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Will rigid insulation above fiberglass prevent condensation in a vented crawl space?

nicolasknapp | Posted in Energy Efficiency and Durability on

Hello Fellow Building Aficionados!
The building I am working on has a vented crawl space with a dirt floor (same grade as exterior) and lives in the hot humid land of Oklahoma City. (Climate Zone 3)

My goal is to meet the code required floor insulation value of R19 with the least amount of skill and cost possible while still getting the job done right. Here are my two questions:
 – In a humid climate like OKC, will fiberglass insulation work to insulate a floor system or is the risk of mold and condensation too high?
 – Would it be possible to use a layer of rigid foam to keep the crawl space warm enough during the summer to prevent condensation/mold in a layer of fiberglass? The system from top to bottom would be subfloor, R5 rigid insulation sealed with foam, R13 unfaced fiberglass batts & insulation supports, and a 10mil poly vapor barrier on the floor of the crawlspace.

My thinking is that this would work the same way that walls with exterior foam work in northern climates by preventing the wall sheathing from getting cold enough to have condensation problems?

I work with a non-profit construction company that uses volunteer labor and donations, so I often need to get creative to get my projects done well with the resources I have.

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  1. Expert Member
    BILL WICHERS | | #1

    If you encapsulate your crawlspace with a poly liner, you can insulate the walls only and eliminate the fiberglass in the floor. This will improve everything, and might actually be easier than trying to make the vented crawlspace work properly. You only need R5 rigid foam on your crawlspace walls to meet code in your climate zone, although I'd use more than that. 1" of polyiso would be R6, or 2" of EPS would be a bit over R8. Either would be better than only R5. I would probably go with 1.5" of polyiso here myself, which could be reclaimed polyiso to save some money, and would get you around R8-R9 or so.

    If you want to leave your crawlspace vented, which I don't recommend, putting rigid foam (polyiso again) under the joists and air sealing the rigid foam layer is probably your best option, but that's probably going to be more work than a proper crawlspace encapsulation project. The reason it would likely be more work is that you'd probably have to seal around a lot of penetrations, where if you insulated the walls of the crawlspace, you'd mostly be dealing with an unbroken flat surface which is much easier to insulate and seal.


  2. Expert Member
    PETER Engle | | #2

    Among the benefits of a sealed and conditioned crawl space is the fact that mechanicals are brought into the building envelope. Even in OK, it gets cold in the winter. Improperly insulated pipes will freeze in a vented crawlspace, but not in a conditioned one. In some cases, a thorough job of insulating the floors actually increases the risk of freezing pipes hanging below the floors because of the lower energy flow from the house. Also, with insulation between the joists, there is nothing protecting the bottom of the joists and any exposed beams from condensation. i have seen entire floor systems rot out from this effect. Sealed and conditioned crawl spaces are just a better solution for all climates and locations, except for flood-prone areas. If you are in a flood zone, then you have to work harder to really protect any exposed systems.

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