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Community and Q&A

Converting a Crawlspace into a Slab on Grade

marleyandbowie | Posted in General Questions on

I just bought and moved in to a single level 1900 sq ft mid century house in southwestern NH with air quality and rodent problems, and would be so grateful to get input from this community. The house was originally built in 1955 as a 3-season cottage and then expanded and converted into a year round structure in 1980. We are situated about 150 yards uphill from a large pond, so there are elevated levels of ground and air moisture here. The house is built on a combination of foundation types, floor plan and some photos can be seen here: The basement part of the foundation is actually pretty clean; it is the dirt floor crawlspaces that seem to be the main source of our issues. They are full of debris, mold, rodent carcasses, potentially asbestos fibers from old ducts, and emitting foul odors that we can smell in our living areas.  After reading Martin Holladay’s article on crawlspaces ( and doing a lot of my own research, I am thinking of eliminating the crawl spaces rather than improving and trying to preserve them. I am proposing to do this by first having the basement and crawl spaces professionally remediated for asbestos wrapped ducts and flooring, then tearing out our first level floors and joists above the crawl spaces (some of the floors need to be replaced anyways), filling the crawl spaces within the existing foundation walls (sand/gravel/vapor barrier/insulation/concrete), and topping it off with a polished slab embedded with pex for hydronic radiant heat. 

Why would I do this? Currently the crawlspaces contain old ductwork and a few electrical runs, but seeing as how we are removing the ductwork anyways and moving to a combination of radiant and wall mounted mini splits, the crawlspaces will offer no functional purpose moving forward. The other reason why I think this makes sense for us is that we need new flooring anyways due to an old moldy carpet on top of plywood subfloor. So rather than spend a ton of money on improving the crawlspaces and installing new flooring, I thought why not just feed two birds with one scone and turn our problem into a solution. If you’re curious about access logistics, there is good access for a truck to pull right up and basically into the house through a giant sliding glass door that can be removed, so we should be able to do this relatively efficiently and do not have to lift the structure.

For those that looked at the floor plan, the first leg of this project will just be crawl spaces 1, 2 and 3. That will be the new slab. Crawl space 4 will have to be addressed separately next year.

In my mind this crazy plans makes a lot of sense, but I’d love to hear from anyone with pushback if there are things I am not considering, and any general info on how to make sure this project is a success. Thank you!

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  1. walta100 | | #1

    Generally the floor joists are what supports the walls if you remove the joists you will need to support the walls somehow.

    I got to ask what are you really saving with this plan?

    Everything you can touch is likely covered in lead paint and is a hazard, rotten old siding, drafty single pain windows an old roof.

    If you must redo almost ever thing it may cost less to start over.


    1. marleyandbowie | | #2

      Thanks Walter. Good points. The house would have been first painted inside and out in 1980, didn't they stop using lead paint in 1978? And unless I am misunderstanding something, I don't see why we would have to touch the walls, windows or roof, as all the walls are on top of the existing foundation wall, which is like 80% CMU blocks and 20% poured. The roof is 15 years old, it's a mix of old and modern windows, and everything is actually in pretty good shape. It's just the foundation and HVAC system that are causing problems.

      I'm extremely new to building science so forgive me if I'm sounding dumb!

      1. Expert Member
        MALCOLM TAYLOR | | #4


        "all the walls are on top of the existing foundation wall"

        I think the main complication is that all the walls sit on the floor which is on top of the exiting foundation wall.

        1. marleyandbowie | | #7

          Oh I see. I guess I did not understand the anatomy of a wall. Do you think there is any reasonable way to do this and leave the walls alone? Could the perimeter of the subfloor that supports the wall somehow be supported during the demo and then integrated in the slab? Or is this crazy talk, you can be honest with me haha. Perhaps the concrete guy I am speaking with and I are not on the same page, because he didn't mention anything about this yet

  2. user-2310254 | | #3


    Your proposed strategy sounds a bit drastic and expensive. I would focus on cleaning up and conditioning the crawlspaces. Once that is done, you can simply ignore them while preserving the option of using them for wire, plumbing, and duct runs down the road. For example, while you are considering radiant heat at this point, the load calculations you will hopefully do before finalizing your design may suggest that a ducted mini split will deliver more comfort at less cost.

    A ducted system will need ducts, and a conditioned crawl space is a good place to put them.

    1. marleyandbowie | | #6

      Thanks Steve. I get what you are saying, but I'm looking at a very expensive project either way. The quotes I am getting to thoroughly remediate, encapsulate and condition the crawlspaces are in the same ballpark as filling them in and converting to a slab. Also, we have a rodent entry problem and that would not be solved by simply cleaning and conditioning the spaces (sorry for not making that more clear in my post). I am also considering laying rat slabs with a good drainage system, but again that doesn't seem to be substantially cheaper than my plan. I also really don't like forced air so we wouldn't re-install ductwork again, but having the spaces for future plumbing and electrical is a good point.

      1. Expert Member
        NICK KEENAN | | #9

        There are a lot of charlatans in the crawlspace encapsulation business. A lot.

        A rodent problem is solvable.

  3. Expert Member
    NICK KEENAN | | #5

    Concrete makes a lousy floor for residences.

    If there's ductwork in there they aren't inaccessible.

    It's a fairly recent concept that the bottom of the house needs to be sealed. Looking at the pictures I can see that it's not sealed, but I don't see anything to suggest it's not sealable. The question with crawl spaces is where do you put the edge of the building envelope: do you put it at the sides of the crawl space or the underside of the joists? Wherever you put it, it should be air-tight, vapor-tight, insulated and critter-proof.

