GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Exterior Insulation For Elevated Cabin Floor Assembly? (Zone 6)

VT_Cabin | Posted in Energy Efficiency and Durability on

First post here and first build, so thanks in advance for the help!

I’m designing a floor assembly for a 16’x30′ cabin in Vermont (Climate Zone 6). The assembly is supported by three 6×6 skids held 2’ off the ground with helical piles.

I plan to construct the floor upside down in four 16’x8′ sections, flip them, then join them (similar to this

Here’s my proposed assembly, from top to bottom:

1. AdvanTech subflooring (glue and rink shank nailed)
2. 2×6 floor joists with R-23 Rockwool insulation
3. Taped foil faced 1.5″ polyiso rigid insulation for vapor barrier and condensation control. (R8ish when cold, 25 psi compressive strength)
4. Plywood to protect the polyiso
5. Galvanized hardware cloth for mouse and porcupine protection
6. Pressure treated 6×6 skids
7. Helical Piles

The 1.5” foil faced polyiso provides the vapor barrier and necessary R-8 (35% interior/exterior ratio for Zone 6) below the assembly to control condensation. 25 psi compression should be fine supporting the building weight on the skids.

Also I’d like the building to last, so I’m thinking to use stainless fasteners for anything that goes into the PT skids.

A few questions:

1.  I’m concerned about attaching the joists to the PT skids through the 1.5” polyiso insulation layer while maintaining structural integrity and ease of construction. I don’t think I can toe in 3.5” nails anymore… (what the plans called for without any exterior insulation). Thoughts on this?

2.  Are PT skids necessary or if it’s up 2 feet are they more trouble than they are worth in terms of fastener material? If they aren’t PT, then I have more fastener choices.

 3.  Should the plywood on bottom be painted or have any membrane covering it for protection?

4.  Is there some totally different way that would be better?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    1. Are the "skids" the beams the floor joists bear on? I would run a 2"x3" on top of them at the exterior for the floor system to bear on, and not rely on the foam. It won't be much of a thermal bridge as it would be under the exterior walls.

    2. They don't need to be PT.

    3. Two feet above grade it should be fine, especially if the ground underneath has a vapour-barrier to limit moisture rising. Do you need hardware clothe over it? I've never found it necessary here, but maybe you have different pests to worry about.

    4. It's not a bad way to build. I've done a few outbuildings that way. Recently I did one this way, which I liked too. Six of one, half a dozen of the other.

    The attached photo is of an outbuilding with a foam insulated floor showing 2"x2" under the load-bearing wall.

  2. VT_Cabin | | #2

    Thanks for the reply Malcolm. I've drawn up and attached what I think you mean. Does this look correct?

    Lots of mice around, but also plenty of porcupines and they can get chewy on plywood.

  3. walta100 | | #3

    Generally, if you insulate the perimeter wall it will have a smaller surface area and the conditioned crawlspace is a good place for ductwork and drain pipes.

    Do you have a idea for the drain pipes in your current plan?


  4. VT_Cabin | | #4

    Hi Walta - I should have originally said it's a workshop, not a cabin, so there is no plan for plumbing or ductwork. It will have electricity, a minispit, and a woodstove. I want it warm enough to spend the night out there if need be.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |