Radiant Barrier – Either Here or There

A roof radiant barrier has been the industry standard for a number of years in residential construction in keeping an attic cool in hot climates. This is done by applying a low emissive foil to the bottom of the rafters in a ventilated attic. More recently, we have been sealing our attics and applying spray foam insulation at the bottom of the rafters for an even cooler attic. This is a highly effective solution to cooling attics in our climate as long as the radiant barrier is adjacent to an air space. Applying any type of insulation so that it is in contact with the bottom of the radiant barrier renders it ineffective. The insulation must not touch the radiant barrier in order for it to reflect radiant heat properly.

A radiant barrier is a material with low emissivity combined with an air space to block or reflect radiant heat gain in a home rather than absorbing it. A material with low emissivity is one that does not absorb heat, such as aluminum foil. Radiant heat, such as heat from the sun, is transmitted through space rather than conducting heat through contact with another medium.

This is why the air space is so important. The radiant barrier blocks the radiant heat as it is traveling through space. If there is contact, then the heat will continue through the foil rather than be blocked, nullifying the effects of the foil. When radiant heat is absorbed by a material, heat then moves through conduction. If radiant heat is not allowed to be absorbed then conduction is greatly reduced. The air space prevents the conduction of heat while the foil reflects the radiant heat.

A better application of a sealed attic is to create an air space between the insulation and the radiant barrier. This can be done by using baffles to create the air space or by using a metal roof installed on furring strips creating the air space. In the case of the metal roof, the metal acts as the radiant barrier.

Some people have built with spray foam insulation and a radiant barrier foil without leaving an air space. The cost of creating the air space can be a deterrent because there may be more labor and more materials involved. While the radiant barrier is ineffective, the results are still good as compared to a conventional vented attic. When using a composition roof in this application, there is no point in applying the foil if it will be in contact with insulation.

However, the metal roof is a far better choice of a roofing material than composition and the air space cost nothing extra. By choosing metal and a sealed attic, you are creating the best possible application for your home.

Green Living Tips!

I welcome inspirations from your home to put into future newsletters. Email them to me; Cammi Klier. Thanks!!

If you happened to have an unfinished beer (maybe some people don’t have that problem), sprinkle it over your compost pile and turn it over to give a boost to the micro organisms. A beer can also be flushed into a septic system for the same micro organism boost.

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