not an expert on this, but will tell you what i know.
if you enclose the space while leaving closeable / openable ventilation, you'll have something that works with seasonal variations in temperaure. It'll keep out cold winter air and / or the hottest heat wave air -- but only for temporary cold spells and heat waves.
Long-term average temperatures are still going to seep in and establish another average temperature inside the enclosure, which is still better than before. Even with ventilation open, the temperature inside will be better than before, since air (ventilation) does not equalize temperature immediately.
Extreme swings in temperature get dampened, and a new average temperature gets created inside your foundation space belwo your floor.
Two seasons out of four, you can close the vents for a certain length of time, which you manage and control. Most of the time, you'll have to leave the ventilation open. I don't know the answer to jimbo's question, but I think a vapor barrier won't work well enough to do what is required over a "long" period of time being enclosed, because there is no or little air circulation if you have a 99% airtight space.
Humidity eventually gets through or around any barrier. Ventilation removes the ever-higher humidity.
Foam hung under the floor is good too. It's heat insulation applied at the right place. If the house enclosure is not made totally of foam but is made airtight, it will do the job.
David