How It's Made

• You can make solar window boxes from a variety of scrap materials such as old storm windows and spare wood. Regardless of what you use to make it, the key components are always the same: A shallow box about 6 to 8 inches deep, and as wide and long as the window you plan to use it in; black backing to absorb the heat of the sun; and a glass covering to allow sunlight in while trapping the heat.
  The black backing of the window box is shorter than the box itself. One- to 3-inch spaces are left as airflow vents at both the top and bottom to allow air to enter and exit the box.
  
  

How It Works

• The glass covering of your solar window box allows sunlight to enter the box space. The black material at the back of the box heats up as the sunlight hits it, and this heat transfers to the air contained within the 6- to 8-inch depth of the box. The glass cover also works to help contain the heat and allows the air to warm further.
  The open spaces in the black backing act as inlet and outlet vents on your window box heater. The bottom opening allows cool air from the room to enter into your window box. Since the sun is shining into the box through the glass, that air immediately starts warming up. As the air gets warmer it rises within the box between the glass and black backing, until it eventually reaches the top vent. Once the air has warmed enough to reach the top of your box, it escapes out the top vent and back into the room as heat.
  
  

Tips

• While solar window boxes can be made with almost any type of material, a few simple adjustments can make it work much more efficiently.
  Use metal for the backing. The box frame and backing can be made of wood, but since metal heats up faster and hotter you'll get better results by using metal as the heater backing. Copper is an excellent heat conducting metal and it can be painted black.
  Insulate the frame. Building a window box frame with insulating materials can help prevent heat loss.
  In North America, you'll get the best results from a solar window box heater by placing it in a south-facing window that is not shaded by trees or awnings in the winter.