How Aluminum Foil Insulates
This entry was posted on August 11, 2014.
Many insulating materials come with a layer or multiple layers of aluminum foil. In fact a basic radiant barrier can function properly with a single layer of aluminum foil. Given that metals conduct heat rather well, especially aluminum, it might seem strange to use it as an insulating material. Here's how it works.
Heat Transfer Basics
Heat is really a measure of the energy of the atoms in a material. The more energy they have the higher the material's temperature. This energy is transferred in three ways: by conduction, convection, and radiation (and don't forget that heat always flows away from its source, from hot to cold.)
Conduction happens when thermal energy is transferred through a material. When atoms are tightly packed, as they are in a solid metal, the energy moves easily.
In a gas the atoms have more space, and the energy moves more slowly. That's why bubble materials are effective like down in a winter coat, which creates similar micro-air pockets - they slow the rate at which heat moves.
Convection takes place when a liquid or gas expands. This makes it less dense and it rises, letting cooler fluid take it's place. That's why we say that heat rises.
This leaves us with a puzzle: how does heat from the sun reach us through the vacuum of space?
The answer is as light, both visible and invisible, or infrared. The sun radiates light, some of which passes through the earth's atmosphere to be absorbed by plants, soil, water, and so on.
Absorbing Radiant Energy
Some materials absorb light, and hence energy, better than others. A shiny material like aluminum tends to reflect a lot of heat while wood and concrete absorb a much higher proportion. That means aluminum is slow to warm up when exposed to radiant heat.
Re-radiating, or emitting
Everything that absorbs heat tends to give it off again. (Stand near a brick wall that's baked in midday sun and you'll feel the heat being radiated.) And just as some materials reflect better than others, some radiate heat better. This property of being a heat radiator is termed emissivity and it's given a value from 0 to 1.
(Scientifically speaking, emissivity is the ratio between how much heat a material emits and the amount emitted by a 'perfect' emitter, otherwise known as a black body.)
Iron and steel are quite good emitters and have emissivity numbers in the region of 0.5. (This is why they glow red hot when heated in a furnace or by a welding torch, they are radiating heat.)
Aluminum has an emissivity of around 0.04. That means it radiates very little heat away from its surface, which is one reason why radiators aren't made from aluminum!
Insulating with Foil
Aluminum foil can be an effective insulating material because it doesn't radiate heat out into the environment. That's what makes it effective directly under a roof: although it will warm up through conduction from the shingles, it won't radiate that heat out into the attic space. In HVAC applications for example, the foil blocks heat from entering or escaping keeping the temperature where it is wanted inside the ducts. This same concept is used in many other residential and commercial applications.
And that's how foil insulates.