Crawlspace dilemma and recycling fiberglass batts
Hey Fellow Builders and Consultants,
I’ve got a semi-unique weatherization project that I’m about to start and need some advice.
The building in question is a 24′ x 24′, single level, four-season cottage on piers, ranging from 3′-4′ tall from grade, with a loose-fitted board skirting on 3 sides, and open on 1 side; very high and dry and well ventilated “crawlspace”……sort of.
The floor deck is 12″ I-Joists with 8″ fiberglass batts and 1′ rigid foam rips, loosely stuffed into the joist bays. (Very leaky!)
The ceiling inside, is tongue and groove pine, nailed directly to bottom chords of roof trusses, with no air barrier, (very leaky!) and 8″ fiberglass batts over it, with gable end vents, (very well ventilated attic).
My attic plan, is to pull aside the fiberglass batts, apply a 1″ layer of closed cell spray foam against pine boards from above (R-7), re-install the batts (R-24), and recycle the good-conditioned fiberglass batts from the floor deck, installing them cross-hatched over first layer (R-24), for something like R-50 total.
My floor deck plan, is to apply a 3″ layer of closed cell spray foam in I-Joist bays, against the subfloor from below (R-21), then cover bottoms of I-joists with 1-1/2″ Poly Iso, to combat thermal bridging through I-Joists (R-9.5) and tape the seams, for something like R-30 total.
My questions are:
1. If I’m careful to air-seal the layer of rigid foam on the joist bottoms, is there any type of insulation that belongs in the remainder of the I-Joist bays, other than spray foam, that isn’t too risky? (there will be 9″ unfilled space left, after 3″ spray foam) (I’ve read Martin’s article “Building an Unvented Crawl Space”, but unsure this scenario fits various prescriptions therein.)
2. Should I consider another type of rigid insulation board to apply to the joist bottoms, due to poly-iso’s poorer cold-weather performance?
3. Should I not worry about thermal bridging through the I-Joists, which is minimal compared to dimensional lumber joists, and just add more closed cell spray foam?
4. If I omit the layer of rigid foam on I-joist bottoms, and spray thicker closed cell in the joist bays, do I need an ignition or thermal barrier? ( I’ve read Martin’s article “Thermal Barriers and Ignition Barriers for the Spray Polyurethane Foam Industry,” and I’m a little unclear how to handle this particular scenario, given that there isn’t really a crawlspace…..sort of.)
5. Does my attic plan makes sense; should I “flash and batt”, re-using the fiberglass (both layers), over the closed cell foam, and call it good?
Appreciate any thoughts you all have!
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