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Shed floor insulation in Climate Zone 4a (Virginia)??

shedworkshop | Posted in General Questions on

I’m planning my 8×12′ shed/woodworking space, and am wondering what a reasonable option for insulating the floor and joists is?

I’d like to avoid fiberglass insulation and would prefer either rockwool or some sort of foam board (or some combination of the two).

Foundation: Gravel
Base: Concrete Deck Blocks
Joists: 2×8″ pressure treated wood at 16″ OC
Flooring: 3/4″ pressure treated plywood
Floor insulation: ???

I’d like to heat/cool the space with a ductless mini-split. Something like this:

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  1. shedworkshop | | #1

    Would it make sense to put foam board underneath the joists to fully insulate them?

    Or would that cause moisture issues since the joists wouldn't have airflow if I do that?

    1. shedworkshop | | #2

      Or should I insulate above the floor sheathing using rigid foam? If I do that, would the sill plate / framing go directly on top of the rigid foam?

  2. Expert Member
    KOHTA UENO | | #3

    Some ideas for possible floor assemblies in this column by Joe Lstiburek.

    BSI-009: New Light In Crawlspaces

    The difficult thing will be how to build these assemblies with the joists close to the ground (I assume inches away from the ground, not feet). There was a recent FHB or JLC article on building a floor assembly higher and lowering it to the ground on lifts.

    And if you wanted to go in a completely different direction, you could abandon the floor framing and do a "wood slab" per Josh Salinger's FHB column:

    Assembling a Concrete-Free Slab
    An innovative plywood slab-on-grade means less time, money, and carbon

    1. shedworkshop | | #5

      The New Light in Crawlspaces article is perfect. I'll likely go with an approach similar to Figure 6 or Figure 7 (except using Rockwool instead of fiberglass). Thanks!

  3. Expert Member


    8'x12' is small enough you can easily flip the floor. Frame the joists with the hangers upside down. Cover with 1 1/2" of EPS foam (omit the foam and provide solid blocking where the supports will land). Sheath with 1/2" PT plywood, then turn the whole thing over. Fill the joist bays with batt insulation before installing the subfloor - which doesn't need to be PT.

    1. shedworkshop | | #6

      I'm going to go with this approach or the ones mentioned in the New Light in Crawlspaces article (Figure 6 and 7). The article doesn't mention plywood underneath the foam, but I would think it would help in keeping critters out of the insulation. Thanks!

      1. Andrew_C | | #7

        Sheathing is definitely required to protect the foam. Remember that ants are one of the key things that you're trying to deter, so protect the outer edges of the foam on the sides as well.
        Also note, if you use something like Figure #7 from BSI-009 and use an airspace at the top, you do need to insulate the perimeter (like a rim joist) so that you don't short-circuit the insulation below the airspace.

        1. shedworkshop | | #8

          Got it! When you say to insulate the perimeter, do you mean I should insulate on the outside of the rim joist? If I'm doing that, I might as well insulate the whole exterior wall as well with the same foam right?

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