How Radiant Heat Barriers Work

How do radiant barriers work?

Radiant heat barriers work by inhibiting the heat transfer process that is characteristic of your roof and attic setup. Typically, the heat soaks into the roof and heats up your attic space, whereupon everything in it heats up, including your conventional fiberglass or cellulose insulation. Once the materials in your attic warm up, they start to give off heat themselves, just like if you were to put a pan in to a hot oven, after a while in there the pan would start to give off it's own thermal heat. And, to continue the example, when you remove the pan from the oven it keeps giving off heat. This is quite common and as matter of fact, more people burn themselves touching hot pans than do touching the hot oven chamber. That is because you cannot always see thermal heat, and therefore it is easy to forget which pans have been in the oven and which have not. This form of heat transfer is called "emissivity" and it is a quality of all materials. Scientists use a scale ranging from zero to one to determine how much thermal home energy any particular substance will give off; it varies depending on the material and the surface temperature (which is directly influenced by the ambient temperatures. And the same is true of the relationship between your attic heat and your living space heat. The roof heats up because of the hours and hours of exposure to direct sun. The heat transfers through your roof and begins to heat up your attic space and everything in it. Once that happens, then everything in your attic serves as a heat source, like hot lava on top of your ceiling.

This is where the real value of a radiant heat barrier really shows. Basically, the radiant heat barrier is able to deflect this thermal, or "radiant" heat. This is known as "reflectivity" and is also measured on a scale of zero to one, again with one being the high reading.

. For a material that is opaque (that is, it does not allow radiation to pass directly through it), when the emissivity and reflectivity are added together, the sum is one (1). Hence, a material with a high reflectivity has a low emissivity, and vice versa. Radiant barrier materials must have high reflectivity (usually 0.9, or 90%, or more) and low emissivity (usually 0.1 or less), and must face an open air space to perform properly.

When the heat from the sun penetrates your roof, a radiant heat barrier is able to reflect a portion of that (usually between 70% and 90%) back towards the roof and out, thus keeping your ceiling and insulation much, much cooler. In these cases, your air conditioning has to work much less vigorously. This is why we say that the radiant heat barrier reduces the stress placed upon your A/C system.

Under simlar conditions, a roof mounted radiant barrier can work by reducing the amount of radiation present on the insulation. Because the amount of radiation hitting the top of the insulation is less than it would have been without a radiant barrier, the overall insulation temperature is significantly lower and the amount of heat running throughout the insulation is also curbed.

Radiant barriers tend to also reduce indoor heat losses through the ceiling in the winter. Radiant barriers reduce the amount of home energy radiated from the top surface of the insulation, but can also reduce beneficial heat gains due to solar heating of the roof. The net benefits of radiant heat barriers for reducing winter heat losses are still being studied.