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Community and Q&A

Remodel–insulating floor

Kc54w36XVZ | Posted in General Questions on

I am looking to tile my den floor and before I do that I want to insulate the floor. It is over slab with only about 6-12 inches between the bottom of the floor joists and the slab so I cannot access from under it, but I do know that the batts are sagging. The room tends to be cold, I have already air sealed the rest of the room, and would like to address the floor before I tile. My thought was to tear up the subfloor, put in cleats at the bottom of the joists to support rigid foam boards, seal them with expanding foam, then lay out batt insulation in the caviity.
Is there a better, easier, quicker, and cheaper way to accomplish the same thing?

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  1. GBA Editor
    Martin Holladay | | #1

    You have an unusual crawl space. If you have a wood-framed floor over a crawl space, and the height of the crawl space is only 6 to 12 inches, your floor assembly is likely to have long-term problems.

    I don't know who built the crawl space, but the builder made a fundamental error. The crawl space height should be sufficient to allow access; otherwise the crawl space should be filled with compacted granular fill and a slab should be installed.

    If you can afford to correct the problem, your wood floor framing should be removed. Then install compacted fill, a layer of horizontal rigid foam, a layer of polyethylene, and a concrete slab at least 4 inches thick.

  2. Kc54w36XVZ | | #2

    We bought the house about 2 years ago, the den was an addition at some point I believe. From the basement window I can see the insulation sagging. The problem is that there is a concrete window well that goes within a few inches of the floor joists, therefor I cant really tell the clearance from the ground/slab to the joists. I really dont even know what is benieth the floor--ie concrete, gravel, or skeletons. I will only be able to tell once the floor is ripped up. Idealy I would like to be able to insulate without tearing up the subfloor, but I dont know if this is possible or feasable. Hoping to have a few ideas, and a few options to go on if I find those skeletons.

  3. GBA Editor
    Martin Holladay | | #3

    Obviously, it's hard to determine the best way to proceed without knowing what's under the floor. But if the situation resembles the description in your original question, you'll either need to dig out the crawl space to lower the dirt floor below the floor joists and create an access hatch, or you'll need to fill up the crawl space with compacted gravel and install an insulated slab.

  4. davidmeiland | | #4

    It sounds like someone built the addition over a former patio or garage slab.

  5. user-659915 | | #5

    Questions for Joe:
    1. the usual - location?
    2. is there ductwork in the subfloor space?

    depending on the answers to both, you might want to open up the subfloor around the edges and just insulate/seal the perimeter of the subfloor area.

  6. Kc54w36XVZ | | #6

    location: NH
    There is no ductwork, the house has FHW

  7. albertrooks | | #7


    A couple of more questions:

    1. What are the dimensions of the room?
    2. Can you get down to the subfloor surface easily and inexpensively?
    3. Are there any "built in" cabinets or fixtures that would be hard to work around on the interior side of the floor?

    Ok that was three questions. :)

  8. Kc54w36XVZ | | #8

    1 The room is about 20x16
    2 I will be tearing out the carpet--yes subfloor easily
    3 the only thins that is in the room would be the fireplce hearth that is situated kitty corner in the room, but that should not pose an issue

  9. albertrooks | | #9


    This one is straight out of left field, and I'm sure I'm going to get a "collective groan" from many readers on this, but never the less, I'll risk being pigeon holed as "a wee bit eccentric".

    This application could possibly be accomplished quicker, simpler and "possibly" cost neutral with Vacuum Panels. I really don't have any solid basis to know if this would be a good application because we don't have enough application experience here is the US with them.

    We are learning that the "right fit" for a new product from Poraxetherm is a basement floor where you can't increase the floor level due to height restrictions (bumping your head), and it's far to invasive to add insulation to any surface that's not vertical. The Vacuum Speed system is a series of various panel sizes from 12" x 12" to 24" x 40". They lay out on the floor with staggered seams. They obviously don't fit wall to wall. For the gaps to the walls at ends of the runs there is a matching thickness foil faced polyiso panel that can be cut and fit. From there, add the same foam sheeting that's used under laminate floors, and then add some kind of floating floor as your finished floor.

    I brought this up because your application will really make it hard to get under the subfloor and get any really good insulation in without having to rip everything up. Subfloor and all. Since I don't know much about your project, I don't know what it will wind up costing to get meaningful (whatever that is..) insulation in there.

    The positives: Amazing insulation values: R-18 for a 5/8" thick panel. R-20 for a 3/4" panel, or R-30 for a 1" panel. You had said that you've got your air-sealing done. So… It would be: Just get down to the subfloor. Add Foam sheeting, add panels, add foam sheeting again, add a floating floor. Done. (I think!)

    The Negatives: Vacuum Panels have a life expectancy of up to 50 years. They will need to be replaced at some point. You'll build up the floor height and have to deal with floor transitions from other rooms. I think you wanted a tile floor. The floating floor may be a problem for that.

    I'm guessing that this would be in the range of $16/ square foot or $5k for the insulation. I only bring it up in case you see the cost really climbing due to how much of your floor has to be R&R'd. This is a new product area that we (The Small Planet Workshop & Peak Building Products) are starting to work with and importing. As I said, I really can't say if it makes sense cost wise because I don't know your job. However, I do think it make sense in application and end result: Let the batts hang where they are. Don't disturb how the vapor is moving now, just add the insulation ontop of the subfloor.

    I attached a piece of literature if your curious.

    Best of luck!

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