DO NOT INSULATE THE FLOOR JOISTS! That isolates all plumbing below it from the source of heat. Even if the pipes are insulated, if left unused, it will loose all latent heat and freeze.
I recommend the following. 1/seal all crawlspacw venting air tight; 2/ Insulate the foundation walls (Foam board works well); 3/include your crawlspace in your heating (Your floor is only as warm as your crawlspace, unless you have in-the-floor heat. I've used these measures with dood success.
As far as your furnace running a lot, I suspect that your cold crawlspace may be a symptomn of a greater problem. Air is probably leaking out of your house (Probably into your attic)and being made up by outside air being draw in low (Your crawlspace). Other syptomns would be snow melting & ice building up on your roof. Solve the leakage problem, and other problems will be gone or minimized