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Rigid insulation beneath floor on shed

user-5872725 | Posted in Green Building Techniques on


First time posting and would like to thank you in advance for at least taking the time to read this.

I am building an insulated shed/writing studio for my wife. Looking to make it 12’x8′ and was thinking of building it on concrete blocks with 2″x6″ studs for the bottom frame. On top of that I was planning on 2″ rigid insulation followed by plywood then flooring. Will I be able to put the sill plate on top of this and drill through to the 2″x6″‘s? I haven’t worked with rigid in this context before.

Putting it on blocks seems the best option in case we wish to move it later.

Any advice would be greatly appreciated. Thank you for your time.


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  1. GBA Editor
    Martin Holladay | | #1

    We need to clarify your vocabulary and your meaning.

    You wrote that you are "thinking of building it on concrete blocks." Does that mean concrete block piers? Or a crawl space with concrete-block walls?

    You wrote that you are thinking of building it "with 2x6 studs for the bottom frame." Do you mean "studs" (vertical members used to frame walls) or joists (horizontal members used to frame floors)?

    You wrote, "On top of that I was planning on 2 inches of rigid insulation followed by plywood then flooring." Does that mean that you want to sandwich the rigid foam between the top of your floor joists and your plywood subfloor? If so, you can't do that (because the tops of the floor joists will dig into the relatively soft foam).

  2. user-5872725 | | #2

    Hello Martin,

    Thank you for your prompt reply. I apologize for the vague language. The attached image is what I was thinking of. You are correct it was 2"x6" joists I was referring to.

    Your answer to the sandwich was what I was looking for. I am trying to figure out the best way to insulate the floor. Would it be to put OSB down on the joists, then insulation then flooring? Appreciate your time and patience.

  3. GBA Editor
    Martin Holladay | | #3

    You haven't told us your climate zone, or whether you intend to heat this room.

    Minimum code requirements for floor insulation vary from R-13 minimum in Zone 1 (southern Florida) to R-38 minimum in Zones 7 and 8 (northern Maine and northern Minnesota).

    If you want to install a site-built SIP -- in other words, rigid foam insulation with OSB on both sides -- you can. To achieve R-30 (the minimum requirement in Zone 5), you would need about 5 inches of polyiso or about 8 inches of EPS.

    For more ideas on insulating this type of floor, see How to Insulate a Cold Floor.

  4. user-5872725 | | #4


    Thank you again, I am on Prince Edward Island and it is Zone 2. There will be a 6' wide patio door to let light in, it will be south facing and I plan on spacing studs 24" O.C. filled with mineral wool. IN addition, insulating the roof as well. There will be electricity run to it. For heating methods I didn't think it would need much more then a simple space heater for the really cold days.

  5. GBA Editor
    Martin Holladay | | #5

    Of course, it's up to you to decide how much insulation you want to install, as long as your building complies with local building codes.

    Prince Edward Island is actually in Climate Zone 6 (using the climate zone map established by the U.S. Department of Energy). Here is a link to the relevant map: Climate Zone Map Including Canada.

    If you were building in Zone 6 in the U.S., building codes would usually require your floor insulation to have a minimum R-value of R-30.

  6. KeithH | | #6

    Making a foam sandwich seems hard.

    Why not put batts in the joists bays, put osb/plywood/etc as a critter guard on the bottom (tape the seams), and call it done?

    It is pretty easy to insulate inside the 2x8 joist to R-30 using Roxul mineral wool batts. In the USA, you'll need to order this material from a big box pro desk as 2x8 isn't stocked. You could also, with slightly more effort, use 2 R-15 2x4 batts. In the USA, you may find 2x4 thickness stocked at very competitive prices at big box stores. Obviously you could use fiberglass batts for lower cost but might not be able to get to R-30 fill with fiberglass.

    Be sure to follow a standard 16" or 24" o.c. layout (except for the 1 odd bay you'll likely have) or you'll have a high waste factor with fiberglass or mineral wool.

    Bear in mind you will likely need a hard layer of material to protect the insulation batts from critters (plywood etc) and a plan to air seal the joists as well (a building tape on the plywood joints is probably adequate). Otherwise, your batts will get destroyed by animals eventually.

    Your overall performance will be significantly lower than R-30 because the joists themselves will be closer to R-9. You could mitigate this with rigid foam under the joists but you might struggle to break the thermal bridge to the structural lumber sitting on the blocks or piers. I suspect you would find filling the joists to R-30 performance pretty good without worrying too much about thermal bridging.

    If you need help understanding whole assembly R-value, here is a GBA QA that might help:

    I'd guess GBA talks about all of this in the link Martin included but I thought this brief reply might help.

    On the other hand, if you are basically building a small house, I'd start with reconsidering the pier blocks on grade foundation.

    Hope that helps!

  7. Expert Member

    Easy - remember this is a 100 sq ft shed. The heating load is going to be tiny. It isn't subject to any code requirements either for structure of insulation, and it is for occasional use, not to be lived in.

    The only advantage to putting the rigid insulation on the underside is that he can fill the joist bays too - but it complicates the connection between the piers and the structure above. If he is happy with 2" continuous rigid insulation on the floor, I'd be inclined to do as he suggested and put it on top. Because there needs to be a perimeter of wood to protect the "sandwich", I'd use a 2"x4" and secure the bottom plates of the wall to it.

  8. user-5872725 | | #8

    Thank you all for your feedback. I will post a drawing of what I have decided after making some changes. I truly appreciate the help, take care.


  9. user-7020837 | | #9

    I have a similar question. Building a 10x10 shed In coastal Los Angeles, mild marine climate, heating source is a direct vent gas fireplace.

    What is the downside if I just put a zip-system-like sandwich of "construction plywood---rigid foam--- pre-finished plywood" on all interior ceiling/wall/floor as a continuous envelop of insulation on the INSIDE? Plus Roxul in between studs/joists/rafters? Is XPS the right rigid insulation to use?

    (1) It's a 10x10 shed on concrete stem wall foundation, with maybe 10 inches of space under
    (2) interior surfaces (ceiling/wall/floor) are all pre-finished plywood, no drywall
    (3) Exterior cladding is tongue and groove wood siding. On top of 15 minute building paper or tyvex housewrap?

  10. GBA Editor
    Martin Holladay | | #10

    You can install rigid foam on the interior side of your framing if you want. For more information, see this article: Walls With Interior Rigid Foam.

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