Insulating floor on posts
Hi, i am building an small bump out addition on helical piles. The floor stack looks something like this (top to bottom):
3/4” ply
vapor barrier
2×10 floor joists with R30 batt
3/4” ply
4” extruded foam board inside of 2×10 floor beams (walls are covered with 2” CI down to match)
My main two questions are:
1) Vapor barrier – good/bad/indifferent? I know if this was a wall that vapor barrier would be a bad idea, but I suspect the different convection profile for a floor makes for a different conclusion.
2) the plan is to build a skirt wall around the perimeter to the ground hiding the piles, with a floating anchor to the ground so that it’s reasonably air tight. Because of this the plan is not to cover the 4” foam board with another layer of PT ply (or cement board). Any problem with that?
Any other suggestions would be appreciated.
Thanks!
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Replies
Hey,
First, it would be great to know your name. Second, it would be helpful to know where you home is located. Without knowing your climate, this is all speculative.
That said, I don't think you need the interior vapor barrier. I do think you should air seal the assembly at the exterior sheathing layer. You could tape the plywood seams or tape the rigid foam seams or both, but you'll have less risk of moisture issues if outdoor air is not getting into the assembly. Some builders put vapor barriers on the ground below grade level decks to stop ground moisture from wetting the deck. I'm not sure how effective this is, but I doubt it would hurt.
Right, sorry about that. This is in Toronto. Contractors around here are trained to add vapor barrier, and inspectors are used to seeing it, so if it’s okay to have I’d rather just let them add it.
All agreed on doing a very good job sealing from underneath. I’ll probably use spray foam instead of tape because all the seams are at inside corners.
Thanks!
Andy