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Can I build a standard wood joist floor on top of an unheated slab?

Studio_Home | Posted in General Questions on

Hi 

New home builder here!
I’m building a small studio retirement pad for myself.
It’s a 768 sq ft house (24′ wide x 32′ deep) double wall construction.
Just a bathroom, kitchen, & a living space.
I’m assuming that a slab is the cheapest way to build?
Can I build a standard wood joist floor on top of an unheated slab?
Floor will have R-36 rock-wool batt insulation.
My region normally requires a minimum 6 foot to grade basement.

Any comments?

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Replies

  1. joshdurston | | #1

    If the slab hasn't been poured yet, but I would consider...
    -Insulating under the slab and around the perimeter
    - install a proper barrier under to slab to prevent ground moisture from migrating upwards. (cheap and very effective, but sadly missed in many slab pours).
    -If you have wood touching the slab even with a barrier and insulation I would consider some sort of gasket/barrier/poly to prevent moisture from migrating into the wood
    -I don't think you'll need a floor joists and insulation if you do it properly under the slab, just your flooring and underlayment.

  2. Aedi | | #2

    As far as I know, there is nothing code-wise stopping you, but it is not commonly done for a few reasons. By your double stud walls and use of rock wool batts, I'm going to assume you are one of those anti-foam folks, so I'll tailor my answers accordingly. If you are not anti-foam, just insulate the slab the normal way.

    What climate zone are you in? I am assuming that, since you want a lot of slab insulation and your region requires 6 foot to grade basements, frost heaving is a concern. There are a few ways to mitigate that.

    If you are hoping to save money with a slab, the most reasonable option is to build a frost-protected shallow foundation. This technique involves insulating the ground below the slab to prevent it from freezing. Normally, this involve prodigious amounts of foam, but I believe there are ways to use mineral wool boards in its place, or even that new insulated gravel stuff (search for it on GBA). With this approach, there is no need to insulate the top of the slab, and so your approach is unnecessary.

    Another approach is a raft slab. However, I do not think there is a way to get around foam with that approach, so I'll ignore it.

    The last option is to build a traditional stem wall foundation or monolithic slab. This requires the footings/bottom-most part of the monolithic slab fall below the frost line, which in turn requires a fair amount of excavating. This makes the cost difference between a slab and a full basement rather small, which is one of the reasons basements are still so common up north. With this type of slab, you can proceed with your plan, but it probably isn't wise. Since you are planning on building a complete floor system anyway, There will practically be no additional cost to add a basement and drastically increase the amount of space available to you. Another thing to keep in mind is that lumber in direct contact with concrete needs to be pressure treated. Don't plan to have the joists in direct contact with the concrete, use pressure-treated sleepers.

    So overall, your approach is possible, but it is probably better to go with a basement or a frost-protected shallow foundation.

  3. Expert Member
    Peter Yost | | #3

    Check out the most recent GBA Spotlight on slabs and insulation: https://www.greenbuildingadvisor.com/article/insulating-raised-slab.

    Peter

    1. Aedi | | #4

      I had already forgotten about the possibility of a slabless floor! Seems like a good approach when the plan is to cover the slab anyway. The comments have more resources on that setup (see replies to comment #8).

  4. GBA Editor
    Martin Holladay | | #5

    Grant,
    Several points here:

    1. If the frost depth in your area is 6 feet, you need deep footings (a perimeter frost wall) for most slab foundations, so that a slab isn't necessarily cheap. Alternatives include a raft slab foundation or a frost-protected shallow foundation; you can enter these search words into the GBA search box for more information. Neither of these two latter options is common or cheap.

    2. If you have a slab foundation, the usual way to install flooring on top of the slab is to simply install subflooring followed by flooring directly to the slab. If there is no horizontal rigid foam under the slab, you would need to install a continuous layer of rigid foam above the slab, followed by subflooring. There is no need for wood joists. More information here: "Installing Rigid Foam Above a Concrete Slab."

