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Comment by ajkjk | Hacker News Reader

It still blows my mind that the color of something makes such a difference.

Wait until you hear about how much money we could be saving if roofs were white instead of the typical dark gray. (Locale: en-US)

Wait why aren't roofs white then?

Many newer roofs are white. For example, EPDM roofs are all near-white.

Meh, my EPDM roof is black.. I think you’re thinking of TPO?

If you live in the north, it's so you don't have roof collapses in the winter due to snow buildup.

If you live in the south, it's because people think there isn't good roofing materials that can be lighter colors. Those people are wrong.

Once the snow is covering the roof the color can’t matter? This would only have a marginal effect on very mild snow to melt it before it accumulates. Snowy days also don’t have much sun to catch to begin with, clouds ya know. No, roof collapses are prevented by stronger and more sloping roof.

I can tell you're not from the north. Snowy days aren't dark and cloudy. You'll wear sunglasses sometimes. Dark roofs absolutely help melt snow. The snow starts melting near the gutters and then climbs upward. The snow also sublimates. When it gets really cold, clouds can't form so there are a lot of bright days without a cloud in the sky.

The north could (and is!) solved by metal roofs that are white or light colored and steep - steep enough that snow slides right off.

Because they produce glare for neighbors. Also tradition. Mirrors would be even better, but apart from expense and weight, worse for anyone in the path of the glare :-/ [edit: reply comment to this says mirrors wouldn’t actually be better]

But, for example, California is requiring light roofs and other mitigating issues, so there is some progress.

BTW if you do decide to paint your roof (or any other external surface) please use TiO2 paint as the UV from the sun will then cause it to break down pollutants like NOx and greenhouse gases like methane.

Mirrors have low emissivity. While they accept a bit less heat, they will hold on to it more than something with white paint (which can have pretty good emissivity) and will thus get hotter.

Thanks, that’s interesting. Al is a pretty good IR reflector, and I believe most common mirrors these days use it as the reflective coating. Is the glass the problem?

Given that window glass can get damn hot when exposed to strong sun, and situating a perfect reflector behind it would presumably double the effect, intuition leads me to guess that yes, the glass would be a problem.

Cultural norms are really hard to change. There is no reasonable world in which businesspeople in the US south and southwest should wear suits and ties. But if they didn’t, they would not be taken seriously in our culture.

They are in many parts of the world. The reasons why they aren’t in other parts of the world are many, but they include the fact that bitumen as a petroleum byproduct has been cheap; other traditional roof materials used in Western Europe are also naturally fairly dark; in cooler climates heating has historically been a more pressing concern than cooling; people migrating from said cooler climates have not bothered to adapt their construction methods to local conditions; AC powered with cheap fossil energy has been seen as a silver bullet; people choose to avoid upfront expenses even if that means suffering in the future; cooling is rapidly becoming a concern even in said cooler climates, but adaptation lags behind because people are surprised that the climate change is actually now their problem and not some future people’s problem…

There is such a thing as white bitumen, and Obama's Nobel-laureate Secretary of Energy Steven Chu had suggested we adopt it.

Just built a house in Ireland (moved here from California) and have been very glad we ignored basically all the advice people wanted to give us and went for things that make it easy to cool. High ceilings with roof windows to vent heat have made the last couple days much more pleasant. Really glad it's wood and we're not dealing with all that thermal mass people here seem so enamored of.

I also have a 200 year old stone cottage and when it gets hot it stays hot for a very long time (same for being cold).

Thermal mass makes sense if exposure to heat can be carefully controlled, in a manner consistent with solar passive principles. Ideally this would be in the form of concrete floor/brick wall behind a glazed and shaded south-facing window (northern hemisphere) in an otherwise well insulated house.

In practice, it’s generally better to just have a well insulated house in the first place. (Passivhaus being the attainable ideal.)

True, and thermal mass is amazing if you have a lot of giant rocks and use them to soak up sun and keep fruit trees warm at night, allowing you to grow fruits normally suited to much warmer climates.

My experience was mostly old brick buildings in Dublin (old enough to not be cavity wall) and having all the heat they soaked up during the day radiated back at you at night.

I plan to add retractable awnings over the windows that get a lot of sun at some point too; they can make a big difference.

And, of course, trees! Deciduous trees give you shade in the summer and kindly drop their leaves so you have light in the winter. They're amazing!

Thermal mass can definitely be useful when you don't have air conditioning. Most of the older building in countries like Peru had thick walls (as well as tall ceilings) and were far cooler than outside during the day since the thermal mass effectively buffered the indoor temperature between night and day.

True, I think I'm just a little grumpy from years of boiling hot apartments in very old buildings.

The roof of the apartment building where I live is silvery-white.

It makes totals sense. I also suppose that many people who own single-family houses would not do that, mostly because it looks odd. Looking odd is something that makes people uncomfortable.

When you find a way for a light-colored roof to look cool while also not costing an arm and a leg, people will gladly adopt it. This happened to solar panels when they started to look well and also signal wealth and care of the Earth.

a tricolour dog or cat that's been sitting in the sun is how I discovered this as a child!

How did you decide what 3 colors to paint them with for the test?

It sounds like a natural experiment to me - genetics decided the colors (I'm guessing black, white and brown: colors commonly found in multicolored cat & dog breeds)

This is also why I paint my CPUs and GPUs black.

[1] https://duckduckgo.com/?q=la+painting+streets+white

I suppose you could as well polish them very well, but I think that will last shorter time than paint.

It’s easily relatable, compare wearing a black vs white t-shirt a warm sunny day.

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