Someone on here the other night suggested hot water bottles. Our lows have been -15 to -20 the last few nights - I have a fairly weather-tight coop - but I did try two big tide bottles and a big white vinegar bottle filled with hot water - and the girls are loving them up. I also have hay baled around the north and west sides of the coop (I get free hay
I do have a heat lamp secured but it only has 125 watt bulb in it that I have been turning on for about 5 hours at night when the temperature or the wind chill is below 10 degrees but I think the girls are pretty comfortable. When the weather turns back to temperatures in the 20's or 30's I'll turn the lamp off and just use the water bottles I think. The temperature tonight when I got home was -3 at 6 p.m. The temperature in the coop was just under 20 degrees. I should mention that I also have a big south facing window - so they are like salamanders on hot rocks in there during the day when the sun shines - even at negative digit temperatures. Anyway, my point was, the hot water bottles were a hit and in a small coop that you've made less drafty with the plastic - they may do the trick if you are uncomfortable with the heat lamp.