Exterior Insulation For Elevated Cabin Floor Assembly? (Zone 6)
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 https://youtu.be/W7BB6ZGSOOQ)
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
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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?
Thanks!
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Replies
VT_Cabin,
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. https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
The attached photo is of an outbuilding with a foam insulated floor showing 2"x2" under the load-bearing wall.
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.
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?
Walta
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.