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Community and Q&A

Making a conditioned crawlspace

mcStHUSXoK | Posted in General Questions on

I have been reading about conditioned crawlspaces. Would this be a project that I should even think about taking on myself? Our house has a layer a plastic on the ground and fiberglass insulation between the joist. The reason I am considering this is the florida room tends to stay cooler and warmer than the rest of the house. I have some of the 1 1/2″ blue foam insulation that I was thinking of covering the foundation with. Should I leave the fiberglass insulation in place or remove it? Also, what about covering the floor joist with a layer of insulation board that has the silver on one side? The crawlspace is dry and we don’t have any problems with drainage.

Thanks, Randy

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  1. GBA Editor
    Martin Holladay | | #1

    1. What's a "Florida room"?

    2. Whether or not this is a project you should think about taking on yourself depends on your skill level and self-confidence.

    3. If your crawlspace is dry and problem-free, you might be fine just installing foil-faced polyisocyanurate foam under the floor joists. In some crawlspaces, however, this work is a nightmare, because of all the pipes and ducts that are in the way.

  2. mcStHUSXoK | | #2

    Martin, A florida room is just a finished porch room off the back of the house. Some people choose to make them a screened porch, ours is finished with hardwood floors and lots of windows.

    If I install the foil-faced foam, would I need to remove the insulation that is in place.

  3. Riversong | | #3


    Conditioning crawlspaces is a way to prevent the moisture problems that are so common in those kinds of foundations and, by supplying some conditioned positive pressure air down below it's a way to keep musty air from being drawn up into the living space above.

    If you have no moisture problem in the crawlspace and floor assembly and you just want to reduce heat loss through the floor, then you're better off keeping the thermal boundary where it is and adding an air/thermal barrier below the joists such as foil-faced polyiso board.

    The foam board should be well-sealed at joints and edges and serve as a radiant barrier to decouple the floor from the cooler ground below it. That is why foil-faced board is preferable, and it's also easier to seal with foil tape. The edges will have to be sealed to the foundation or sills with canned foam.

    You can leave the fiberglass in place, unless it's become a mouse nest as is commonly the case. But the additional foam board will not only increase the thermal resistance of the floor but also create a true air barrier (if completely sealed) which will dramatically limit stack effect air movement from the crawl space into the house.

  4. mcStHUSXoK | | #4

    Thanks Robert! Should I try and put some blue foam around the sill, between each joist? Then try to seal it with the foam. What about blocking the foundation vents? Will this be needed or leave them as they are?

  5. Riversong | | #5


    Insulating and air sealing at the rim joists is important, and rigid foam blocks sealed with canned foam at the edges is often the easiest way to do this. The blue foam (XPS) is fine for that purpose.

    You don't indicate where you are (what climate zone) so it's hard to answer the foundation vent question. But if your crawlspace has been dry with the vents open, I would leave them open. In northern climes, the warm, humid summer air can condense on the cooler dirt in a crawlspace and cause moisture problems. But with a well-defined thermal barrier in the floor above, it would probably be better to keep the crawl area vented to the outside.

  6. mcStHUSXoK | | #6

    I'm in VA. I'll get under the house this weekend to see how much trouble putting up the foil foam will be. Is there are special tape to use on the foil foam? Also my opening to the crawlspace is not very big, so could I score the foil on the backside and fold it in half? As long as I don't break the foil, should this work? Thanks for all the help.

  7. Riversong | | #7

    Sure you can fold it or cut it into thinner strips (though that would require more taping. A building supply house should have aluminum foil tape for this purpose. It's incredibly sticky and creates an air-tight seal.

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