I've been contemplating how to best insulate the coop against the coming cold weather. I have heard good things about Foil-Bubble-Bubble-Foil insulation and was planning on purchasing some. Then I remembered I have a garage full of thick, multilayer corrugated cardboard! The stuff traps air, so it should be good for some insulating properties and it's free, so it cost nothing to try. I lined the ceiling and East & West walls of the coop with it. Since the coop is on the East side of a hill, it gets a fair amount of shielding from our Northwest winter winds, but every little bit helps.
I can't say how it compares to "normal" insulation, but on one of the nights last week where the outside freshwater tub froze over, I had put the bird waterer in the coop and it emerged frost-free the next morning! So the inside is definitely above freezing when the outside was in the upper teens/low 20's. My next experiment will be to line the inside of the cardboard on the ceiling with cheap aluminum foil so it will reflect radiated heat back down. I also plan to line the inside of the North wall with plastic film and fill the space with shredded newsprint for more insulation. Need to set up some temperature sensors so I can see how much difference all this is making. That's something I can do tomorrow if I have time.
I'm still keeping the fresh air vents, but I need to block them from direct wind, or there will be strong, cold drafts inside come winter since they face North-South.