My breeding pens have Pine pelleted horse bedding in them. They are never wet. But I'm a freak about fresh air and sunshine with all my livestock and never closing them up. Even in the coolest winter with deep snow my ewes lamb outside. They have barn access on the worst nights but it's totally open. Buildings (coops, sheds, run ins, barns), especially closed up buildings, harbor bacteria and increase humidity and decrease air circulation. They also create more work for the human. I have no desire to clean up manure. I have cattle, sheep, poultry and rabbits. My animals aren't even allowed on the part of the property where the barn is except for those 2-3 months of winter. And the main reason they are all close to the barn is not to provide shelter, it's for convenience for me for watering and more importantly to get all livestock off of the forage when foot damage will do more harm than good.
I remember way back when I had horses that even tho they had access to shelter they usually only went inside when I fed them inside. I haven't been thru cold weather yet with the chickens but now that it is cooler they don't show any signs of being bothered by it. But I think the plastic will be great for the really cold months. I am not sure if my vents are in the best place to not put the birds in drafts. I will probably try changing the location of some of the roosts so they can be further from the vents. The window is half the size of the front of the coop so it will need to be covered at times.