It does sound crazy that a few dollars worth of aluminum foil can help keep the heat out of your house. Putting any sort of reflective material between a heat source and a target will reduce the transfer of radiant heat. The same principle is used for everything from survival blankets to spacecraft.
As aluminium has excellent thermal properties, it is also ideally suited for protection against heat and heat radiation.
Aluminum foil keeps food colder than plastic wrap when food is removed from the refrigerator or freezer. In both cases, food must be wrapped tightly with no holes. Aluminum foil is also better for keeping in oxygen and moisture.
The reflective surface will reflect heat and the matte side will reflect less heat . If you're baking or defrosting, the matte side will absorb more radiant heat and reflect less infrared heat while the shiny side will reflect more of both, so it makes more sense to bake and defrost with the matte side facing up.
Aluminum has a high thermal conductivity, so it is a good heat conductor when placed in direct contact with another hot object. However, aluminum can also be used as a reflective insulator because it can reflect radiation from the sun.
The trick is to use the right type of window shades to block the heat and to install them as close to the glass as possible to get the maximum benefit. For insulation purposes, cellular or honeycomb shades are the most effective. The honeycomb shape helps to trap air, reducing your solar heat gain.
Block the heat
Shade windows and walls using external coverings, like blinds, awnings or large potted plants. Plant deciduous trees that cast shade over your home in summer, but still let the sun shine through in winter.
Aerogel is more expensive, but definitely the best type of insulation. Fiberglass is cheap, but requires careful handling. Mineral wool is effective, but not fire resistant. Cellulose is fire resistant, eco-friendly, and effective, but hard to apply.
The kitchen foil on windows trick
If you're desperately attempting to keep cool, then take a roll and spread it on your windows - especially those that get direct sunlight. Windows magnify heat, so by using foil you're reflecting the sun off the window, sending it away from your home.
Materials with a low thermal conductivity like cardboard are perfect for making insulation. These are used to keep heat from transferring between objects or locations. A good example of this is when faced with a window being broken and left without replacements on hand.
Water is a one-of-a-kind substance for many reasons. An obvious one is its unique ability to absorb heat. Water is able to absorb heat - without increasing much in temperature - better than many substances.
Insulators: Non-metal objects are used to slow down the transfer of heat. Different materials are better insulators than others. Good insulators include: wood, cloth, Styrofoam, cork, and plastic. (Glass and air also act as insulators.)
Stay in an air-conditioned indoor location as much as you can. Drink plenty of fluids even if you don't feel thirsty. Schedule outdoor activities carefully. Wear lightweight, loose-fitting clothing and sunscreen.
Foil does not work by preventing the conduction of heat as other insulators do. It works by reflecting radiative heat back towards the source. So it drives your heat back into your home…and not out through itself and into the ether.
Objects with a high heat capacity absorb a lot of energy before getting hotter, while things with a low heat capacity only need a little energy to change temperature. Since the aluminum has a lower heat capacity, it contains less energy and thus cools more quickly to room temperature.
So, now that you know the reason behind the different appearance of the two sides, you're probably wondering if you should use aluminum foil with the shiny side up or down. Quite simply, it doesn't matter. Since both sides are made of the exact same material, they will perform exactly the same way.
Materials with a low thermal conductivity like cardboard are perfect for making insulation. These are used to keep heat from transferring between objects or locations. A good example of this is when faced with a window being broken and left without replacements on hand.
If you are in a pinch, bubble wrap is an excellent insulator for summer windows. Simply spray a light mist of water on your windows, and lay bubble wrap against them to act as another layer of glass. The bubble wrap will stick as long as you need, and it works to keep the hot out and the cold in.
Emergency management agencies specifically recommend using aluminum foil-covered cardboard that goes between windows and drapes.