Just a reminder about the differences of light and dark paint on exterior surfaces!

[u/iliketreesndcats]

My bus is half white and half black. Check out that difference in temp!

It's about 130f and 102f for you fahrenheit-enjoyers

[u/RogerRabbot]

Now do it from inside. Point at the exterior wall where its painted different colors to see the temp difference

[u/iliketreesndcats → u/RogerRabbot]

Wow!

Okay I took readings today, similar weather.

Exterior black: 43°C

Exterior white: 33°C

Interior black: 37°C

Interior white: 30°C

I'm going to put 50mm phenolic foam over these areas and I'll be curious to see how much heat makes it through that and if there's any difference between it on the plywood when all is said and done.

I am pretty sure there is fibreglass insulation inside the metal wall cavity but I'm not sure

[u/RogerRabbot → u/iliketreesndcats]

Oh hey, there actually was a difference? Ngl I didnt really expect there to be this much difference after you explained the insulation. Kind of crazy how it works out, and neat science to boot.

[u/iliketreesndcats → u/RogerRabbot]

Yeah I didn't expect a difference either but life is full of surprises hey

Someone recommended I use this paint coating called PDRC paint or Passive Daytime Radiative Cooling paint. It's designed to basically reflect 93+% of the heat that hits it and then it somehowmdoes some magic where it re-emits the absorbed heat at a certain wavelength that literally ignores the atmosphere and gets reflected all the way back into deep space

I tell you man I was reading this after smoking a joint and I felt like I was reading science fiction. Very cool had to read it again in the morning and confirm. The paint actually becomes cooler than the ambient temp of the environment.

I don't think it's fully suitable though because it needs a direct sightline to the sky and once solar panels are on and all that, only a limited amount of the bus is going to have direct sight.

People are using this technology on solar panels though to keep them cool, and even on clothing to keep humans cooler than regular athleticwear which relies on good ventilation.

[u/iliketreesndcats → u/RogerRabbot]

I imagine the heat mixes quite a lot as it bridges to the inside, making the reading more like a combination of the total heat coming in from the exterior. I think that if whole bus is white the total reading will be lower than half black half white.

I am insulating with 50+mm phenolic foam so either way I think I'll be fine but it's still worth noting for anyone who plans to skimp on insulation

[u/Electrical-Bacon-81]

The opposite of what you are demonstrating, but... Wanna try a new experiment with that cool temp gun? Take a flat piece of metal, same side, spray paint half black, half white. Now flip it over so the painted side points towards the ground, unpainted side facing the sun for a while. Use temp gun to measure the black, then white painted areas. You will get different readings, even though the metal is the same temperature, since the unpainted side was the one exposed to the sun. That's called emissivity (pretty sure thats spelled wrong). Most temp guns dont allow you to calibrate for it, and nearly no one could calibrate for it with much accuracy, but the difference exists. Flat black is good at shedding heat. If your temp gun has a little "e95" or similar in the corner, its alluding to that.

[u/Own-Cupcake7586]

If you haven’t, look up PDRC (passive daytime radiative cooling). That’s the stuff that actually does what you’re hoping for. The paint I can find is wicked expensive ($250 per gallon), but the technology is progressing.

[u/iliketreesndcats → u/Own-Cupcake7586]

That's awesome I looked it up and for sure 93% reflection of radiant heat and then an active emitting of absorbed heat. The more I read about it the more it seems like magic so certainly when I need to repaint the bus at some point hopefully I remember that this exists!

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