    What you're proposing will be a lot more work than just sealing the crawlspaces properly. Since you're OK with ripping the floors out, do that, leave the joists, do the work with the floor out and room to move, then put the floor back in.

    1. marleyandbowie | | #10

      Thanks DC. You and everyone else who has chimed in are making excellent points. I think the main benefit to my plan is that is alleviates the need to have to spend a ton of money on:

      1) thoroughly mold remediate roughly 600 sq ft of crawl space dirt, walls, subfloor and joists
      2) encapsulate/seal and or install rat slabs
      3) install new drainage system(s)
      4) dehumidify the spaces
      5) critter prevention (if not installing rat slabs)

      Those 5 items are looking like they will cost as much, if not more, than demolishing the floors and filling in those spaces up to grade. Now where I think my plan could fall apart is if my walls are being supported by the floors joists rather than fully anchored by the sill plates. We'll need to do some preliminary demo to figure that out. This is a single level ranch and all the walls are exterior, there are no walls that would not be sitting right on top of the foundation wall.

      1. Expert Member
        NICK KEENAN | | #17

        Alternately, you could put down heavy plastic on the floor, spray 2-1/2" of closed cell foam on the walls and rim joists, spray the underside of the floors and joists with Kilz, and call it good.

        If the rodents are just field mice the foam will keep them out. They can burrow in but generally won't. If they do you'll see it and you can fill their holes with steel wool and more spray foam.

        1. Expert Member
          NICK KEENAN | | #18

          Maybe wash the joists with bleach and water before spraying if the dust is thick.

  4. Expert Member
    NICK KEENAN | | #8

    Have you tested for asbestos? If it wasn't winterized until 1980 I wouldn't expect it on the ducts. Floor tile is only a problem if disturbed.

  5. walta100 | | #11

    Come on they built a cottage in 1955 and did not but the first coat of paint on before 1980?

    Let’s say you managed to remove the floor and find a low cost way to support the wall install new plumbing under the new slab, lower all the doors and window to the proper height above the new floor, repair the exterior siding, rewire and bring the electrical up to the current code install new drywall paint inside and out, new finished floors new kitchen and bath fixtures.

    I say you could level it start on a fresh foundation and rebuild for less money that it will cost to make the 70 year old stuff work.


    1. marleyandbowie | | #12

      Walta - There is no plumbing to be installed under the slab, the kitchen is on top of the basement part of the foundation and will not be touched. There is no plumbing running through the crawlspaces. The areas on top of the crawlspaces that need work are just living rooms. The new slab would end up at the same grade as the current sub floor, so there will be no need to change window and door heights, and I don't understand why the interior walls or exterior siding would be touched at all, as this is all done from within the house.

      You're right in that there could be some lead paint under the new paint, but again I don't see why any painted surfaces would need to be modified. The only demo is to the floors above the crawlspaces.

    2. marleyandbowie | | #13

      Maybe this will be helpful, here are a couple photos of the living areas above the crawlspaces I would like to fill in. It's the carpeted area and the small are with blue tile under the dining room table. The terracotta style tile is on a slab and will not be touched as part of this project.

  6. user-2310254 | | #14

    Is there something magical about this site that can be replicated in another, possibly nearby, location? Why not buy another piece of land and build the house you are envisioning?

    1. marleyandbowie | | #16

      I hear what you're saying, but this is a really cool house on a magical piece of land with waterfront and mountain views. Everyone that sees it loves it, even though it has issues it has a bit of a wow factor and I think it's worth preserving. I was going to do all the demo myself, so the main expense would be filling the spaces and topping with a slab, which doesn't seem to be prohibitively expensive so far.

  7. Expert Member
    NICK KEENAN | | #15

    Here's another consideration: you mention being near a lake and having high humidity as a result. I don't think that's necessarily true, you're just in a part of the world where it's humid in the summer. But the winters are cold and the ground temperature stays in the low 50's year-round. In the summer when dew points get into the 60's and 70's, any surface that is in contact with the ground will have condensation, that's where the mold in your crawl space is coming from.

    Slab floors are not popular in New England, because in the summer they can sweat. You might find that with a slab your mold problem is worse, as now you're moving the moisture up into your living space.

    How do you prevent condensation? One answer is to separate your living space from the crawl space with an air barrier, a vapor barrier and insulation, and just let the crawl space sweat. Another answer is to put the air, vapor and thermal barriers on the walls of the crawl space so that the hot, humid air never comes in contact with a cool surface. You can also condition the air in the house so it is less humid -- but air conditioning doesn't work that well in places like lake houses because it's humid but not that hot. And part of the joy of a lake house is being able to live with the windows open.

  8. Coltrabagar | | #19

    I know this is an old post, but I found it while researching to possibly do this on my own home. I realized something from this thread. The walls sit on top of the floor which sit on top of the rim joists and the ends of the floor joists which themselves sit on the foundation wall. The rim joist is re-inforced structurally by the perpendicularly attached floor joints. Using a wall as an analogy, the rim joist is like the top or bottom plate of the wall and the floor joists are like the studs of the wall. They help hold each other in place. So cutting out the floor joists would actually structurally be a problem if their function was not somehow re-created. The rim joist, which is typically 1 1/2 inches wide would be sitting on its edge unsupported. This would be a very unstable situation unless retrofitted somehow.

    I suppose you could attach the rim joists to the new 4" concrete slab with bolts through the rim joists. This would be similar to how wall sill plates are attached to slab foundations. The trick would be making sure the rim joists don't shift while the concrete is setting up. Perhaps doubling or trippling the rim joists with new treated lumber could work well enough to get the cement poured and cured.

    Anyhow, I would like to know what route you ended up taking and how it worked out for you.

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