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Metal roof: pros and cons of going white

My shingles need to be replaced and I'm having a standing seam roof installed. Buy once cry once... Anyway, I'm a little bit torn on the color. Aesthetically, I'm pretty flexible, since I'm also having the stucco re-done soon and will be able to color it whatever will look good with the roof. I live in zone 5b in New Mexico. We need more heating than cooling, but the summer heat can be pretty unbearable. I use evaporative cooling due to the low humidity so my cooling bills are fairly low.

Obviously a white roof would be the best choice for reducing the cooling loads. But what would the winter heating penalty be? I'm having a hard time finding anything other than extremely general information like "the winter heating penalty will be small and offset by the summer cooling gains." This seems like a very California-centric point of view!

I tried the energy-star-affiliated roofcalc.com but it predicted that the increased winter heating loads would result in the consumption of an extra 100 therms of natural gas. Can this possibly be right? Is there a better way to figure this out?

Asked by Nathaniel G
Posted Sat, 06/21/2014 - 20:38

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Nathaniel,
If I were you, I would make the decision on aesthetics, not energy performance.

There will be an energy penalty during the winter if you go with white roofing. But the effect of solar radiation on your roof during the winter is less than it is during the summer, because the summer sun is higher in the sky than the winter sun. So you may still want to choose white roofing, in spite of its winter disadvantages.

The effect of white roofing really only matters if your ceiling is poorly insulated or if you have ducts in your attic. And if you have either of those problems, it's best to address those problems directly -- by creating a conditioned attic, by moving or insulating your ducts, or by increasing the thickness of the insulation on your attic floor.

If your house is well insulated, and your ducts are in the right location, the temperature of the air in your attic is irrelevant.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 06/22/2014 - 05:16
Edited Sun, 06/22/2014 - 05:18.

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That's sort of what I was leaning toward. I do have poorly-sealed ducts in the attic, but they're only used for the furnace, which I'm hoping to replace with a ductless mini-split heat pump once the building envelope is better-insulated. The attic has R-11 fiberglass batts between the lower truss chords and about 5" of cellulose piled on top that somebody in the past added. Not great, but not abysmal, either.

However, thermal imagery confirms that the ceiling drywall gets hot when the sun's shining, and that heat radiates down into the house. So clearly the insulation up there isn't enough.

Answered by Nathaniel G
Posted Sun, 06/22/2014 - 10:49

3.
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Nathaniel,
You wrote, "Thermal imagery confirms that the ceiling drywall gets hot when the sun's shining, and that heat radiates down into the house. So clearly the insulation up there isn't enough."

It sounds like it is time to call up your local cellulose installer.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 06/23/2014 - 05:34

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