I actually built a similar system as an experiment, basically a 1 x 2 x ¼ meters (multi-layered) cardboard box painted all black inside and covered with a sheet of transparent plastic on one of the larger sides. It was really cheap, around $10-15 in materials. Then I added two holes (in and out), and put some cheap flexible air duct hoses with some old CPU fan, which was another $8. On a sunny day it heats up the air incredibly well, where it actually could be beneficial during the winter season.<p>Unfortunately there are big problems that I encountered that I couldn't solve cheaply.<p>1) Mounted on the roof, it has to be sturdy enough to resist heavy rains and strong winds. We get winter windstorms, and one winter it got up to 55 knots with 70 knot gusts. I have no idea how to cheaply mount this box on the roof so it doesn't fly off and kill someone.<p>2) Intake and exhaust hoses would have to go either through the walls or through the windows. And since I don't own the house, my only real option would be to cut circular holes in the windows, and then replace the glass when we move out. Again, not cheap.<p>3) For the winter months I would have to also insulate these hoses, or they would lose most of the heat on the way to the house.<p>EDIT:<p>I also think if you don't care about what your house looks like, you can either paint it black or wrap it in black plastic.