Cari, I believe aluminum foil will just conduct the heat and make it hotter, but I’m not sure. Even plain cardboard will work, AS LONG AS you put spacers of at least 1/2 inch between the house and the barrier – it must have a way for air to flow between the two pieces – otherwise it just makes it hotter. I’m a firm believer in at least 1/2 inch Styrofoam, which is cheap and reusable. Again, you need to have dead air space between the two items. Check out Sialis.com for complete information. When the temps get close to 90 degrees is when I begin to worry about the heat, especially because my houses do not have any shade at all.