How to Insulate a Room With Aluminum Foil. Aluminum foil will completely block out sunlight because it's opaque and reflect it because it's silver. Many commercial insulators include aluminum foil. You can use the foil rolls from the grocery store to quickly and cheaply insulate any room in your home, especially in the hot summertime.
Determine how much aluminum foil you need for each room in your home. No one window should take a full roll, but a full-plat window could come close. The amount of foil also depends on how many windows you have.
Pull off strips of the foil long enough to cover one window end to the other. You'll want to go 6 inches to 1 foot past the window's actual height or width. Tear off enough strips to cover the whole window.
Tape the foil strips onto the wall, covering up the window one section at a time. You should use duct tape or some other sturdy adhesive. Tape down every edge along the wall for holding strength and to seal out the light.
Seal with more tape any places between strips along the windows where light comes through. This step may not be necessary if only very small cracks of light are shining through.
Repeat the process for all windows in rooms you want insulated. The foil will block out the sunlight, reduce the heat buildup, help keep the rooms cool and lower your energy bills.