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Insulating an Off-Grid Crawlspace

mountaincabin | Posted in GBA Pro Help on

How do I finish my crawl space after exterior mineral wool energy retrofit?

First the important details…

-We’re completely off-grid
-In zone 5c in the Cascades of E. Washington
-My wall assembly photo is attached (with proposed basement finishing)

-I would like help determining how to complete the interior of my crawl space.  Ideally, I would like to add insulation to the crawl space floor and maybe to the rim joist too??  The insulation in the crawl space floor is to help the performance of our masonry stove (we’re installing one this summer).  It seems that insulation against the soil isn’t ever recommended for crawl spaces but since off-grid, if installing floor insulation means we can eliminate the use of propane heat, then I’m all in.
-My attached assembly drawing shows the details of our exterior and the interior crawl space details are proposed but not executed.
-I would like to use mineral wool but understand many here propose foam and most likely will here too.  Is there anyway I can get around this?
-I have added the dimple mat detail to create an air gap to equalize the water pressure should there be any.

Thanks for the help!

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  1. mountaincabin | | #1

    I'm reviewing my drawing details, I'm guessing that if I leave in the dimple mat, it will need to go up foundation wall so that any moisture that comes in can exit down to soil if needed.

  2. mountaincabin | | #2

    Anyone have thoughts?

  3. mountaincabin | | #3

    Asking one last time...any advice would be appreciated.

  4. Expert Member
    BILL WICHERS | | #4

    You appear to have put most of your efforts here into the floor, but not the wall. That's opposite of how it's usually done. Are you thinking that the exterior mineral wool is all you are going to use on the wall, so you don't need to do anything on the interior? That could make sense, but you might still want to run the dimple mat up the interior of the wall to catch any potential water that makes its way through the wall.

    You want your poly to tie into your rim joist air seal, you don't want any gaps there. If you want to leave some wood exposed on the sill plate to help with drying (hopefully you have a capillary break there too), then I'd seal the poly to the edge of the sill plate with urethane caulk to ensure a good air seal.

    Your footing is a pretty big thermal bridge. You will want to at least wrap the footing itself with your mineral wool panels. Ideall you'd go up the wall too, but you could get away with less there since you already have a lot on the exterior. I'd run polyiso on the walls, since this should be a dry area (polyiso will wick mositure), and polyiso is easy to work with and also the greenest of the commonly available rigid foam materials.

    For your floor, is there any particular reason you want to use rigid mineral wool panels? Those are probably the most expensive option for this. EPS would be a lot cheaper, even if you put a subfloor over it for protection. The one potential downside with EPS relative to mineral wool is that you may be required to provide some fire protection -- that's something to check with your local building department (if you have one -- sounds like you're in a pretty remote area).

    If you're in an area where you don't have code compliance issues, you could run the EPS alone, or run some plywood or OSB over the top as a basic ignition barrier for some extra fire protection. If your crawlspace isn't used for storage and isn't really accessed for anything besides infrequent maintenance work, you have pretty minimal risk of a fire starting in there.


  5. Expert Member
    PETER G ENGLE PE | | #5

    I'd skip the dimple membrane on the inside, and Stego wrap is probably overkill. Reinforced poly would work fine to keep vapor at bay, and if you've got bulk water welling up through the floor you've got bigger problems to deal with. Wrap the poly right up to the bottom of the sill plate. I agree with adding a couple of inches of polyiso foam to the foundation walls and over the footing. Those are still thermal shorts in your plan. You could use rockwool if you just don't want foam. If it's environmental concerns, look for recycled EPS or polyiso foam - the environmental cost has already been paid on recycled content.

  6. mountaincabin | | #6

    Bill + Peter thanks for your suggestions and for responding to my cry for help.

    Peter - I like the idea of the membrane because it allows any moisture or condensation that gets on the concrete walls back to the soil. It also allows any moisture on the soil to equalize and dissipate. To your note about water, the soil is soft from water during this time of year due to the massive amount of snow melt (I believe we had ~130"+ of snow this winter). It's normal for us during winter to have a continual 3-5' standing snow on the ground. When we did the exterior mineral wool last year down to the footer, we replaced the footer drains also to help with drainage. We probably didn't do it perfect since we hand dug the trench but I believe we took advantage of the insulation project. Most of the crawlspace is water free except this one side during this time of year and there is no standing water only soft soil. I wonder sometimes if I should add an interior drain pipe for this reason but also think this may not be needed since it's seasonal. My thinking is the dimple mat will allow the drying (or rather elimination of standing snow) that occurs to continue to happen naturally. Thoughts? I like your idea about recycled foam. I added your suggestion to my updated drawing (see attached) that incorporates insulation around the footer. Did I get this correct? Also, see my second picture, is this where I should attach the mat, bottom edge of sill plate into the wood?

    Bill - My biggest concern with using no foam is partly because we are in a wildfire area and when foam burns it is toxic. It also eliminates me adding other materials to ensure code compliance for fire blocking locations. If I use the dimple mat + Stego Wrap, will this create condensation between these layers where the water won't have the ability to exit? This was my only concern with doing both.

  7. mountaincabin | | #7

    Snow last month for visual :)

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