In moderate or dry climates without the threat of sustained subfreezing temperatures insulation between floor joists makes sense.
Insulate floors crawl space.
Department of energy recommends r 11 3 insulation under floors in warm climates and an r 25 6 to 8 in cold climates.
That doesn t mean it s a problem but it isn t needed.
Unlike fiberglass rigid foam insulation does not absorb water support mold or allow air to pass through providing that joints between boards are sealed with tape.
If as you say the floor is only dirt covered with a vapor barrier then there would be heat loss there.
Insulation is to keep heat from moving from one place to another so if the crawl space is heated then the insulation is not needed.
Where winter temperatures are extreme opt for insulating the walls and sealing off the crawl space entirely.
This type of crawl space also allows insulation with an r value of 19 installed in the floor joist if the crawl space and has a permanently installed dehumidifier.
A crawl space under a house helps protect against certain pests but a crawl space with abundant moisture can invite problems including mold wood rot and vermin.
Insulation also helps to preserve the air quality and reduce energy costs.
Fiberglass insulation batts or rolls are the most economical and easiest diy choice for insulating between the floor joist in a crawl space.
Crawl space insulation as a part of overall home air sealing and insulation helps maintain your entire home s energy efficiency without insulation heat and cool air are easily lost through the floor.
However the insulation needs to be treated with a fire retardant.
Floor insulation for moderate climes popular reads.
Cold air that sweeps through a crawl space meanwhile can lead to cold floors inside the home whether or not the space is wet.
Instead of installing the foam insulation between floor joists in the crawl space the foam board is installed against the crawl space foundation walls.
In those days common building practice was to insulate the floor above the crawl space and to leave the crawl space s wall vents open so any moisture buildup would vent to the outside a.