  5. Studio_Home | | #6

    Grant Stocker Decides
    After much wailing and gnashing of teeth ;( I've decided on forgoing basements and slabs altogether ;) I'm going with twelve 18" round piers on a monolithic 9" thick footing (poor soil), with rebar all around. My reasoning, slabs are too low to the ground (we can get 4' or 5' of snow on the ground!) and ultimately require heat leakage to keep the underside from freezing, and an unheated basement is just a a big crawlspace. Yes the beam/joist floor is expensive but it gives me R-36 in the floor and walls.

    1. GBA Editor
      Martin Holladay | | #7

      Grant,
      It's true that "an unheated basement is just a big crawlspace" -- but it would be useful, and it would keep your pipes from freezing. If the upcharge for a basement isn't much, consider a basement -- you'd get twice the square footage.

    2. Aedi | | #8

      Hi Grant,

      Your solution is a workable one, but some of your reasoning is faulty. Most notably, not all slab designs require heat leakage to keep the underside from freezing. For example, there are design guidelines in place for frost-protected shallow foundations meant for unheated buildings. These designs use wing and sub-slab insulation to prevent heat loss from the ground, allowing the earth's natural heat to remain, and eliminating frost heaving concerns. Another approach is an insulated raft slab, which focuses on modifying the ground beneath the slab to reduce water retention, and thus ice and frost heaving. A raft slab also has the benefit of elevating the slab -- about as much as your included pier details would.

      I too am biased against basements, but sometimes they are the appropriate solution. Even an unheated basement will remain well above freezing, and if nothing else it is a useful place for utilities and storage. If you are interested in keeping your first floor elevated, that provides the opportunity for taller basement windows, which in turn makes the basement much more usable and livable.

      Your fixation on having R36 walls and floors is also not based on sound reasoning. If your home is in direct contact with the ground, there is little reason to have as much insulation in the floor as in your walls. The ground never gets as cold as the air, especially when there is a house in the way preventing contact between the two. As a result, you have a much smaller temperature difference, and so need less insulation. If you had an unheated basement, the temperature would probably never drop below 50F, whereas outside it could be -20F! It would make more sense to put less insulation in the floor, and use the money you saved to put more insulation in the walls -- say, R20 sub-slab and R45 for the walls. This is especially true when you plan to use double stud walls -- making those thicker to accommodate more insulation is trivial.

      In fact, I hope that R36 is not meant to be center-of-cavity, as it is unusual to make a double-stud wall that thin. That's scarcely better than a 2x8 wall (and equivalent to a 2x10)! It would put your whole assembly value at about ~31-33, for both the wall and the floor (the reduced thermal bridging from the double studs is practically a rounding error at that depth). If you are going through the trouble of a double stud, you should aim for better. On a whole-assembly level, you could beat R36 center-of-cavity with a 24" OC 2x6 wall and 2-3" of foam (or comfortboard IS) if you were so inclined, and it would likely be cheaper.

      The only reason to put as much insulation in the floor as the walls is when the floor is exposed to the open air, as your solution ultimately requires. Even with that extra floor insulation, you will almost certainly lose more heat through your R36 floor than you would with R20 sub-slab or above a basement.

      I highly recommend running a few different insulation options through a modeling program like WUFI, or hiring a professional to do it for you. It will allow you to have a much better sense of what areas to put your insulation budget towards, and strike the proper balance.

  6. Studio_Home | | #9

    Eureka moment!

    OK, the point I was missing (blush) is that we're not necessarily insulating the slab from house heat loss but we're insulating the ground to keep the ground heat in the ground, thus no freeze-up. This makes far more sense to me now. Most heat rises, except for conduction losses. D'oh!
    I've already decided on 24" OC double wall with offset studs . My wall is 2x6, 1/2" gap/wire chase, then 2x6 with 5/8" drywall for an R44 rating. I'm giving serious thought to your R20 sub-slab suggestion.

    Thanks

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