I have not read all responses, but let me say large ducks like yours will be very cold tolerant. It has been below zero F at night for about a week here, and in the teens in the days at best. I neither heat nor insulate my coop for my flock of Blue Swedish and Khaki Campbells. It was 7 below zero F this morning with fresh snow on the ground and the ducks came barreling outside as soon as I opened their door for breakfast and probably would have slept outside in the snow last night had I let them. Really, your ducks will be fine. Pekins are large and fatty and have a great down coat on, and their waterproof oiled feathers are a great defense against the cold. It sounds like you have done a great job with their house, but I wouldn't lock them in it 24 hours a day ever really, they should go in and out as needed. Also I'd be sure the water is in their run and not their house because you risk an icy mess inside that you can't easily clean up until spring. Good luck! It's hard to leave our flock in the hands of others at times, but we just need to take comfort knowing that they have been set up for success.