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Earthen crawl space

MarcoD | Posted in General Questions on

My condo has an earthen crawl space.  I am planning a renovation and need to address the uninsulated first floor deck.
I am thinking a polyethylene vapor barrier over earthen floor of crawl space cleated to the inside face of crawl space wall and sealed.  I would apply an impermeable R10 continuous sheathing to underside of floor joists, taped and sealed.  I do not want to remove entire subfloor. Any recommendations are welcome, thank you in advance.

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  1. Expert Member


    There are two types of crawlspaces: vented and un-vented. People usually insulate the floor of vented crawlspaces, and the concrete walls of un-vented ones. If the crawlspace is un-vented it needs to be conditioned - that is kept close to the same temperature and humidity as the space above. You can get more advice here:

  2. Expert Member
    BILL WICHERS | | #2

    I would seal the floor as you indicate (most codes require at least 6 mil for this, but 10 mil or more is much better). You want a "crawlspace liner" for this, which is heavy polyethylene made for the purpose and available in large sizes so you can do most of the work with a minimum of taped seams. I used a 20 mil reinforced liner on the floor of my own crawl space.

    It's often easiest to run the liner up the walls too, or to use thinner material (I used 6 mil here) on the walls, rather than trying to detail the insulation as part of the crawlspace liner. I ran the liner up the walls, then installed rigid foam (polyiso) over the liner.

    You don't want to put anything impermeable under the floor joists after you seal up the crawlspace. Insulate the walls of the crawlspace, sometimes with a "skirt" on the floor around the perimeter if your crawlspace is shallow, then leave the underside of the floor open. Your crawlspace is now conditioned space inside the building envelope (and needs some ventilation to connect it with the interior of the home), so there is no need to try to insulate or seal the floor which is now an "interior" partition of sorts, with conditioned air on both sides.

    This is what is known as a "crawlspace encapsulation" project and there is a lot of info about doing those here on GBA.


  3. walta100 | | #3

    First read your condo documents. You may well not own the crawlspace or the dirt floor.

    The next choice you need to make is will the crawlspace be vented or conditioned.

    Vented has a long history and works well if you can pay the heating bills and live with the cool floors. Your plan the insulate the floors risks cooling the crawlspace to the point the pipes may freeze if it remains vented.

    If you try closing the vents without conditioning the air in the crawlspace mold and rot are very likely.

    If you chouse to condition the crawlspace you want the insulation on the exterior wall and not the floor.


  4. Jud_Aley | | #4

    We've done a lot of crawl spaces like this.
    If I were you I would block up all the vents put down a double layer of 6 mil poly vapor barrier & two inches of rigid foam insulation and pour a concrete slab over it, you can hire a pump to get the concrete in, you could run the hose in through an open crawl space vent before you finally close them up, if there are no open vents you could cut a hole through the rim joist or knock out a block to get the pump hose in.
    I have been in many crawl spaces with poly only over the dirt floor and after time the poly gets brittle and cracks and stops doing its job.
    If you’re not going to pour the slab at least get the string re-enforced poly, it will hold up much longer than the regular poly or vinyl pool liner.
    We caulk or foam the plate or sill to the foundation and then only insulate the rim joist, we usually do a 2 inch layer of foam board and then a layer of Rockwool insulation over it but we've also done a double layer of R23 Rockwool, you can figure that out what will work best for you. We have never insulated the walls of the crawl space but we do introduce conditioned air into them, if you think about it, heat rises and so if you put heat in there in the winter it's going to rise up to the underside of the wood floor and the insulated rim joist is going to hold it in where it is most needed, then the floor above will be warmer. The biggest reason I see to insulated the walls is if the duct work is uninsulated, but the best solution to that is to insulated the duct work and of course there's no problem with insulating the walls but it's another step you might not need to take.

    If you stay with a dirt floor and insulate the walls with rigid foam be careful you don’t create a path for termites to travel from the dirt floor, up behind the rigid foam and into the wood framing above. Keep the foam a few inches above the floor and a few inches below the sill plate.
    If there is No duct work in the crawl and you do want some air movement in there you can set up a bath fan in an adjacent basement or a closet above and use it to blow air into the crawl using 3 or 4” bath fan duct work.
    You can set up a thermostat in the crawl so the fan comes on when things get down to a certain temperature, you can also set it up with a humidistat if high humidity is the problem.
    Whatever you do don’t insulate the underside of the floor with fiberglass or Rockwool, it will become a huge mouse hotel and toilet.
    I would also not insulate the underside with rigid foam board as mentioned above that seems like a high-risk scenario for trapping moisture between the rigid foam and the subfloor above and as the other person said likely rot.

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