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ComfortTherm plastic-enclosed batting above radon retarder sheathing in crawlspace?

agurkas | Posted in GBA Pro Help on

I have encapsulated my crawlspace (5-7′ tall walls) with proper poly I got from radon mitigation specialist. Everything all the way to band joist is encapsulated. I also have good above code level ventilation going between crawlspace and basement. Basement walls I was able to cover in polyiso, but I don’t have large enough access hole to the crawlspace to get even 1/4 size sheets of polysio.

Would attaching ComfortTherm plastic-enclosed batting work in that kind of situation? I am in Zone 5.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't advise using fiberglass batts in a crawl space. In most cases, fiberglass insulation doesn't hold up well to the typical conditions (varying humidity or occasionally high humidity) in a crawl space, and it is hard to secure in place.

    The best approach would be to use closed-cell spray foam insulation or rigid foam insulation, as advised in this GBA article: Building an Unvented Crawl Space.

    Someone must have gained access to that crawl space to install the poly liner and take the photo. I'm not sure how big the access hatch is, but access hatches can be enlarged. You might want to talk to a spray foam contractor.

  2. charlie_sullivan | | #2

    You can buy EPS is 14.5" wide strips made to fit between studs. That's more work to install than wider foam boards but it might fit into your hatch.

  3. agurkas | | #3

    Another question comes to mind on this topic: how for crawlspace that is 20'X25' with average of 5' tall cement block walls, how much energy am I losing by not insulating? If I am heating rooms above with heat-pump minisplits with COP of 2.7, I wonder how much cash am I burning per year. Under$100? Above $100? I have many projects to handle on the house, so I am looking at feasibility of spending $400 to cut a larger access opening or just leaving it alone.

  4. agurkas | | #4

    I did all the work in that crawlspace. It was actually only 3' deep, so I dug out another 2' after consulting with an architect. Not fun for 6'5" guy with a bucket for many weekends on his knees.

    I really fear spray foam, since I have very sensitive lungs and even most reputable spray foam folks I know have made mistakes before that end up with lingering smell.

    Crawlspace is only under the addition to the house, so I may cut an access door in the foundation separating those two spaces.

    Question: how about Roxul ComfortIS boards? I have bunch of it left from my last project. Would that work? I would also get flammability barrier with those.

  5. GBA Editor
    Martin Holladay | | #5

    A good energy modeling program can provide the answer to your question. But it's quite possible that the payback for the investment you're talking about ($400 to enlarge the access hatch, plus the cost of the insulation) will have a very long payback period.

    Since you have many areas on your house that you want to address, it's perfectly OK to prioritize other types of retrofit work, and to leave the crawl space the way it is. Homeowners make decisions like that all the time.

  6. agurkas | | #6

    I think I will skip it then. Question: floor above the crawlspace is always cold. Would dense packing the bays between floor joists make a noticeable difference or is there a more cost effective solution short of carpeting over lovely hardwood floor?

  7. charlie_sullivan | | #7

    Yes, dense packing under the floor (or even simple fiberglass bats) would make a difference for comfort as well as energy savings. Often insulating the crawl is preferred because the area of the crawlspace walls is so much smaller than the area of the floor, but if it's easier for you to insulate the floor it's a good solution.

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