Neighbor painted his house black. Will my house be hotter or cooler?
June 25, 2021 11:49 AM   Subscribe

Am I a tiny tiny tiny bit warmer, now because his house absorbs more heat? Or am I a tiny tiny tiny bit cooler because his house reflects less light? (Yes, I know, for all practical purposes, neither--our two houses are pretty close together, but I'm sure the difference is too small to be detectable either way. I just want to know which way.)

My more-money-than-sense neighbor yielded to the trend and painted his house deep deep gray--essentially it's black. It looks fantastic, and gazing out my windows toward his establishment is more relaxing, now, because I'm not as blinded by reflected light as when his house was light beige. I love it. So I'm not asking because I'm unhappy with this development.

We live in Florida and the entire, vast, exposed back wall of his now black house faces the rising, blazing, merciless sun. His previous tenant moved out, partly because it was too hot in the house. (Other two reasons: too many roaches, garage door inconvenient, thus too hard to stow his brand new BMW.)

I realize that my neighbor has just spent thousands and thousands of dollars to render this house unrentable/unsalable because it is now the towering inferno in there and he will shortly regret this decision and have to spend many thousands more dollars to repaint. So he's in a bad, bad way because of the decision, no question. But as always my first thought is what about me? Am I now an infinitesimally small increment cooler in my house because there's less reflected light? Or am I an infinitesimally small increment hotter in my house because his smoldering charcoal briquet retains more heat for a longer time?
posted by Don Pepino to Science & Nature (14 answers total) 11 users marked this as a favorite
I can’t answer your question exactly, but this scenario strikes me as the inverse of the “cool pavement” problem.

Some cities such as LA have been experimenting with installing “cool pavement” that reflects most sunlight instead of absorbing it as heat, with the goal of reducing the temperature of the urban environment. However, it seems that in many cases, it actually has the opposite effect — the reflected sunlight gets absorbed by people walking on the pavement, making them feel hotter.
posted by mekily at 12:08 PM on June 25, 2021 [3 favorites]

I'm talking out of my butt, but it'll depend on how far away their house is from yours, instantaneous air temp, and the duration of sunlight.

You might be infinitesimally cooler during the day but radiating heat during the night might make you infinitesimally warmer during the night.
posted by porpoise at 12:09 PM on June 25, 2021 [12 favorites]

Reflection is likely to be more significant than emission, as mekily observed. Taken to the extreme, you get 20 Fenchurch Street, London, which literally cooked cars and the Vdara death ray in Las Vegas.
posted by SPrintF at 12:25 PM on June 25, 2021 [6 favorites]

Not a scientifically rigorous answer, but the most logical guess I can come up with.

In the first scenario (light colored house nearby) your house is heated by (mostly) visible light, including some reflected from the nearby house. Your house absorbs some of that energy, reflects some away. Of that which is absorbed, some eventually is radiated to the environment as infra red. Given that some energy is transferred to your interior and heats it, the infra red energy radiated away must be less than the total absorbed.

Your house, even though light colored, has been a heat sink.

The same is true of the nearby house. In the second scenario (dark colored house nearby) the black house is a bigger heat sink, reflecting much less light, absorbing more. The infra-red radiated toward your house is less than what had been reflected as visible light. Your house should be cooler. Measurably so? I don't know.

But how about over the long term? After the sun goes down, and visible light isn't heating your house anymore, is the stored energy in your neighbor's house keeping you from cooling off as fast? A totally different question, to which I also don't have a definitive answer.
posted by wjm at 12:36 PM on June 25, 2021 [2 favorites]

But how about over the long term? After the sun goes down, and visible light isn't heating your house anymore, is the stored energy in your neighbor's house keeping you from cooling off as fast? A totally different question, to which I also don't have a definitive answer.

The answer would depend on how close your home is to your neighbors. Across the street? You probably won't notice. But right next door? Your wall (depends on insulation if the heat of his house will make it to the interior of yours) will be hotter due to thermal mass and heat transference during the night. This is why buildings in hot climates should either be built with slight materials, limiting heat transference, or should be incredibly thick and well-insulated.

Plants not used to the higher heat will probably suffer for a while.
posted by The_Vegetables at 12:43 PM on June 25, 2021

Hmm. In the minority, and certainly could be wrong, but I think hotter, assuming your houses are of roughly the same height.

Most direct light hitting a light-colored house will move harmless away from your house. Some will hit pavement, etc., and be reflected again or absorbed, but it's not concentrating.

