Is it protected from the wind? I'm guessing the crawl space might get pretty darn cold. They'll stay alive but not active at the bottom of their comfort range. What species are they?

You could try a garden heating mat w/ thermostat under neath to keep them comfortable. It might work better than the light bulb. The light bulb could do the trick in a closed space though. I'd be wary of fire hazard what ever you do. You could insulate w/ straw bales also.

Here is a link that might be useful: Heat mats

I would insulate with insulation. Maybe a water heater blanket that Home Depot was trying to push on me last time I was there.

When I was talking to them at Homedepot I was told that a R-19 insulation should do the trick for an outside bin I was thinking of.

I live in Raleigh. Although I have a similar crawl space I am going to keep my worms in my garage. Now that my babies are eating, mating and just enjoying themselves I don't want to slow them down.

I have had my bin for a few months and I have no smell or flies, so it passes the wife test.

Good luck...