Sizing HVAC Equipment With Poorly Conditioned Crawlspace
I’m in the Seattle, WA area (Climate Zone 4 coastal) with a minimally ventilated crawlspace (~1 ft2 vent area per 1500 ft2 floor space), with poly vapor barrier on the floor, R30 insulation on the crawlspace ceiling, and R12 rigid foam around about 2/3 of the foundation walls. The air handler, HRV, and all ducts (*mostly* separate between HRV and Air Handler) will all live in the crawlspace. Ducts will be hard metal wrapped in R8.3 insulation (3″ foil backed fiberglass). I’ve considered encapsulating the crawlspace and not insulating the crawlspace ceiling, but my building dept. says I’d need to put R10 as a 2ft perimeter covered in cement slab on the inside of the crawlspace to make this work… can this possibly be true? Are there any less expensive 2018 IRC-approved options?
Anyways, I’m not finding any options on the CoolCalc Manual J tool that fit this condition, and am trying to understand how to size my Heat Pump w/central air handler (no wall units) setup. Given the partial insulation and minimal ventilation, I feel like assuming ducting is in an “open crawlspace” is going to lead me to oversize, but am nervous that choosing “closed crawlspace” could lead me to undersize. Switching between the two puts me right on the line for a 32 kBTU/hr Mitsubishi hyper heat.
Is oversizing as a “buffer” poor practice here? Any ideas or rules of thumb?
Difficult crawlspace encapsulation requirements; concerns about duct losses in vented crawlspace.
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