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Community and Q&A

Insulating a Cabin Floor

MountainManDave | Posted in GBA Pro Help on

Greetings. I recently built a small off grid stick-frame cabin in forests of New York State and moved my family out of the growing chaos that my once loved nation has become… Anyways!

I have rarely ever had to build something that wasnt on a proper foundation. On the few occasions I have, the insulation process was subbed out to insulation contractors and I devoted my attentions elsewhere.

The cabin is on stilts. I have used ALL code Compliant methods and r-values throughout, mainly because they are what I know best. That is until the floor… the underside of the cabin is open. The floor being 2×8 joists, I opted to put r-19 fiberglass in the floor bays and covered with a good layer of plastic sheeting. I left a 2 inch airspace between subfloor and batt tops. I know it isn’t right but hell, the materials were free and I was on a budget! My floor stayed warm and comfortable all winter despite the lack of skirting. Wood heat is an amazing thing! In the spring I climbed under the cabin and discovered condensation had gotten caught by the plastic and the batts were one wet mouse house. I had expected rodent issues, it’s a forest, but the moisture was a problem.

This year I bought 3inch foil face rigid foam board and was going to install in place of batts and spray foam the seams for a good air barrier. I can only afford so much board. Do I insulate the floor again, the batts kept us perfectly toasty, and avoid the plastic or do I use the foam board around the perimeter of the cabin and secure with lattice or something? If the later, do I vent the space? Should there be plastic on the ground under the cabin? Each time I ask someone I get a different answer. Winter is back. My floors are too cold and I need to get the most out of money. Help very much appreciated!

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Replies

  1. DCContrarian | | #1

    To avoid condensation, either the vapor barrier has to be on the warm side of the floor, or there has to be roughly 30% of the insulation value as foam on the cold side of the barrier (the exact number depends upon your climate).

    I think insulating the floor for a house on stilts is the right way to go. I'd go with faced fiberglass batts with the facing facing up. Then something mouseproof across the bottom. My preference would be 1/2" plywood, but I realize that's really expensive right now. 1/2" OSB would probably work, you'd have to be careful not to chip it and create openings for the mice, they only need a hole the size of a dime. Hardware cloth works too. So does Durock or similar cement underlayment board.

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