WATCH NOW: Secondary glazing windows with Clear Comfort.
Windows are the weakesk link (thermally speaking) in the average house. They cause more heat gain and loss than any other area of your home - even when they're closed.
To stop this you have to insulate them - double glazing, heavy curtains or some form of secondary glazing that traps a layer of still air. In this GIY project we insulate a timber sash window with a plastic film called Clear Comfort (this is an Australian product but there are a wide range of similar and cheaper products available in the US).
It is affordable (around $12/m2), easy and really makes a difference.
Trickiness rating: EASY