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Crawlspace Insulation and Encapsulation Questions

jdchess | Posted in General Questions on

2000sqft brick ranch on a crawl space in CZ3 (southeastern NC, warm humid). Built in 1989. The stem walls are brick. The HVAC ducts are located in the crawlspace. I have removed all of the fiberglass insulation from the floor joists. There’s some mold in several places. It doesn’t appear to be very heavy but it’s obvious that there has been significant moisture issues in the past.

I want to encapsulate the crawl space and insulate the brick stemwalls. I’ve heard varying opinions regarding rigid foam vs. spray foam for insulating the crawl space walls and rim joists. Which would be the best option for my location? Major disadvantages of using spray foam? Are there any major risks in using spray foam for this application?

Also, once the crawl is sealed and insulated, which is the best option…a dehumidifier or a supply duct with conditioned air to control humidity?

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  1. jdchess | | #1


  2. walta100 | | #2

    The way I see it there are three types of crawlspaces.

    1 Vented works fine in most climates but is an energy pig.
    2 Conditioned the air in the crawl space is more or less the same temp and humidity as the rest of the house.
    3 Something in between often called encapsulated a code violation that sometime becomes a moldy mess.

    Most any thickness of poly will stop the moisture thickness is about durability if you need to get in there and work you need thickness survive.

    I maybe the most anti spray foam person but the rim joist of a crawlspace is one of the few places I recommend it.


  3. Expert Member
    BILL WICHERS | | #3

    Code requires a 6 mil minimum thickness for a crawlspace vapor barrier. I would recommend at least 10 mil on the floor, but you can go thinner on the walls to save money. Be careful though -- sometimes, due to the minimum order quantities, it's cheaper overall to use the floor material for the walls too instead of buying two different thicknesses and having a lot left over.

    I like polyiso here for more R per inch, ideally reclaimed polyiso if you can find it in your area. After that would be EPS. You can use XPS, but it is usually more expensive and also less green, so I'd put it down as a last choice here. I like to use the "tap in" plastic insulation fasteners to attach the rigid foam to the walls, similar to these:
    You just drill a hole through the foam and into the wall, insert one of these anchors, give it a few whacks with a mallet and you're done.

    I prefer to run the poly all the way up the walls and tie it into whatever is used to seal the rim joist, but you can use the rigid insulation on the walls in place of the poly too. I just find it easier to put poly up completely, then insulate over the poly.

    If you only have a little mold, it's probably not worth worrying about. If you have a lot, I'd remove the worst of it with mold killing cleaner, then possibly run some mold killing primer over the moldy spots to make sure it doesn't come back. Once you've sealed up the crawl space from the moisture, the mold shouldn't be a problem anymore.


  4. jdchess | | #4

    Thank you guys for the replies and info. Unfortunately, termites are an issue where I'm located, so I have to leave a 3" gap between the top of the vapor barrier and the sill plate. I guess either separate pieces of rigid foam in each bay or spray foam like Walta mentioned. Any issues with rigid cut to fit tight in each spot?

  5. jdchess | | #5

    Anyone have other thoughts or advice regarding rigid foam vs spray foam? I understand Walta's point about spray foam being an easier option for the rim joists. Are there any risks or drawbacks to using spray foam for the stem walls and rim joists vs rigid?

  6. Expert Member
    BILL WICHERS | | #6

    Spray foam is generally more costly. Spray foam is a good fit for rim joists because of all the connections in that area, so using spray foam greatly reduces the labor that would otherwise be needed to make lots of cuts if using rigid foam board. For the flat walls, rigid foam panels go up reasonably quickly. You can easily to do a hybrid insulating job, by putting up the rigid foam panels first, then applying spray foam to the rimjoist area and tying in the top of the rigid foam panels by getting them in the spray foam at the top too.


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