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We are building a roof over garden table: sort of like a pergola with roofing. We are using OSB panels for insulation and fiberglass roofing sheets for weather protection. We already chose translucent roofing for availability and price, but we started to wonder whether a dark opaque roofing would offer a better heat protection from the sun.

Dark opaque roofing would become way hotter than a translucent one, but would also protect the underlying OSB panels from direct sunlight. Hot air between the roofing and OSB panels is free to circulate and convey the heat away, but some heat would nevertheless be transferred to OSB panels including with heat radiation from the roofing.

Translucent roofing enables most of the sunshine to be reflected from the bright OSB panels, but I guess a greenhouse effect comes into play and traps some heat, again heating the panels.

So more out of theoretical curiosity than out of need for actual advice I ask: Which roofing would offer cooler environment under the roof: translucent or opaque?

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Why not a light-colored opaque roof? Get a can of white spray paint, and ... –  Alex Feinman Jun 7 '11 at 13:46
    
I like Alex's idea: you can easily paint the underside of the fiberglass, or even the top of the OSB, and reduce the greenhouse effect. Seems like the OSB should be able to take it either way, if there is ventilation at the bottom and top. –  Shimon Rura Jun 7 '11 at 15:52
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You could even add some reflective insulation between the transparent roof and the OSB - even kitchen foil would do. –  mgb Jun 8 '11 at 19:43
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I measured the temperature under the roof (with translucent roofing) and in the nearby shade. The difference was one and a half degree (C) hotter under the roof. So I guess for practical purposes it doesn't really matter what kind of roofing we have ... –  SinkovecJ Jul 10 '11 at 11:05

3 Answers 3

up vote 2 down vote accepted

Technically speaking, the coolest thing to do is to have an opaque roofing surface that's a perfect mirror. Short of that, it should be painted somewhere in the red spectrum, reflecting the infra-red range. Beyond that, lighter colors are better.

Personally, I'd avoid the translucent, since I don't know whether long-term exposure to UV rays will damage the OSB.

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Based on the same physics as a greenhouse, I would expect it to be cooler under the opaque roof because:

A translucent roof allows radiation through and when this hits a surface, a portion of the energy is re-radiated in the infra-red range, which can't get back out, thus warming the air.

But for practical solutions - the comments give some nice ideas.

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OSB is sheathing...not insulation. You need sheathing, of course, but note that it won't really be providing any insulative abilities.

If you want to increase airflow under the panels, add furring strips and attach the fiberglass to those. Be sure to put down roofing felt or ice/shield over the OSB to ensure water resistance.

A dark panel would be fine, though the lighter the color, the less it will absorb the sunlight.

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