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Community and Q&A

Leveling and insulating a basement floor

MattJF | Posted in General Questions on

I have 1200 SF of basement and I am thinking about insulating the wall of using 3″ polyiso/furring/drywall. I will be using the space primarily as a shop and “lab”, with a section possibly being sectioned off as a playroom. I am trying to figure out what to do with the floor. The house was built in 2008, but the basement slab was poorly done and has a bunch of cracks and is far from level. I should probably figure out “what went wrong” before doing anything. I believe the issue is likely settlement of uncompacted substrate, but that is just a guess. Most of the cracks define high points on the floor, with variation as much as 1 1/4″ over 8′, with 1/2″ being more typical. What steps are needed to diagnose what happened?

I would like to seal the cracks as I have to implement radon mitigation after air sealing the rim joist. It would be nice to level and insulate the floor (rigid foam, plywood on top) as well. What techniques exist to level and insulate the floor? I was thinking about self leveling compound, but it is expensive for the area being covered. Leveled deckmud might be an option under foam, but a lot of work. Profiled sleepers on top of foam is a lot of work and I worry about concentrating the loads on foam with equipment on the floor.  Is there a reasonably priced way to do this I haven’t considered?

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  1. Expert Member
    BILL WICHERS | | #1

    Is the slab still settling, and the cracks still propagating, or has all the movement stopped? If things are still shifting, then you have a big job to fix things. If the settling has stopped, then you can put a new surface (I’d normally use a self leveling concrete for this, but I’ve never needed as thick a layer as you appear to need).

    If there is still movement in the slab, you might try contacting one of the specialty foundation repair companies that can do bentonite injection. I’d want to do a full inspection of the foundation too to make sure there wasn’t any settling of footings, or voids under anything.

    You’ll want someone expierienced with this type of work to come out and look at your site. With the amount of settling you have, I would have it professionally inspected even if you think the movement has stopped. You don’t want any surprises with issues like this.


  2. Expert Member
    Peter Engle | | #2

    Mudjacking is somewhat expensive, very messy, and occasionally imprecise. The cost is particularly high for small jobs. It would also fill any stone under the slab with cement.

    If the slab is really broken up, it might be best to rip it out and replace it. That would provide an ideal opportunity to insulate it, reinforce it, pour a proper slab and install a proper vapor barrier underneath the slab. At the same time, you can check the stone, column footings, etc. for settlement, debris and all sorts of other bad things. It would be messy but you would end up with what you want/need. You could put plumbing in there for lab sinks and a bathroom at the same time.

    1. MAinspector | | #3

      I agree with Peter, the best approach might be to rip it out and start over. The cost of the products and/or time needed to try and level and repair might be just as expensive and might prove to be wasted if the slab is still settling. While very labor intensive, just about anyone can rent a jack hammer, crack up a slab, compact the sub base, and lay down rigid foam and poly. I would suggest calling a concrete contractor to pour the slab but DIY is always possible.

  3. Expert Member
    BILL WICHERS | | #4

    I’ll add that replacing the existing slab is probably the best option, especially if things are still settling. With the old slab out of the way, a proper inspection can be done of the remaining concrete structure. Everything can be done right when the new slab goes in.

    I’d only do bentonite injection if you can’t replace what is already there. It’s generally more difficult to fix existing things than it is to just build it right the first time, or replace the damaged stuff completely, but not everyone has that option.


  4. MattJF | | #5

    I think I would be inclined to live with the floor as is and leave it uninsulated before breaking it up and hauling it out. At least at this time. It is not something I want to do myself. I will have to price it out. I found a foundation/slab expert and am waiting for a call back to see what if they can make a recommendation.

    How do you tell if the floor is still moving or not? We have been in the house for 9 months, it's 10 years old now. I recall a method of gluing glass slides across a crack. Are there any other tricks?

    I guess I was thinking that if the floor is stable, but not flat, there might be some alternative leveling options when floating a foam/plywood floor over a slab. All the loads on the slab are now well distributed by the flooring system (should be well below 20 PSI) so maybe that opens up other ways of doing this.

    1. Expert Member
      BILL WICHERS | | #6

      I usually watch for crack propagation. Mark the ends of the cracks with something like a grease pencil and see if they grow. You can also glue fragile small things (glass capillary tube works pretty well) across larger cracks. You need something brittle that will break if the crack widens or flexes. If the cracks grow, or glass tube breaks, then things are still moving.

      It’s possible everything has stopped settling in the ten years since your house was built. If that’s the case, you just need to use something like floor leveler to give you a new flat surface over the existing slab. My only concern with the self leveling stuff is that you said your slab has some really severe elevation changes and I’ve never worked with something that extreme before. You’ll end up with a pretty thick layer of the self leveling stuff in the low spots.


  5. MAinspector | | #7

    Matt, The way forward really depends on the finished product you are comfortable with. What did you have in mind for a finished floor? A slab that out of level really limits your options for flooring. I think if it was my floor I would either crack it up and redo it properly or seal the cracks and put down carpet with a good rubber pad underneath. I would hesitate to put too much time and materials into a "middle of the road" approach that could be trashed if the floor does continue to settle.

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