The usefulness of a radiant barrier sandwich – an exercise in futility?
I’ve been throwing around a few ideas, mostly to myself, that I wanted to open up for discussion. One of them has to do with radiant barriers, and more importantly, a double radiant barrier.
For example, suppose you built a cathedral ceiling, with the layers as follows, inside to outside. Asking for a friend, it completely doesn’t already exist this way on a project in my back yard.
1/2″ drywall, 2×6 rafters, R21 fluffy, 19/32 OSB (taped seams), #15 felt paper, 1.5″ foil faced polyiso (foil facing out, taped seams), 2×4’s on the flat, 7/16 LP TechSheild (taped seams), #15 felt paper, arch. shingles.
In climate zone 4A, or mixed humid, my thoughts were that having a single radiant barrier is good, so two must be better, eh? the foil facing on the TechShield guards against some of the IR transmission from the (160 F+) shingles in the summer, and the foil face on the polyiso reflects another portion of that back up into the TechShield, (also making for one of those optical Fresnel nightmare problems), and so on. In the winter, any IR loss from the outer surface of the polyiso will (should, might) be mitigated by the low(?) emissivity of the foil face on the polyiso.
In the worst case, it seems that it’s only a small waste of money, in the difference in cost between regular 7/16″ OSB and the TechShield. In the best case it helps to any degree, and since the building is small, and mostly unconditioned, the extra $50 will pay for itself in the long run.
Anyone care to offer up a thought? It’s not a permanently habitable living space, and the effective R value seems to be around R27. It’s not code in this area, but it’s probably not far off by U value.
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