Troy at The DIY World made a passive solar window heater for his RV using just a few simple items.
He knew that to make this project work, the heater would need to do two things:
- heat the air on the interior side of the window
- create a passive flow of air to drive the heater: cool air in, warm air out
His idea was to line a sheet of styrofoam or cardboard with aluminum foil, spray paint it black, and then cut a couple of slots in the sheet. The slots would allow the cool air to pass by the hot black panel, heat up, and exit near the top.
In order to heat the air, Troy used some science.
Why Are Black Objects Hotter Than Light-Colored Objects?
You probably know that darker objects tend to get hotter than lighter-colored objects when placed in the sun. But why is that?
Related: Clever Mod for Cold Weather RV Window Insulation
The color – and temperature – of an object depends on the amount of light energy the object absorbs or reflects.
The darker an object looks to us, the more light energy it absorbs – and the hotter it gets.
Here’s an example of why an apple is red,
An apple is red because when white light, which is made up of all visible wavelengths mixed together, hits it, its atomic innards reflect all the red wavelengths more than the other colors and bounce them at our eyes.
A black object, on the other hand,
…looks black because it absorbs all the wavelengths in white light and reflects none. As the shirt absorbs all the light coming from sun and, say, a desk lamp, the energy carried by that light doesn’t just disappear into the shirt, never to be seen again. Instead, as the light is absorbed, it gets converted to other forms of energy, usually heat, and then emitted by the shirt. The darker the object, the better it emits heat, because it’s a better absorber of light.
How to Make Your Own Solar Window Heaters
If you’re having trouble staying warm in a colder climate, you might want to first consider adding some extra insulation around the windows in your RV.
Unlike residential homes, most RV windows aren’t made of double-pane glass. Some higher end motorhomes might have double-pane glass, but for the most part, all travel trailers and fifth wheel trailers have singled-pane window glass.
Troy used just a few materials to make his panels. You’ll need:
- a styrofoam or cardboard panel a little larger than your window
- a roll of aluminum foil
- a can of black spray paint
- some standard clear tape
Step 1: Choose a window that faces South. In the northern hemisphere, South-facing windows get the most sun in the wintertime.
Step 2: Determine if you’d like to completely cover the window, or leave some space open to let light into your RV.
Step 3: Cut the styrofoam or cardboard sheet to your desired size.
Step 4: Wrap the styrofoam or cardboard in aluminum foil. Tape the foil to the panel using simple Scotch tape.
Step 5: Cut two rectangular slots in the top and bottom of the panel. Use the images below as a guide for sizing and placement.
Step 6: Using black spray paint, paint the entire panel in a well-ventilated area.
Step 7: Hang the panel over your desired window!
One of the nice things about this design is that you don’t have to modify anything on the outside of your rig. The panels just sit inside the window frame, and can be easily and quickly removed.
Outside of Troy’s RV with the DIY solar window heater in place.
Close-up of the homemade solar window heater.
Watch Troy explain how the solar window heaters work in his RV.
More resources:
- What effect does color have on heat?
- Color and heat absorption from DOE Ask a Scientist
- Another design using foil solar panels by Instructables
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