The idea of painting a roof or wall white in order to reflect sunshine and keep the building’s insides cool is hardly a new one, and even in the loosest interpretation of the word ‘white’ it generally works pretty well. This is also what [NightHawkInLight] found after using an off-the-shelf silicone-based coating for his shed’s tin roof, though with a few caveats.
One might say that this is mostly a problem for people who live in non-desert climates — like Michigan in this case — yet it’s undeniable that having a cooler indoors in a high-humidity climate will inevitably lead to a higher indoor humidity level. This was the first issue that was encountered, though it mostly meant that instead of running an air conditioner eight hours a day, a weekly dehumidifier session was required, which was at least less expensive in terms of kWh.
While the current silicone-based paint on the roof stays above ambient, in a subsequent test both [NightHawkInLight]’s DIY sub-ambient cooling paint and a commercial option get a sample panel down to around ambient temperature, which could cool down the roof even more. Of course, in this case the humidity issues would get worse, with likely condensation forming that would have to be dealt with.
Overall, a cooling paint on the roof is a pretty thing even if you’re not living in the desert, but you have to be able to tame the resulting humidity and condensation issues.
Given the eventual rising cost of water it might make sense to run the dehumidifier on a regular basis and collect the non-potable water for gardening and cleaning purposes. When I lived way out on the Olympic Peninsula we ran one in the basement constantly with a hose that drained into our half-basement shower pan. It was an older house with no central air and baseboard heaters so I had to make do with box fans to move air up or down the stairwell, but it was cheap and worked.
A simple solar powered fan on one or two of the peak roof vents is usually enough to draw air through soffit vents into attic spaces and prevent mildew/mold build up. I’d also recommend insulated ducting for kitchen hood and bathroom vent fans to prevent dumping heat into the attic as well.
This is all good general advice, but… Dude, it’s a shed. 🤣
The experiment is on a shed, the results can be applied in many and various situations.
Perhaps that paint could be used for some sort of passive thermoelectric generation or a heat pump to increase the efficiency of air conditioning might be possible.
A black painted surface on the hot side and this thermoemissive paint on the other, there could be quite a bit of temperature differential. Now if only you could get a PCB fabrication service to mass produce
modular thermocouple out of jumper wires that could span a roof.
Carnot said it won’t work. You need a huge temperature difference to be able to extract energy from it and this is a placebo effect (you can have maybe 10°C difference at best, while you need > 50°C at least for any power to be generated with a TEC).
A dehumidifier is expensive to run and gives off a lot of heat that’s very expensive water!
Every time I see someone try to do this they all make the same rookie mistake. They all obsess about this idea of the sky window. I get it, it’s a neat idea and it makes for a fun narrative for the video. They get to say look, they’re sending this heat straight into space! But they’re not.
The problem is that when you tune a material to have higher thermal emissivity, you also increase its absorption in the same band. So if you’re trying to target this window where thermal radiation can go straight into space, you’re also tuning the surface to absorb all of that sunlight more. But not a single one of them realizes the error.
The sky window is a senseless distraction. The goal is not to somehow magically beam the radiation straight into space. You just want the surface to emit more than it absorbs. That’s it. What you actually want to do is tune it to reflect the majority of the radiation it encounters, in other words you want to tune it to reject that sky window, and you want it to emit in other frequencies.
Also, I just saw the point where he claims that his little plywood sample acts as a control. But you can tell that it’s a different color than the other surface he is comparing it to, so it really isn’t an adequate control, it is not controlling for changes in emissivity, and I don’t know why they go to all this trouble to justify using error-prone thermal cameras when they should be using thermocouples every single time. They always make the setup more complicated than it needs to be in order to introduce new sources of error.
Hasn’t he done exactly the opposite of what you say? If he was tuning for thermal emissivity with corresponding increased absorption he’d have painted it black.
The majority of the energy from sunlight comes it at wavelengths shorter than 1 µm, and the atmospheric window is at 10 µm where you have very little energy coming in, so it is indeed possible to tune it to both reject sunlight and emit at the suitable wavelength where the atmosphere is transparent at the same time.
He addresses that in a pinned comment: The wavelengths a material emits are also those it absorbs, and as you want to minimize absorption of energy you try to “color” it in such a way that most emittance/absorption is in a region where little energy arrives. Which is exactly that sky window, because the air doesn’t enit much in that band.
