tar peaking above the ground, then it's almost certainly unsealed. But as long as it's dry, that's fine.

    You'll have to check with your building inspector about local ignition barrier requirements. If you seal the passage between basement and crawl, he may not require anything.

    Since the purpose of the ground vapor barrier is to stop soil moisture (and radon gas), it's best to tape or caulk all seams and penetrations and caulk the VB to the bottoms of the crawl space walls (Tremco works well). The XPS on the walls can help hold the VB tight. The insulation must go all the way to the sills (unless you're in termite country) and be continuous with the band joist insulation.

  10. rf_engineer_5 | | #10


    Getting into more details. Do you find that acoustical sealants (I think Termco makes this) have any negative effects on any poly type products? Meaning that, the materials don't play well together and eventually the bond falls apart.

    Funny thing here: Again, very few places around here know about acoustical sealant. It is just not common. I didn't hear about it till seeing the show Holmes on Homes. You definitely cannot get it at HD or Lowes. I have to order it online.

    BTW, I did one of those home radon test kits and it came back a 1.8piCu/L, which is half the value before you should call in a radon specialist. I am good to go for radon and my job can proceed ahead.

    I did not plan on insulating the floor.

  11. rf_engineer_5 | | #11


    Last question: Assuming I am able to use XPS on the walls, do you think it is sufficient to attach the XPS to walls using just an adhesive made for foam OR do I need to use a mechanical means? Or both?

  12. Riversong | | #12

    Adhesive is fine - actually preferable since it avoids thermal bridging. You need some kind of high-tack adhesive like PL Premium to hold the foam in place. You can also use a foam gun with an adhesive foam such as PUR-Stick (and you can use the same foam to seal corners and edges).

  13. lucasdupuis | | #13

    Try Rmax tsx8500. It's meets the same specs as Thermax. I just went through this when the local insulation supplier found out Dow isn't shipping Thermax to my part of the country (Montana). TSX8500 meets all the ASTM standards that Thermax does, so your local building officials should accept it. You might try calling around to commercial building insulation companies. They tend to stock this kind of product because its rated for exposure inside prefab metal buildings.

  14. rf_engineer_5 | | #14

    Hi Lucas,

    Thanks for the heads up. Never heard of this product before. For those others interested in looking at this product, check it out here:

    Pg3 has a warning on it about polyiso being flammable, yadda yadda yadda. I looked through all of their other datasheets for polyiso and all of them state that you need an ignition barrier of 1/2 drywall over the product. This one didn't. Maybe it will work. I will look into it and see if I can get the material here. Somehow I already know the answer...sigh. Things never seem to break my way sometimes.

  15. Riversong | | #15


    You might check with a commercial roofing outfit to see if they would part with a few sheets of something similar.

  16. rf_engineer_5 | | #16

    Robert and otherrs,

    Instead of going the rigid foam board route against the crawlspace walls, what are your thoughts about using something called an insulation blanket? This product is from Cetainteed:

    It is a material that will attach at the sill plate with a furring strip and nails, It hangs like a curtain and can be left exposed. At the bottom of the wall, you leave it long by a foot or two and place lumber/bricks/whatever you have to make the bottom stay tight to the wall. Seems pretty simple but not sure how well this really works. R value is a bit better than XPS.

  17. GBA Editor
    Martin Holladay | | #17

    Although the product can perform well in a very dry crawl space, most crawl spaces are too damp for fiberglass products. After a few years, the fiberglass can be damp and moldy, affecting its performance.

    I recommend sticking with rigid foam board or closed-cell spray polyurethane foam.

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