Straw can also harbor bugs in their hollow centers. Chickens don’t need bedding, they need litter. I agree sand or something scoopable is best for keeping a clean coop. The less poop the less humidity, and humidity is what makes the risk of frostbite higher. Currently, I’m using pine shavings with a poop board underneath the roost that I scoop daily. This keeps things clean and fresh with manageable humidity. The job of litter is not to keep the coop warm, although some may argue that the deep litter method helps keep the coop warm, I have not found that to be true in my experience.