I intend to keep my 10,000 btu window a/c in the window all winter. I bought a Frost King silver cover, but I want to insulate the sides. What type of insulation can I use that will be water proof? I don't want the winter sleet/snow or rains to destroy the insulation.
When I had window units in my NW GA home, I winterised by using homemade covers that completely wrapped around the outside part of the units.
I sew and used vinyl tablecloth fabric to create pockets for that insulated foam board; I attached the cover to each unit by sliding the cover onto the unit then screwing in weatherproofed lathe strips all the way around. After the screws were in I went around the lathe strips with waterproof silicone caulking.
Worked a treat! No draughts, and no drips, kept the rooms those units were in a lot warmer from then on!
In the spring I just peeled off the caulk, loosened the strip and removed the insulated covers - stored those with the storm windows until the next autumn.
Wish I had a picture or two to upload but any home sewer will understand what I'm talking about above and would be able to make the cover for you - all you'd have to do is cut the foam board and slide the pieces into the pockets, etc.
Hope this helps. I think you can buy those wrap-around insulated covers now at most DIY centres but the homemade covers worked great for me.