A radiant barrier should be installed to reduce heat gain during the warmer months and heat loss in the colder months. Radiant barriers reduce the heat transfer across the air space between the roof deck and attic floor by reflecting radiant heat.
Solar energy absorbed by the roof causes the material beneath it to radiate heat into the attic. With a radiant barrier installed on the attic floor, the heat radiated from the roof is then reflected back up, keeping the insulation cool. This then also keeps the rooms below cool by minimizing the heat that moves through the insulation, to the rooms below the attic.
A radiant barrier consists of a thin sheet or coating of highly reflective material, usually aluminum, which is applied to a substrate material. The substrate may be kraft paper, plastic film, plywood sheathing or air infiltration barrier material.
Radiant barriers must have a reflectivity of 0.9 or higher and a low emissivity, of 0.1 or lower. Reflective is a measure of how effective the material is in reflecting heat (.9 = 90% reflected) while emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody (a theoretical object that emits 100% of the thermal radiation) at the same temperature.
There are various ways a radiant barrier may be installed. Preferably for retrofit construction, the radiant barrier is attached to either the faces or bottoms of the rafters or top chords of the roof trusses. In both configurations the space between the roof sheathing and the radiant barrier provides a channel through which warm air can move freely. If the radiant barrier is reflective on only one side, the reflective side should face toward the main attic space, since a surface facing downwards is less likely to have dust settle on it.
In order to function properly, a radiant barrier must face an open air space.
For more detail, safety tips and illustrations see the
US DOE fact sheet for radiant barriers.
The barrier can alternatively be laid atop the existing attic insulation, with the reflective side up. Although this is the simplest method, there are several disadvantages: Dust will settle on it, decreasing the radiant barrier's effectiveness; traffic (if the attic is used for storage) will damage it; and moisture can be trapped where it will soak the insulation and potentially lead to mold problems. Also, kitchen and bathroom vents and recessed lights should not be covered with the radiant barrier
A radiant barrier should not be confused with a vapor barrier. The radiant barrier must allow the passage of water vapor. If there is no vapor retarder at the ceiling, moisture from the living space may condense and could freeze on the radiant barrier of an attic floor. In areas of extreme cold or long periods of cold, a layer of condensed water could build up. If your radiant barrier is applied to the roof, it is not necessary that the material allow the passage of moisture.