Originally Posted by ehmanta
...use plastic sheeting to add an extra layer to overhead hatches and portlights.....
I agreed with the rest of the advice, but caution this. It will work, as you trap air between the plastic and the hatch, which is the idea behind double paned windows. However, we have shades that pull over and do essentially the same thing and constantly find condensation collected between the two and have to wipe down small amounts of mildew. We can keep on top of it, because we open and close them frequently, but if we happen to leave one closed for weeks, you can really see it getting a hold. I guess, one just needs to keep on top of it.