I found a quick and effective way to insulate house windows during times of extreme hot or cold weather. It is called “radiant barrier”, it is about 3/8” thick and comes in rolls of typically 24” wide. There are several suppliers and it is usually found in home centers. Since it has reflective foil on both sides bubble insulation inside, I found it works well on windows to reflect solar heat in summer or keep heat inside during cold winters. Of course since this covering is blocking all light, you will probably want to use it in extreme conditions, or in unused rooms.

Step 1: Cut and Apply to Windows (and Doors)

Materials are pretty simple.
Roll(s) of "radiant barrier" insulation from your local building supply or home center
Double stick tape, or
Velcro

Cut panels to the size of the windows you want to insulate. Use your double stick tape to attach the panels to the window. You can do this to a whole door also, a lot of heat will come through a thin door that is exposed to full sunlight or cold.