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Insulating a shed floor – rigid foam on top or bottom?

Meagan McAleese | Posted in General Questions on

I am building a shed on skids and would like to insulate the floor. I want to put batt insulation (rockwool) between the joists and I thought I would be excessive and add a layer of rigid XPS. My question is should I put it on top of the joists or on the bottom?
On the bottom: Do I start with a layer of OSB on my skids, then a solid layer of XPS, then more OSB, then my floor joists? Is the rigid XPS firm enough to have my whole shed bearing on it?
On the top: Do I put a layer of OSB on my skids, then the floor joists, then osb and a complete layer of XPS and put my walls on top? Once again I have a question about the bearing strength.
Or do I forget about a complete layer and just put the XPS between my skids (bottom) or between my walls (top)??
Thank you

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Meagan,
    If this building is being built on skids, then the skids will be the surfaces that contact the soil when you are skidding your shed. The skids should have a strong structural connection to the framing above.

    You won't be skidding the building on the OSB used to protect your floor insulation. That's why you will be building skids.

    I think that the best location for the rigid foam is on the underside of the floor joists. Install the rigid foam in an airtight manner (using a high-quality tape to seal the seams). The install a layer of OSB on the underside of the rigid foam to protect the rigid foam from abuse and rodent entry.

  2. Meagan McAleese | | #2

    So just to be sure, if I have a 20'x20' shed and the floor sits on five skids (treated 4x6's), the floor joists sit on skids as regular and I only need the rigid insulation and osb under the joists but between the skids, I don't have to worry about the thermal bridging through the 4x6's because a good structural connection is worth a little thermal bridging?
    That was what I was thinking, thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Meagan,
    I think that your five 4x6s could be fastened through the OSB and rigid foam with carriage bolts. That way, you'll reduce the thermal bridging.

  4. Meagan McAleese | | #4

    Oh gosh, now I'm confused.
    Back to my original question....this means the whole shed will be bearing on the XPS. This is okay?
    From the bottom up....4x6 skids, osb, XPS, osb, 2x6 floor joists? Perhaps build the floor, then attach the osb,XPS,osb to the floor joists and then flip it over and secure the whole thing to the 4x6's?

  5. GBA Editor
    Martin Holladay | | #5

    Meaghan,
    You can do it either way. If you feel better having a wood-to-wood connection, without the intervening rigid foam, you can do it that way -- and you'll have a thermal bridge. Either way will work.

    Five 4x6s are pretty rugged -- they help distribute the weight of the building -- which is why I'm guessing that the intervening rigid foam probably isn't a problem. But I'm not an engineer.

  6. Expert Member
    Malcolm Taylor | | #6

    Megan,
    It's worth thinking through your sequence of construction for the floor assembly you are using. Buildings that are intended to be sheathed underneath are typically set on pier foundations to allow access to the underside. There is no way to sheath the underside of your floor joists if they sit on 4"x6"s. On small outbuildings it is sometimes possible to frame the floor, sheath the top and then flip the whole thing over. That would be fairly difficult with something 20'x20'. The knock-on effect of this is that you probably don't want to put any insulation between your joists as it will be subject to the depredations of pests.
    It's also worth remembering that most codes preclude building that close to the ground without using pressure treated materials as the materials are subject to moisture damage. Given that at 400 sf it is fairly unlikely that you will actually move the building on its skids, it might be a better idea to consider a slab as a foundation.

  7. Meagan McAleese | | #7

    Thankyou Martin, I feel better knowing your opinion for the rigid. And thankyou Malcolm for the moisture damage reminder! I hadn't thought of that but of course you're right - I shall rethink this, perhaps use green plywood...

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