How do you make a passive solar window heater?

How do you make a passive solar window heater

Passive solar window heaters allow you to use the heat of the sun to drive up the internal temperature of a room without using electricity. Sound like science fiction? It’s not! All you need is a couple of household items and some free time and you’ll be basking in your free heat, post haste.

A solar-powered passive window heater utilizes the phenomenon of color-based light absorption (the idea that certain colors absorb light and emit heat at higher rates than others) to throw off heat absorbed from the sun’s rays.

While solutions like this one won’t replace your 12,000 BTU window heating unit or your solar attic fan, a solar-powered window heater (also referred to as a solar box heater) will add a few degrees in a small room and can be just enough to make a difference in the right circumstances. It’s like a solar water heater, but for your room!

What You’ll Need

  • One slightly-larger-than-window-sized piece of styrofoam, cardboard, or lightweight sheet of material
  • One utility knife
  • One can of matte black spray paint
  • One box of tinfoil (you won’t need the whole box, just enough to cover one side of the cardboard)
  • One, preferably south-facing, window


  1. Take the cardboard, styrofoam, or lightweight sheet and measure it against the south-facing window. Cut the sheet down to size with the utility knife until it overlaps the edges of the window frame by about one-inch on each side. You don’t have to be exact about the sizing, but it should, at the very least, cover the window and leave you enough room to hang the device from the wall in some way.
  2. Wrap the sheet in tin foil. While you can choose merely to cover one side in tin foil and secure the foil with tape, I find it easier to simply wrap the entire sheet with foil and add some pieces of tape at the end of the process. It’s just easier this way. The foil should be tight around the sheet and there should be no gaps.
  3. Take the entire contraption outside (if you’re not outside already) into a well-ventilated area. You’re going to cover the entirety of one side of the foil-covered cardboard/styrofoam in matte black spraypaint. Make sure it’s a matte finish, not glossy. You want the final color to be as dull, dark, and unreflective as possible. Wait for the paint to dry completely in that well-ventilated area.
  4. Three inches from the top of the sheet cut a one-inch-high by four-inch-wide slot into the horizontal middle of the sheet. Do the same three inches from the bottom of the sheet. By the end of this process, you should have two slots cut into the horizontal middle of the sheet, one near the top, and one near the bottom.
  5. Take the device and attach it to the inside of your south-facing window with tape, hooks, nails, or any other type of fastener that suits your fancy. Ensure that air can flow freely between the two slots you cut into the device.
  6. Make it look pretty. You can decorate it however you choose, as long as you make sure that the black side remains unobstructed and air can flow between the slots.
  7. Have a beer. That’s an order.



How on Earth does this work you might ask? It’s pretty simple. After you’ve got the whole thing set up you’ll notice that the painted side of the cardboard sheet faces outside. You’ll also notice that a south-facing window catches the most sun during the winter. Therefore, in the winter and more often than not, the sun will be shining through this window and striking the matte black side of the cardboard.

The reason black looks black to us is that it absorbs virtually all visible light and reflects very little of it. That’s especially true of matte black shades (glossier shades throw off more reflection). But thanks to the law of the conservation of energy, that visible light has to go somewhere. So instead of being reflected off into the world in the form of light, it’s absorbed by the paint, transformed into heat, and emanates off into the air (or into your pool, in the case of similar solar pool cover or solar sun rings).

So why did I cut two slots into the sheet you continue to ask? You are a curious one, aren’t you?

Depending on how you installed the passive solar window heater this step might have been redundant. The idea is though that you need to keep a constant flow of air around the heater. If you allowed room around the sides, top and bottom of the heater for air to flow then you didn’t need the slots. Air can simply naturally move around the heater. The hot air will flow out of the top of the device while cool air is pulled in underneath. Remember, heat rises.

But if you installed the heater in a fashion that doesn’t allow for air to flow around the device (perhaps your window is recessed, for example), then you will need the slots to allow for proper airflow. Cool air will get sucked in through the lower slot and hot air will blow out of the upper slot. (Want some more interesting DIY projects? Check out our guide to making your own solar still.)

Final Thoughts

Like solar curtains, a passive solar window heater likely won’t solve all of your heating needs, especially if you live in a cold environment. But that’s not the idea. Do-it-yourself tricks like these allow you to shave that little bit off of your energy bills and be a little bit kinder to mother Earth without breaking the bank or taking too much time out of your life.

Solutions like this work especially well in small rooms or areas, like those found in outward-facing closets, bathrooms, or even recreational vehicles (RVs). You’ll find that in those environs, you don’t have space for a full window unit heater. Instead, passive solar window heaters like this can take the temperature in a room up a few degrees and make life just a little bit more comfortable. (Want to see more cool solar products? Check out these solar weather stations.)

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