Crawspace moisture control plan. Is it right?

11 years ago

So, I finally got a quote on properly insulating my beach house crawlspace and would like to run the final plan past the experts here.

The sand floor will be covered with 15 mil plastic which will run up the walls about 6" then sealed on the edge with foam. The fiberglass will be removed from the underfloor above and 3" of closed cell soy based foam will be applied with 2" on the joists. All a/c ductwork will be sealed with mastic and returns and registers sealed within the foam insulation. The space will be vented with unlouvered Smartvents which will be kept closed (we are in a flood zone and vents are required by code).

I could save money by not covering the joists with foam but am worried that they may be susceptible to rot even if I run my Sanidry dehumidifier. I can't treat the space as open pier construction because half of it is below grade. I do have two large openings in the above grade walls, front and back in lieu of flood vents but am closing these. They are about 2 x 4 ft. Would leaving them open and adding a few more open vents be enough ventilation to leave the joists uncovered? Also, I was told by another contractor (different house, different problem) that using foam under the floor was a bad idea because of possible water entrapment from leaks from kitchen, bath etc. The floors above are limestone and prefinished hardwood. Is this true?

I'm getting ready to get started on this project. If anyone sees any glaring errors in our approach please let me know. Basically the space will be unvented and unconditioned. Is this a bad thing? Should we leave the vents open and not dehumidify? The last thing I want to do is spend a ton of money and cause damage to my house. Thanks so much for you help.

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Comments (2)

worthy
11 years ago
This is a continuation of this thread.

As the conditioned space is completely separated from the ventilated crawl space, leaving the vents open or closed and not dehumidifying would appear to be acceptable.

The "foam under the floor traps water" argument is akin to those used against spraying the underside of a roof with foam. Water from occasional overflowing sink or spilled mop bucket is unlikely to penetrate the flooring. Even if it does, the foam is permeable and any concentration of water should be visible from below.
cheri127
Original Author
11 years ago
Thanks for weighing in, Worthy.

I'm trying to address all issues before installing the foam. Maybe this question should be in the plumbing forum but I'll try here first. Because the crawlspace is partially above grade, I'm worried that the pipes down there may freeze once the floor above is so well insulated, especially if the vents are left open. What would be the best way to insulate these pipes?