Create a 'conditioned crawlspace'.

A 'conditioned crawlspace' is one of the most energy efficient types of foundation systems and can usually be done for little cost

In most cases, all that will be required is to insulated the walls of the crawlspace to a thickness of R-10 to a depth of 4 feet all around your foundation, eliminate any vents to the crawlspace, place a vapor retarder over any exposed soil in the space, then 'condition' the crawlspace by heating and cooling it.

This will give you a very warm, energy efficient house, at minimal cost.

Here is a link that might be useful: Conditoned Crawlspaces

I can't access the crawl space. It's not high or wide enough to actually crawl into. In order to add any insulation under the floor I'd have to rip up the floor and that's what I'm trying to avoid. I'm looking for alternative solutions.

I had nearly the same problem in a room addition (although the crawlspace was bigger, it was still completely inaccessible). I did elect to pull out the floor, install insulation, seal the edges, and re-lay the floor over a new sub floor. It really didn't cost that much (my handy man did it, we ran some new wiring for new electric and cable outlets while we were at it), it was done in 2 days, and made a huge difference in the comfort of the room. I didn't do it as well as I could have--at the time didn't think of adding insulation to the walls of the crawlspace too, or wrapping the ducts with insulation.

If you are willing to add a layer to the floor, with all the changes you'd have to make to trim, doorways etc, plus expense of new floor surface (and I don't think you could get much insulation value that way either) it sounds like you don't mind sacrificing the original wood floor. So why not pull it up?

The only other option I can think of would be blown-in insulation, but you couldn't use cellulose, and I do think although I am not sure, that even with fiberglass the space would still have to be protected from moisture. Insulating foam might be an option if there is more than one point of access to allow even coverage of the underside of the floor.

I'd be interested to hear what you decide & how it turns out. Our older houses present many challenges.

As wasteful as it sounds, the cheapest solution may be to raise the heat in the house or in individual rooms. Put space heaters in the rooms you use most. Keep the heat from leaving these rooms by placing drapes on spring rods hung in the doorways.

There is a carpet system called Flor that sells its carpets in 19 inch squares that you piece together. Some of the styles are washable in the sink, so you can just pick up one square, wash and dry it, then set back in place.

raee_gw I am wondering what you mean when you say "it didn't cost too much"? I have exactly this issue. Just enough space under the floor to let the cold air up; I'm pretty sure it's bare ground with no vapor barrier as we live in a dry climate (ave 27% humidity or less unless it's raining). Our room is right in the middle of the house (the original house portion which has been added to multiple times with concrete subfloors).