>>11121553Plastic Duct brand window insulation kit. It is the kind you use the provided double sided tape then put up the plastic over the window and use a heat gun/hair dryer to tighten up the plastic. When properly installed you can't see the plastic because it is so clear and tightens flat so well when heated. That is really great for daytime when you want light. Any window that lets in direct sunlight will help warm the room if the window is double pane or has the window insulation kit plastic over it.
At night, I recommend using a panel of foam board (blue/green Styrofoam panels for building are 4'x8' and perfect for this). Just rig something up so that the foam board lays flat over the window (if the window has an insulation kit over it) or that fits into the recess of the window. Having a recess for the foam to set into allows you to use thicker pieces of foam which will result in a higher R-value and better insulation properties.
Make sure to use caulking around the windows to reduce or stop all air flow through any cracks. Ultra clear caulking is good for sections you don't want showing up. This also helps keep insects from coming through such cracks as well as prevents them from hiding in those cracks. For any window that normally opens up, you can use clear tape over the seams to further stop any air flow. Be careful around wood as the tape may cause finishes to be pulled off the wood when the tape is removed. If that is going to be a problem you can just use a putty/clay to temporary fill the cracks.
Some people use insulated curtains, but those are only useful if they block any air flow from reaching the window/insulation kit. White is the best color to help reflect IR and prevent thermal transmission from materials to the air. So, both sides of your foam/curtain should be white to help further prevent transfer of heat to the outside.
