You don't say what's in the space beneath the floor. If it's earth then I would say this is a good opportunity to do the job right and cover the floor with a layer of roofing felt, and lay 1/4 inch construction grade plywood over this for a sound, draft proof subfloor, onto which you can then lay carpet or other acceptable flooring. Fact is, with 1/8 inch gaps in the flooring boards, it's a wonder you don't have an infestation of bugs and such coming up. Additionally, since heat rises, I doubt you are losing heat down through the floor. Obviously, depending on how weather tight the subfloor space is, you coud be getting drafts, which the suggested fix above would eliminate... Best of luck!