Did you add the light just for getting heat to them? I found my girls are fine as long as they have plenty of shavings. When the weather got so bad they wouldn't come out, I moved the heated waterer into the coop, which gives off a little warmth (probably not very much), and they never huddled around it, so I really don't think they're cold. Do you have some that are especially prone to the cold? I don't, so my situation might be completely different from yours.
The way the weather's been the last few years, we're not far from warming up anyway - the last few years I've been regretting not getting pre-emergent weed killer down in February, but then wound up not putting any down at all as it turned out, it was so dry even bindweed was dying. I don't really hope for weeds, but sure do hope for some rain this year.