Vented crawlspace – Floor insulation
Martin Holladay’s “How to Insulate a Cold Floor” article (https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor) brings to mind a question: if I insulate a floor over a vented crawlspace with a combination of cavity batt insulation and taped rigid foam, will I create a moisture problem in that the cavity would have no way to dry?
Alternately, would the floor surface itself (T&G plywood + hardwood floor) be “vapor open” enough to permit drying inward?
Additionally, if I did add rigid foam below my floor joists would I need to be sure that my rigid foam layer provided at least 16% of my total R-value to keep the dew point out of my floor? (I’m in Zone 4 Marine climate zone).
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Replies
J.P.,
In Zone 4, your crawl space won't be cold enough for you to need to worry about the ratio of exterior rigid foam to fluffy insulation. Everything will be fine.
There aren't any cold surfaces in this assembly to permit condensation.
I've heard of a few (rare) problems when homeowners install vapor-impermeable flooring (like sheet vinyl flooring) in a hot, humid climate, and then aggressively air condition the space (cooling the space to 65 degrees, for example). If there is no insulation in the floor assembly, and outdoor air has access to the floor assembly, then the subflooring can get damp in those circumstances. But you've got rigid foam in your assembly, which makes everything perfectly safe.
JP -关于你的计划对自己有益;I assume you have your plumbing and some electric runs on the bottom of the joists. With the rigid foam, are you worried about cutting around that? Or losing access to it?
Max
Max....good thought - one I hadn't considered. Washington State requires R-30 min in a floor, and I can get extra energy code compliance credit if I bump that up to R-38. Right now I'm trying to determine a) the best (easiest, cheapest, and most effective) way to do that and b) if it's worth it. Your point about utilities would certainly be a consideration, though I wouldn't be any more concerned about these utilities than other electrical or plumbing runs in my elevated floor assembly or within a wall.