Is it worth putting insulation between the joists with a craw space under the floor?”
Consider two cases. Vented and unvented crawl spaces. If the crawl space is vented then it’s a must to insulate between the joists. Vents in the crawl space bring cold air flow in the winter. Non-vented crawl spaces may also require floor joist insulation. If the crawl space is not insulated…walls and floor, then you will still get a loss of heat above. Heat “flows” from warm areas to cold areas. What makes the floor and room above cold is the loss of heat into the crawl space, not cold coming through the floor from the crawl space, except in cases where air currents come through the floor. The best way to insulate a non vented crawl space it to attach rigid foam insulation to the walls and cover the floor with plastic, and then putting insulation down on the plastic covered surface. Insulating between the floor joists, increases the your savings in heating costs but is not strictly necessary for comfort.
However, I have neither insulation nor vapor barrier under my floor. I have NO moisture problems, and I have never had my incoming water line, which lays on the dirt under the floor; freeze, in one of our short cold snaps. Insulating above (as in your other question) makes good sense, as it will slow the escape of heat in the cold times and slow the encroachment of heat in the hot times; but the soil temperature remains pretty constant, so the only times your underfloor insulation is holding heat where you want it is when the outside temperature is between room temperature and soil temperature.
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