Quote:
Originally Posted by Piwoslaw
^^ The only comment to the above is that the IR radiation has to penetrate the roofing to get to the radiant barrier, after which it gets reflected and returns the same way. In effect, it goes through the roofing material twice, leaving some of its energy (heat) in it each time. This can make the roofing warmer than without the barrier, and the higher temperature can shorten its lifetime.
This is why it is better to reflect heat closer to the source.
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No wonder I have seen many houses with bright aluminium foil over the roof instead of under it. But anyway, when those reflective materials were introduced here, they were also meant to provide some degree of watertightness when applied under the roof.