Atlanta is embracing a cheap, effective way to beat urban heat: 'cool roofs'
Atlanta is embracing a cheap, effective way to beat urban heat: 'cool roofs'
White or reflective roofs significantly reduce temperatures. Atlanta is the latest city to mandate their adoption.
Walk outside into 100-degree heat wearing a black shirt, and you’ll feel a whole lot hotter than if you were wearing white. Now think about your roof: If it’s also dark, it’s soaking up more of the sun’s energy and radiating that heat indoors. If it were a lighter color, it’d be like your home was wearing a giant white shirt all the time.
This is the idea behind the “cool roof.” Last month, Atlanta joined a growing number of American cities requiring that new roofs be more reflective. That significantly reduces temperatures not just in a building, but in the surrounding urban environment. “I really wanted to be able to approach climate change in the city of Atlanta with a diversity of tactics,” said City Council member Liliana Bakhtiari, who authored the bill, “because it’s far easier to change a local climate than it is a global one.”
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This seems as good a place as any to ask this question that has recently been on my mind.
Would a spaced secondary layer act as a sort of barrier to the direct heating effect of sunlight hitting the roof, effectively shading the entire thing and reducing the load on an interior AC?
I understand insulation exists, but keeping the largest upward facing surface of the building from direct sunlight feels like a no-brainer to me.
That's called an attic. And yes, attics do help the floors below get less warm.
When you have an attic, you can go further by insulating the roof - this keeps the warm day air out, and during the night you can open windows to let the cool night air in. Historically roof insulation was done with thick layers of thatch, though light-weight synthetic alternatives are more common in modern construction. A well-insulated roof won't let through any appreciable amount of heat.
Then as things get hotter, build the roof taller, allow for natural air flow to dissipate the heat, and finally put the building on stilts so air can flow under it.
Retrofitting existing buildings to have space for good insulation is expensive, especially with the atrocities the US has been building in suburbs for the past 80 years.
Yes, I have an attic. Through it runs my AC ducts which are insulated pretty poorly from what I can tell. And yes, additional insulation above the ceiling of the interior spaces. The attic itself is at deadly temperatures pretty much all summer long, because attic ventilation does not adequately clear the heat. This has been true of every house I've lived in since I was a kid. (Which is a lot of houses because I'm old)
It's difficult for me to imagine that permanently shading the roof and leaving an air gap above it would not improve things in addition to the presence of insulation and the attic itself, since it would lower temps in the attic. Regardless of the presence of insulation, reducing the delta between the attic temp and interior temp seems like it would be a win to me. My question, ultimately, is how much would a spaced layer above the roof impact this. To me, it feels like it would impact that delta a lot, but maybe not.
A shade structure? Yes, a shade structure will dramatically reduce load on the AC of whatever is under it.
Would that help the larger city climate? Well sure, if it's trees. A giant umbrella over the top of a skyscraper? Who knows
I have to admit I was really thinking of houses - it seems to me a small expense for such a potentially substantial improvement, especially if done at construction time.
(None of this is to disagree with the idea of using lighter/more reflective roofing materials, FWIW.)
Edit - and I don't mean a giant umbrella or similar, I feel like a typical home could easily just have a structure the size of the roof and spaced a few inches out from it.