We discovered pretty decent wood floors under the carpet in our home. As DIYer's, we bailed in hands first! After removing the carpet in the dining room, my boyfriend questioned the concern of the hardwoods having a subfloor for extra support and insulation? I've jumped around the room and the whole house starts to rattle. So the flooring is pretty solid! Refinishing the flooring can be done. But how do you insulate to prevent air loss due to no subfloor? Do you have to have a subfloor? They didn't use them with the floor when it was built and the flooring is still in really nice condition? By the way, we live where the ground shifts/cracks to settle. A dry, hot climate. Thanks for reading and replying

No subfloor was a common practice in the 50's and 60's here. Many older homes have no subfloor and the floors are up to 90 years old and performing well.
Finish will fill gaps and you could fill any gaps prior to the last sanding sequence.

You can also seal up the crawl space and have it conditioned through the HVAC system.

Stephen,
Thank you for responding! We live in North Texas and believe the house was built around 1920's. There is about 10 to 12 inches of crawl space with only small cut outs to access under the house. I did crawl under to see if there was a subfloor. The support beam run patella and the floor boards run perpendicular to them. The floor from underneath looking up appears very tight and no signs of damage. From the inside, the floors have good character wear and have some give in doorways. How should I best address my boyfriends concern over air/heat loss and matter of no subfloor? Any and all suggestions are greatly appreciated. I have remodeled a pier and beam house and insulated with R30 and chicken wire. Would that be a solution? Thanks again.

Since you have no sub-floor there will be more deflection between the floor joists.
As the floor gets thinner with sanding, this weakening because the flooring is thinner will show up in doorways with the top of the groove separating from the rest of the plank after it gets stressed over the years.
We refinish fir floors a lot and after the floor has been sanded a couple of times there are broken edges of planks in doorways.
If you are planning to insulate your floor which involves crawling around over the dirt I recommend that you strengthen the floor by adding some blocking between the floor joists. This involves adding a wood strip that is glued with sub-floor adhesive to the underside of the planks. Nails, staples or screws to hold the strips up until the glue dries will keep the strips against the flooring.
It would only take a 2 inch wide rip of one-half inch plywood to make a big difference. You will notice the difference immediately.
Then you would add the insulation, using stiff wire between the floor joists to hold up the insulation.
With all the crawling around under the floor on the dirt I would include a moisture barrier before doing any of the sub-floor/ finish floor work. 6 mill plastic sheeting is the right moisture barrier to use, cutting carefully around any piers, just laid on the dirt with rocks or dirt clods to hold the edges down, overlapping the edges by 12 inches.
You may be able to get free plastic sheeting from a roofing company as they cover orrfs when rain is threatening and pitch it when the job is done. I have put a couple of layers down over the years. It makes it much more pleasant to lay on your back to work under there.