The problem with cold floors is almost universal in old buildings and cottages with crawl spaces like yours. And while the typical approach is to stuff batts between the joists, there are several reasons why this is a bad idea.
First of all, it doesn’t work. Batt insulation needs to be in a wind-free area to perform well. If there’s air movement through your crawl space (and there should be, these places need ventilation), then batt insulation offers little benefit.
The problem with cold floors is almost universal in old buildings and cottages with crawl spaces…
Another problem is the need to support batts physically. Even though they’re held in place by friction between the joists right now, vibrations caused by people walking on the floor will probably jiggle it loose in time.
Then there’s the distinct possibility that rodents or squirrels will set up house in the insulation, or use it on their own DIY projects elsewhere. Any moisture rising from the earth could also seep into the batts and cause mold to form on your floor frame. Like I said, fiberglass in floor frames isn’t good. Just about the only positive thing is that the existing floor covering in the room usually acts as a sufficient vapour barrier on the so-called ‘warm’ side of the installation. No need to worry about plastic.
Do you really want to make your floor warm? It’s not easy but it can be done. The very best option involves laying down 2 inches of extruded polystyrene foam on top of the old floor, then adding a layer of 1/2″ plywood on top, secured with screws driven down into the floor frame. A new finished floor goes on top.
You can also achieve similar results by wrestling one or two 2-inch layers of extruded poly foam between the floor joists. Cut each piece so there’s a 3/4″ gap all around, glue the foam to the underside of the subfloor with construction adhesive, then fill the peripheral gaps with expanding poly foam.
Even with all this, you might still find the floor cool. Electric radiant heating mats installed underneath a new finished floor will work for sure, though it’s only economical when used in conjunction with great insulation.