Try wrapping the coop with insulation on the outside, then cover that with plastic, and bungy the whole thing on! I did that, not pretty, but it makes me feel better. Your chickens are probably fine, I wouldn't completely close off the ventilation, leave a bit open near the top. I also put a thermometer inside (without the red stuff) to check the temp. I leaned boards around the outside of the run to cut on snow drifting in, they liked that, and put a (chick) feeder inside the enclosed area so they didn't have to come outside to eat if they didn't want too. A heated concrete paver under the water keeps it from freezing quite as quickly. I'm about done with this weather!