How much heat can be saved by insulating under the floor, in the crawl area of my house?
closed as too broad by Niall C.♦ Jan 25 '16 at 15:04
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According to what I've seen, insulating basement ceilings (which is roughly the same as what you asked) has relatively little effect on energy use, though it will make the floors a bit warmer. If you wear slippers or have carpets/rugs, that may not be something you'd ever notice. The usual suggestion for cost-effective energy saving is air sealing first, then making sure the ceiling of the top floor is deeply insulated, then look at what else needs attention.
In the US, many utility companies will pay for a specialist to evaluate your house and make recommendations. Of course you may or may not agree with those conclusions, but more data is always good, especially when free.