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Radiant Barrier

Mark P

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If I've ever seen this before I don?t remember and if I?ve ever know anything about it I have forgotten. It is a reflective radiant barrier covering the attic floor of a 1967 ranch. There is 3-4 inches of fiberglass underneath. My initial reaction is someone wasted a lot of money buying a product that provides little or no benefit. Is there an R-value calculation for this product? Anyone have some boiler plate on this product they would like to share? Opinion, Thoughts?.?

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Well, it only works if the aluminum is sparkling clean. Dust eliminates its ability to do what it's supposed to do.

It is now forming a cold side vapor barrier over the insulation in a part of the country where it gets cold enough to condense water vapor in that attic. Now, as moisture from the air migrates upward through the insulation it's going to hit that cold vapor, cool to dewpoint, turn to water and accumulate. When enough has accumulated, it will drip down onto the ceiling below and soak the ceiling and then they'll have a really nice little biological experiment growing in a very large petrie dish.



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There's literally dozens of these products out there. Some will do exactly what Mr. O said, but many of them are perforated to be vapor permeable. They are electrically conductive and have relatively low flash points so I would be more concerned with fire than fungi.

All of the technical data I have seen on these is bogus. The R-values are derived from simulations that don't jive with their intended application (E-Shield's published R-value of 11.4 was attained by a sandwich of '3/4" plywood, E-Shield, 6" dead airspace, 3/4" plywood at 70 degrees F) or calculated from extrapolations and unrealistic assumptions. Anecdotally, I have heard people swear they reduce cooling costs. I can't measure that because the foil renders my IR camera useless.

Aluminum is expensive. They could have blown in a foot of cellulose for about half the cost. Lucky for you they didn't, it looks pretty close in there as it is.

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Go to the research


for unbiased information.

Basically, though radiant barriers work if installed properly and they are useless when covered with dust. They must have at least one side with an air space or they are "short circuited".

I like radiant barriers but much prefer it factory installed on the roof decking.

It makes a big difference in sunny climates, especially when the builders insist on sticking the a/c equipment and ducts up in attics.

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