A lot of people have window unit air conditioners to keep specific rooms of the house cool during the hot summer months. With temperatures dipping, it's a smart and energy-efficient idea to uninstall the A/C so you can properly close the window to prevent cold air from coming inside. For many people, this isn't possible, so do the next best thing: Keep it cozy. Start by wrapping the unit with fiberglass insulation (there are eco-friendly recycled varieties at home improvement stores). Then cover the insulation with plastic sheets and keep it all in place with duct tape. This will block drafts, protect the unit and prevent moisture from building up during icy winter days.