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Radiant barriers can be extremely effective at lowering radiant heat transfers when used in exterior siding applications but again, it MUST have an air space on at least one side of the radiant barrier for it to work. In this application, a radiant barrier is installed like a typical house wrap yet before the siding is installed, you must attach 1 X 2 furring strips over the top of the radiant barrier to which to attach the siding. This creates the required air space for the radiant barrier to be effective.
A radiant barrier can be very effective at BLOCKING radiant heat when installed in conjunction with roofing and siding material, however, the radiant barrier MUS T be installed with an air space on at least one side of it. As long as there is at least a 3/4" air space on either side of the radiant barrier, the application will be effective at BLOCKING radiant heat.
Roofing systems that utilize purlins or batten/counter battens are ideal for the addition of a radiant barrier because the roofing easily provides for the required air space already. Metal, tile, and slate roofing systems are examples of such roofing systems where a radiant barrier can be easily incorporated.
Equally as important as the air space on a radiant barrier is the requirement that a radiant barrier remain clean and dry in any application.
The picture below shows a batten/counter batten system with a slate/tile material with a radiant barrier on top of the counter battens. The radiant barrier is installed on the top of the counter battens in non-watertight applications. This allows the radiant barrier to remain dry while also achieving the 3/4" air space requirement.
Because a radiant barrier needs an air space on at least one side, installing it directly below felt and shingles will NOT allow the radiant barrier to block any radiant heat. Instead, the heat from the shingles and felt will conduct all the way through the barrier, into the attic, and eventually in the living spaces.
If an asphalt shingle roof is being installed, you should install the radiant barrier to the underside of the roof decking or roof rafters in the attic space below.