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Community and Q&A

Insulating a crawl space on ledge

eddo234 | Posted in General Questions on

My 1840 house has a 400sf addition with stone foundation and a dirt crawl space on ledge. The addition was added sometime in the 1800s and is timber framed with log joists. The west side of the addition has exposed ledge and about 1 foot of space from ground to the bottom of the joists, the east side has about 2 feet of soil over ledge and 2 feet of space from ground to bottom of the joists. So, not much access to insulate the space. What to do? There’s a little moisture in the Spring, and whatever I figure out for insulation will also include a poly vapor barrier on the ground.
Here’s what I’ve considered so far:
– Try to get sheets of EPS attached to the subfloor and foam the gaps. But space is tight.
– Insulate the walls with EPS/XPS. But walls are irregular.
– Spray foam the floor or walls. Foam contractor says not enough height for access (he hasn’t seen the space, but my email description turned him away). Maybe a DIY spray foam kit.
A question about insulating the subfloor vs the walls: If I insulated the walls, what about the floor? I know it’s generally suggested to just do the walls with a vapor barrier on the floor, but the west wall literally ends at grade level because it’s sitting on ledge (the East side wall ends 2 feet below grade). It seems like the conductivity of the ledge would make the crawl space ground as cold as the outside in the winter, which wouldn’t be desirable for a conditioned unvented crawl space. Thoughts, suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    It's an old house, and it hasn't fallen down yet. There may be no need to change anything. But if you are willing to spend money, you can address any issue that your budget is fat enough to tackle.

    Are there any signs of joist rot or sill rot? If not, you could just leave well enough alone.

    If crawl space access is a problem, and there is no way to lower the dirt floor, there are two choices:

    1. You can jack up the entire building and raise the foundation. It sounds daunting, but people do it all the time.

    2. You can remove the flooring and subflooring in the rooms above the crawl space, and work from above.

    Or, as I said, you can leave everything exactly the way it is.

    1. eddo234 | | #3

      1. "Lift the whole house" C'mon, really, lift the house? Geez. The cost would far exceed any lost heat savings in my lifetime.
      2. Yikes! But not as bad a suggestion as raising the house. Just hoping for a way to not have to rip up finished floor without damaging too much of it.
      3. Leave it the way it is, occasionally damp, no insulation, wasted heat out the crawl space. The fact that the house hasn't fallen down yet isn't much help. Maybe more accurate to say, "Even though heat is leaking out your floor & being wasted, and your feet are cold, you can leave everything exactly the way it is." How "green" is that?
      It might have helped if there was an answer to the last part of my post. If it's feasible to spray foam the walls, which I can probably reach, even though it wouldn't be as good as creating a fully insulated conditioned crawl space, I would probably do that. And if not insulating the exposed ledge on the ground will seriously negate what I sprayed on the walls. But what I got was "you could lift the house"

      1. GBA Editor
        Martin Holladay | | #7

        It's certainly possible to install closed-cell spray foam directly on a crawl space floor. I've heard of many spray foam installers who do that.

        It sounds like you don't like the suggestion that you might simply do nothing. I mentioned two ways to improve access -- and it sounds like you prefer to open up the floor (compared to the more expensive option of jacking up the entire building). So, if you're willing to open up the floor, you may be able to gain access that allows you to install closed-cell spray foam on your crawl space walls.

        But it's your house. If our suggestions sound disruptive and expensive, that's because they are. Only you can decide what you want to do.

        1. eddo234 | | #8

          To be fair to the other answers, it was only your 1st suggestion that was expensive, even far-fetched. :-) I also wouldn't waste money on spray foaming the crawl space floor, I would use rigid foam if I had the confidence that it would be an effective solution to keep out the cold.

  2. Expert Member
    Peter Yost | | #2


    When you insulate the crawlspace in any way, you will almost certainly make some parts of the existing assembly colder and therefore have less drying potential. So even if existing wood framing members in direct contact with ledge is currently dry/not damaged, you can shift that balance with insulation.

    How do you determine the condition of the wood framing in direct contact with ledge? You really need to measure % moisture content by weight with a pin-type moisture meter, measuring in multiple locations and at different times of year. I sometimes take measurements in the spring during a building assessment and tell the building owner that I will need to assess again in the early fall. If that is not possible--they can't wait--then I do the best I can to present them with how not knowing what happens over time with their building can make suggested solutions more uncertain.

    If the ledge is non-porous rock--igneous or metamorphic--and it does not support wicking, you may be able to keep the framing resting directly on it safe with really good site and surface water management on the exterior of your building. With an addition, I would seriously consider jacking up the whole thing at least enough to get a cap break between the ledge and your floor assembly.


  3. eddo234 | | #4

    Thanks Peter,
    I didn't say any wood framing was in direct contact with ledge. I said, " The west side of the addition has exposed ledge and about 1 foot of space from ground to the bottom of the joists..." That 1 foot of space is a stone foundation wall. The white oak beam sits on that, not ledge. So, knowing that, do you think there's a good cost/benefit ratio to spray foaming the walls, but not doing anything about the exposed ledge on the floor? I know insulating the walls will help, but I'm thinking that exposed ledge is such a huge heat sink that it may still keep it very cold in the crawl space, so, a waste of time and money spent on the foundation walls. Another possible option is a DIY foam kit to spray the underside of the floor and leave the walls alone (I'd have to look into whether I'd need to cover that foam for fire rating though, I haven't looked in that) Any thoughts?

  4. Expert Member
    Dana Dorsett | | #5

    Insulating the subfloor means the average temperature of the joists and sill beam will go down, which means their average moisture content would rise. If you're going to insulate the floor it's better to use rigid foam on the under side of the joists to keep all of the wood inside the thermal & pressure boundary of the house. You'd still need to air seal the band joist/sill beam, and the sill beam would still be exposed to the now lower temperatures in the crawlspace.

    Insulating the walls (even though imperfect, due to the thermal bridging of ledge) would have the opposite effect- the temperature in the crawlspace would rise, lowering the moisture content of the exposed wood.

    It may be possible to insulate the most egregious strip of ledge with sheets of EPS, perhaps with a bit of sand for smoothing/leveling.

    Jacking up the sill beam by 1/4 to slip in some EPDM as a capillary break sounds more major than it usually is. Try measuring the moisture content of that white oak sill beam to see if that's likely to be necessary.

  5. eddo234 | | #6

    Thanks Dana,
    Until I crawled into the space, I planned to insulate under the joists. Then I saw that they're log joists, every one a different diameter. I considered shimming the undersides for a flat attachment area for the insulation, but it's pretty tight on the west side (about 1 foot from bottom of joist to the ledge). The sill beam doesn't really extend below the joists, the joists are notched into it (1840s timber frame), so if I could fit under it all to do the work that part would be covered. Do you think that if I laid down some EPS insulation on the floor, perhaps just a few feet in from the walls, that it would help enough with the cold sink affect? I've rebuilt the east wall of the addition, and although part of the sill was rotting, it was entirely due to bad flashing at the juncture of addition to main house and an old entry door, the rest of the 30' of white oak sill resting on the stone foundation is solid, so I'm not concerned enough about it to jack it up for a capillary break. Thanks again!

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