Insulating a pier foundation floor in a marine climate
What’s the best practice floor insulation assembly for a structure built on a very low pier foundation in a cold and wet marine climate?
I’m rebuilding a very small, very poorly-built, addition in south coastal BC (Canada). The addition rotted out, in part due to poorly managed roof runoff.
The climate here is damp year-round, with 50%-100% humidity. Temperatures go much above 60 F for eight months of the year. Winter means nine feet of rain with temperatures in the 40s (F) while summer means a few weeks of temperatures in the upper 70s (F).
The floor joists are 6″-18″ above soil that’s pretty close to waterlogged in the rainy season.
I’m planning to install tiles over Ditra, which is impermeable.
I’m leery of installing fiberglass so close to waterlogged soil without a vapor barrier, but a vapor barrier on the underside of the joists will be wrong-side and will make a moisture trap between it and the Ditra.
Is XPS or closed cell foam the only way to insulate this floor or is there a more cost-effective technique?
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