Quote:
Originally Posted by jeff5may
....I would recommend adding at least 4" of foam board or spray foam to fill the cavity under the radiant barrier to increase this value to above r-20.
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I never thought of spray foam under a floor but it makes sense if you cover the joists. Most people neglect the thermal bridging from studs and joists. I've seen ice form on an "Oregon code insulated" wall at 16" intervals when the temperature in the middle of the room was 70F.
Why heat the upper edge of a joist to 80F when the lower half is exposed to a cold crawl space?