As for the IR camera: It makes for a nice visual for a video. The control seems to be made from the same plywood, isn’t that rasonably close? I do agree that using thermocouples would make a lot of sense tough.
Yeah, it’s the same thing done for spacecraft coatings, except there (since you don’t have the atmosphere) you just tune it to have the highest emissivity in infrared where your material is going to be sitting at temperature-wise.
It’s also just… the entire basis of the greenhouse effect. If the Earth emitted infrared the same way as visible, we’d be much colder. The Earth is much worse of an emitter in infrared than in visible, so we heat up.
I’m not commenting on the window, but:
“So if you’re trying to target this window where thermal radiation can go straight into space, you’re also tuning the surface to absorb all of that sunlight more.”
The object is at 300 K.
The Sun is at like 5500 K.
These are not the same bands. You want the material to be white (reflective) in the visible and black (absorptive) in the infrared to emit.
Also, the sun may be an almost-ideal black body radiator, but your roof isn’t, and trying to use the properties of a BBR to debunk this fails since this material works specifically by not behaving like one.
It’s not “they” who are making the error.
You’re making the naive assumption that these surfaces purely radiate energy in exactly the same bands they receive it in. To wit – the cooling comes from converting the incoming energy from outside the sky window to inside it.
Also thermal emissivity has little to no correlation to a material’s colour in the visible range!
how does this qualify as a hack In any possible way?
beyond cooling painting in the roof any color extends it’s life (The exact same reason you paint your walls)
FFS some articles are just for general learning of random stuff. As usual the hack is in the comments
+1
But in the next video, he will install Doom on the roof paint.
Sir, you’re on youtube-a-day. The site where you get 2 paragraphs about a video you’ve already watched.
+1
I don’t even read the articles anymore, I just come here for the peanut gallery.
It is a followup of his previous hack. A chemistry hack to create the paint by himself.
it needs to be noted that heat is reflected year round. if you paint a roof over a living space in Michigan, you can expect higher winter heating energy usage. Recent research from Oak Ridge National Lab showed heating energy increase was greater than summer air conditioning energy savings as far south as St. Louis and Baltimore.
So we need paint that can flip color from white to black!
Or cover with a black plastic film for winter.
I wonder how viable it would be to tune a thermochromic coating for the purpose. You absolutely have coatings that change color in response to temperature; I’m just not sure whether that’s a ‘might work in theory; tons of practical issues’ or if there’s something reasonably cheap and durable that goes white/clear at “high” (relative to human occupancy buildings) temperatures and black at “low” ones.
This would be less likely to be a paint; but for tiles or similar prepared modules electrically flippable states would also be potentially viable if they are either passive when in a given state or suitably low energy to keep in one state or the other.
https://www.joedoucet.com/climate-responsive-paint
It would look white and black, but it’s not actually white and black that matters. There’s white paint that’s crap for thermal management because it’s got low emissivity in both visible and infrared. And there could be black paint that’s poor because although it absorbs well it could emit infrared even better.
That’s what really makes it hard, it needs to “change color” in two bands in opposite directions.
Don’t know the specifics of Michigan but if your concern is about negative temperature in winter you’d probably have snow on your roof so it doesn’t make a difference.
In fact it does, but this is adressed in the video.
The next step would be color changing paint, white in hot, dark in cold
first paint it black, then paint it white.
instead of painting the roof, install solar panels: the panels will shade the roof, and also produce energy to run A/C and heating. It costs more, but it is a win/win choice.
I live in Florida. I changed from a dark roof (melted candles in the attic) to white elastomeric roof coating over the shingles (no melted candles). Utility bill was reduced to nearly half. My parents and wifes parents painted their roofs white and also had a substantial reduction in their utility bills. Utility costs in the Bahamas are so high that usually only the bedroom has a window AC unit which is only on at night. I have been in homes there that were in the process of painting the roof white. Rooms which the roof had been painted were 10 to 15 degrees cooler than rooms which were not. In addition painting shingle roofs has a two fold benefit. (1) The shingles do not degrade. (2) The elastomeric coating prevents hurricane force winds from peeling the shingles away from the roof. This is evidanced from post hurricane observations Renewing the coating in 10 to 15 years is a huge saving over installing a new roof. For Bahamians the import duty on roof paint is nothing compared to shingles. On that note, many homes now glue and screw 3/4″ plywood directly to the roof trusses, then apply roof coating. No shingles to blow off.
if it generates water condensation, use the water for evaporative cooling, to get even more cooling.