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.”
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.
This has always been one of those, “why the fuck don’t we do this already?” things. Also, NighthawkInLight has a video on how we could make self cooling buildings with infrared cooling paint. https://youtu.be/N3bJnKmeNJY
This is the video i was looking for!
I don’t get why we don’t do it either. I live in a dark brown brick oven… the sun literally hits the entire house because neighbors didn’t want a tree in their yard so they cut all the big ones down… even when outside it cools down, my house stays hot. Even when i leave all the windows open, as soon as i close them, it warms back up. The house itself just stays hot for a very very long time…
This has always been one of those, “why the fuck don’t we do this already?” things.
Because the black roof lobby has spent a ton of money to keep roofs black.
“It isn’t cheap” seems to be the biggest issue. Saw another video that showed it working, but he did say it was pretty expensive for paint.
sorry, I meant why aren’t we using white paint. There’s no way white paint is expensive compared to black paint.
The infrared cooling paint has other issues, like it doesn’t withstand damage easily and the surface texture needs to be maintained very well. But like, white paint is the cheapest kind of paint, just due to commonality.
Alternatively, why aren’t we growing plants on all the roofs, that would absorb even more heat without requiring air conditioning, and it would improve air quality, it could be used for farming, etc. Now that one I can understand being more expensive, but you might be able to offset it with sales of whatever you’re growing up there, but that’s really unlikely. So white paint really seems like a no-brainer.
deleted by creator
Editing to add this link from another comment because I had no idea that (of course) there’s organized opposition by entrenched interests to prevent anything for the public good https://www.motherjones.com/politics/2025/06/dark-roof-lobby-white-reflective-roofs-laws-lobbying-urban-heat-islands/
I don’t think the current roofs are painted black - they’re naturally black because roofing tar and asphalt shingles are black by default (a lot of sheet metal roofing is too but you can at least get that in most colors). So it’s not a matter of swapping out paint but adding something new. It also adds a new maintenance cost - keeping the white roof clean/maintained. Paint flakes off, tarps etc wear and become tattered, dirt and pollen collect on the surface. None of this is a dealbreaker by any means but our society seems to run on defaults and there’s a lot of inertia in construction and a lot of pressure on builders to keep materials costs down (even if doing so costs the owners or occupants more in the long run).
I hope this continues to take off because it really is a big improvement.
Yes EPDM rubber and asphalt are black. They can also just keep using those, just add a layer of white gravel on top, gravel makes the rubber last longer too.
Has anyone done studies to see if placing solar panels on the roof has a similar reflective effect?
Common knowledge round the Mediterranean.
Wasn’t there grass roofs something like 10 years ago?
Grass roofs have been around for ages, but they require a lot of structural planning to deal with weight, water, and when they inevitably have to be replaced, they are more of a hassle. Whereas paint probably just needs to be repainted, or reflective metal roofs last longer while not being too hard to fix (need a radiant barrier, but still easier to work on).
Wasn’t this debunked? I remember something about African tribes wearing dark clothes in the desert and a conclusion that the color doesn’t make a difference, it’s all about wearing loose clothing etc…
Loose dark clothing encourages airflow. Covering up prevents sunburn and direct solar heating.
For buildings, airflow can cool them too. But it doesn’t need to be done through dark roofs or dark exterior walls. And buildings could have more sunshades and awnings, and smaller windows to prevent solar heat gain.
