I just ran across this thread. I built a house in 2002 in Ashland, WI. I estimated the heat loss without any allowance for heat loss through the crawlspace floor. The crawlspace walls have 4" of XPS foam.
The estimated heat loss was 13,000 BTUH for a 1320 square foot house based on 70 deg F inside and -20 deg F outside. The actual heat loss came in at 18,000 BTUH.
Then I got my hands on a newer copy of the ASHRAE Handbook of Fundamentals. That book, unlike previous editions, had a calculation for heat loss through basement floors. The calculated heat loss through my crawlspace floor was almost 5,000 BTUH.
I vote to insulate.
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