I'm looking for suggestions on how to best insulate and protect an exterior office (shed) floor from the elements. The structure base will be built on top of helical piles, 6-8" above the ground using pressure treated lumber. I know it'll be impossible to have a "warm" floor, but I want to add insulation (batts/rigid foam) to the cavities between the joists but am having a hard time figuring out how to best construct everything on top of the piles and have the bottom properly sealed. (This will be in Canada where conditions can be rain and snow.)

I've considered constructing the entire, lets call it, 'sandwich', upside down, away from the piles, starting with 1/2" pressure treated plywood as the bottom with the joists, and then flipping it up onto the piles. I'm concerned the size/weight of this approach will make it near impossible to move. Constructing it in place, on the piles will make it almost impossible to attach plywood to the bottom. Any suggestions?

Crude sketchup, with only 1 helical pile for context: enter image description here

  • 2
    6-8" above grade is the problem, here. If you can't crawl in the crawlspace, it's impossible to maintain anything in the crawlspace. Go up 10 more inches (or dig the dirt down 10 more inches under the building footprint) and it's a whole lot easier. I did once live in a house that had trenches dug for access, and you had to crawl in the trench and reach across the areas with no space as needed. It was awful.
    – Ecnerwal
    Sep 7, 2023 at 0:52
  • I do plan on excavating out 1-3” and filling with gravel for drainage, but I really don’t want this thing 3’ off the ground. With 6” piles, 8” joists, and subfloor, it’s already getting closer to 2’
    – KHibma
    Sep 7, 2023 at 1:05
  • 1
    Doesn't Canada have building codes that must be satisfied? Consult Mike Holmes before you proceed. At only 6-8" above grade would snow blow under the shed and keep the floor joists damp during the winter and spring? Or do you plan to have skirting tight to the ground to keep snow and critters out? Sep 7, 2023 at 10:07
  • @JimStewart Yes- will absolutely be skirting it for those reasons.
    – KHibma
    Sep 7, 2023 at 12:22
  • What is the purpose of the plywood on the bottom? I would simply build the floor system in place then put rigid foam insulation into all the cavities. If the foam is a bit loose and isn't staying put due to friction, a couple of 3" framing nails through a joist and into the foam will hold it.
    – FreeMan
    Sep 7, 2023 at 19:24

2 Answers 2


Rather than flipping it, build it elevated above the piles far enough so you can attach the plywood, then lower it onto the piles (having made appropriate cutouts for the piles when attaching plywood.) Then hope nothing ever needs to be touched again.

Standard building-jacking type supports of timber cribbing - fairly lightly loaded if you only build the frame and attach the plywood before you lower it onto the piles to build the rest of the building. Hydraulic jacks to lift the frame so you can pull out one level of cribbing, lower it to the next level, and reset your jacks to repeat until it's in place on the piles.

  • Also, technically, you only need to construct an open-faced sandwich. The top slice of bread can be added after the insulation is in place.
    – Huesmann
    Sep 7, 2023 at 12:19
  • The only thing requiring elevation is attaching the under-side plywood to the frame. Personally, I'd dig down to make an actual crawlspace I could crawl in under the building footprint (with a drainage trench leading away) and build it on the piles, then crawl under and attach the plywood.
    – Ecnerwal
    Sep 7, 2023 at 12:27
  • @Ecnerwal That's an interesting idea; It's only 8' wide, a 2-3' trench down the middle, excavated a bit lower than the rest, allowing me to squeeze through and reach each side with a nailer could be the path of least resistance
    – KHibma
    Sep 7, 2023 at 12:34
  • Make sure you have room to use a nailer - if you're going to stay with 6-8" outside the trench, it might have to be a palm nailer, as a nailgun needs considerable room.
    – Ecnerwal
    Sep 7, 2023 at 12:58

I solved the problem by using 10” beams and 8” joists. This 2” difference allowed me to set a perimeter around the bottom of the interior along with a couple 2x4 cut to fit within that run the same direction as the joists. I could then put 1/2 PT ply on top of these braces. Obviously this wouldn’t support any sort of weight, but it allowed me to run the joists and added some toenailed screws from the joists to the ply for a little more stability. I could then insulate as desired and finished with the subfloor.

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