A dark house physically close to yours will absorb the direct light as heat, diffuse it down across the whole structure, and then warm the surrounding air or emit it back out as IR and warm your house that way.

I'd reverse this opinion if the house was much taller; in that case it would be reflecting more light directly on you if it were light colored, but be radiating out more it's heat far from your place if it were dark colored.
posted by mark k at 12:48 PM on June 25, 2021 [2 favorites]

My intuition agrees with mark k, but also wants you to set up a simple experiment. Three light boxes and a dark one, set up in two matched pairs in the sun. Measure their temperatures morning, midday, and night.
posted by clew at 1:14 PM on June 25, 2021 [2 favorites]

I assume the neighbor has AC, so also consider that is now working harder to offset the hotter house and the heat pump is dumping more heat energy into the neighborhood. So that may also warm you up an infinitesimal amount depending on where your house is relative to the condenser.
posted by willnot at 1:45 PM on June 25, 2021 [4 favorites]

Hotter. A very teeny tiny bit, but hotter nonetheless.

The way your neighbor's house affects yours is by its infrared radiation. Convection and conduction, the other two modes of energy transfer, will not be in play here as the air around it, stoked by the temperature of the outside walls, will just waft up on calm days, and on windy days will get mixed with lots and lots more air that hasn't been warmed up by his house. Plus, air is not that good an energy transfer medium.

Now the amount of energy transmitted by infrared is related to the absolute temperature of an equivalent black radiator, mumble mumble Boltzmann constant mumble Kelvin mumble fourth order mumble. So as he has turned his house from already too hot to live in into an oversize pottery kiln it will radiate more infrared. And as the layout of your and his house hasn't changed (unless his starts deforming or melting) the amount of incoming energy at your house will rise.
posted by Stoneshop at 1:54 PM on June 25, 2021 [2 favorites]

I would expect the result to depend mainly on how much glazing you've got facing the dark house.

If you used to get a lot of reflected light coming into your house from theirs, and get less now, your house will now be cooler; their dark paint will be shifting more of the incident radiation down to infrared, which is much less good at getting through glass (which is why greenhouses work).

On the other hand, the total amount of heat energy absorbed by their house will now be a fair bit higher, and if your place is close enough to theirs you might get some of that inside by convection, if the wind is right and your windows are open.

And depending how good your wall insulation is, the infrared radiation from their place might warm yours a tad more than the reflected light used to, purely because their house will now be radiating for more of each 24 hour period.

I would also expect the net effect of all of these changes to be so small as to make distinguishing their signal from the noise of normal day-to-day temperature variation close to impossible with anything short of a truly heroic measurement effort.
posted by flabdablet at 3:04 PM on June 25, 2021

Fun question. I think the answer depends on the both the geometry and also the wavelength dependence of the absorption (albedo) of your house. A white house next door is reflecting more 0.5 micron radiation and emitting less 10 micron radiation than a black house. In equilibrium and ignoring other forms of heat transfer, the total power leaving their house will be the same if you add it up over all directions. Convection (of the formal kind and also the "warming air that happens to be moving past anyway" kind) probably reduces the long wavelength emission by a factor of something more than 20% and less than 90%.

My naive guess is that your light colored house paint, like many common materials, is probably a pretty good absorber at 10 microns, and that the amount of the day that directly reflected sunlight bounces of their house and hits your house is also small. In that case your house is probably a tiny bit warmer than it would otherwise be. If you've got left over house paint and a thermometer you don't mind destroying, you could try to test it.

Covering your facing wall with aluminized mylar is the obvious solution. (There may be aesthetic reasons not to do so .)
posted by eotvos at 3:04 PM on June 25, 2021

The black house will heat up but it doesn't have enough mass to store a lot of that heat. So it's not like a big old heated up brick because there isn't any way for real long term storage and slow release of heat.

Pavements that are dark do increase the heat island of cities, partly because they are coupled with the earth and consequently warm that heat-sink.

Your neighbor's dark house will only slightly radiate heat because it isn't going to absorb enough heat energy to make much difference for you. If a coin was flipped, I would bet on you seeing a bit more heat but I cannot give any real reason unless that house is built out of masonry.
posted by mightshould at 3:12 PM on June 25, 2021 [2 favorites]

It’s probably cooler during the day, but a bit warmer at night.
posted by Grandysaur at 12:12 AM on June 26, 2021

The urban heat island effect causes the whole of urban areas to get higher temperatures than nature. One of the solutions is lighter roofing color. Seems likely that the opposite is raising the temperature in your neighborhood.
posted by flimflam at 8:55 AM on June 26, 2021

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