Cabin floor assembly
This is not a question pertaining to “green building” per se, but I have an account here and respect the opinions of many users here, so here goes:
I’m a contractor building a personal off grid cabin for my family. It’s not going to be remotely close to code insulation for Maine. I’m not concerned with energy efficiency or even comfort to a degree… this will be rustic and aiming for inexpensive. Floor insulation is my question. The joists will be 2x10s and I don’t want to fill that space with insulation (cost). Nor do I want mice invading. My thinking was 1/2″ PT CDX on top of the joists, a layer of 1.5″ or 2″ XPS then 3/4″ plywood underlayment on top as the finish floor screwed through to the joists. Has anyone heard of a floor built like this? Basically a SIP on it’s side. My main concern is the XPS compressing over the years and then having a kind of bouncy floor. Secondary concern is the flammability of XPS with a wood stove right above it (with a stone hearth between them).
Good or bad idea?
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Replies
conifer525,
Yes, I've done it successfully on two backyard offices.
On this one the foam extends under the exterior walls, with a 2"x2" perimeter to take the loads from above. No problem with settling. The floor is very firm. These types of assemblies get used to insulate floor slabs, and in concrete-free slabs, without issues too:
https://www.greenbuildingadvisor.com/article/the-concrete-free-floating-plywood-slab-assembly
The only time you notice some softness is, perhaps surprisingly, when you screw the perimeter of the subfloor, and it can compress slightly under that high point-load. Taping the joints of the plywood underneath, and sealing the perimeter of the foam is a good idea to keep carpenter ants out.
Thanks Malcom! Good idea about the perimeter load bearing rim.
At my local Home Depot 2" XPS is $63 per sheet and a 25' roll of R30 fiberglass is $49. That works out to $1.97/square foot for the foam and $1.47 for the fiberglass.
Put the 1/2" CDX on the underside of the joists and the 3/4" on the top and fiberglass in between. You'll get three times the insulation for less money. It will be less work because you won't have to fight a non-standard approach. You want the underside of your joists sealed anyway to keep rodents out.
DC,
"You want the underside of your joists sealed anyway to keep rodents out."
The advantage of moving the insulation above the subfloor is you no longer care what happens in the joist cavities because they completely open and no pests can use them.
You don't end up with a well insulated space, but for small outbuildings in our climate, 1 1/2" of foam on six sides is more than enough to reduce the heating to an incidental amount, and provide comfort. Any complexity is offset by not having to add interior finishes to the walls and roof.
Malcolm --
My experience is that the joists bays are never completely empty, and rodents will nest if there's anything they can perch on, like a pipe or a piece of romex. And once they're nesting they start exploring their environment, and they way they explore is by chewing on things.
DC,
Yeah it only works if they are completely bare. The one I posted does get robins nesting on the dropped beam. But where I particularly like it is when the outbuilding will be close to the ground. Then having the joist bays empty effectively moves the vulnerable parts of the floor further above grade. I wouldn't have been comfortable closing off the bays on this one I did for an architect, which can be picked up and moved (And no, I didn't pick the colours).