Floor Insulation and Moisture Concerns in Hot Climates, Vapor Barrier Sandwich
In a house with a *fully* open crawlspace in a hot/humid climate, is it safe to insulate a plywood subfloor from below – even when the finish floor is impermeable (laminate, most tile, some hardwood ULs)? The dreaded “vapor barrier sandwich”…
I know that plywood is a vapor retarder… but if it has no way to dry out (because of foil-faced rigid foam below it and, for example, an impermeable tile underlayment like Ditra above), what happens to the moisture it takes in? “What moisture?” Well, s*** happens… maybe warm, humid summer air finds its way into the joist cavities. Maybe a big spill in a kitchen or bathroom makes its way to the subfloor (boards separate, grout wears out). Maybe someone leaves an AC setpoint too low.
Here’s Dana and Martin discussing the matter (Replies #3-5, 18)… Dana seems to share the concern, Martin not so much, but I wonder how they’d feel about a hot climate? Reply #14 is scary. https://www.greenbuildingadvisor.com/article/insulating-a-wood-framed-floor-assembly
Also, could a too-thin insulation layer cause problems in a warm climate? Dana once wrote: “In an air-conditioned house the bottom edges of 2 x 8 or shallower joists can drop below the dew point of the crawlspace air unless you add an inch of unfaced EPS.” It’s one thing to say the joists are thermal bridges (if the joist bays are spray-foamed), code only requires R-13 floors in Zones 1-2 – and 7 ¼” of lumber (9.06) + ¾” subfloor (0.94) + ¾” hardwood (0.68) + air film (0.92) is nearly R-12 by itself! With 2×10 joists, these “thermal bridges” would be *above* code-mandated R-values. Am I missing something?
EDIT FOR POSTERITY: I’ve since learned that I was missing something obvious — if only, say, 2″ of a 7 1/4″ D joist is covered with foam, then the thermal bridging does not start at the bottom edge of the joist (for that whole R-12), but rather at the point where the foam ends… so, you only get 2″ of the R-value of lumber, not 7 1/4″. So, for each time I run the numbers for a given foam thickness, I’ll need to enter a different u-value for the amount of joist that I’m leaving exposed.
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