Roof paint for a car? (Repel heat / keep interior cooler)

[Jez77]

I remember reading an article on different paint colours on cars.
Essentially there was no difference in the maximum internal temp any car reached regardless of the colour, the only difference was the amount of time it took to get to that temp and there was only about 10min in it.

[Shaneajanderson]

Originally Posted by Jez77

I remember reading an article on different paint colours on cars.
Essentially there was no difference in the maximum internal temp any car reached regardless of the colour, the only difference was the amount of time it took to get to that temp and there was only about 10min in it.

I guess I have a hard time believing that. I have a white pickup and we just sold our black van. I could get in the pickup in the summer sun, roll the windows down and it was fine immediately. The van however was always dramatically hotter inside, though I never took temperature readings, it's very obvious from just what I've seen with my own cars it makes a difference.

[Shaneajanderson]

I'll also add the van also had a black dash and black leather seats, possibly the worst combination anyone ever came up with.

[Ecky]

Buy a $5 IR gun on Amazon and go around a parking lot one day, pointing it at people's cars.

[California98Civic]

Originally Posted by Shaneajanderson

So here's what I think I'm going to do.

I need some of this roof coating for my camper anyway, so I'm going to get a bucket of that. Then I'll get some automotive paint in a few colors: I'm thinking red and blue (a light and dark variant of each), black, and white. I'll get some test coupons of some thin aluminum from work, paint each color on one of them, and coat one with the roof stuff. I'll let them sit in the sun for about 3 hours (or until they all reach a steady temperature) and record their resting temp.

Any input on this test? should be some interesting data.

Edit: I will paint ONE color on each coupon.

Tests are always welcome around here, but this one might not tell you much about what you would actually experience in your vehicle: the greenhouse effect from all the glass and closed interior as well as the cooling effect on the metal of hurtling down the roadway at X mph are not accounted for in this planned test and they will effect actual heat and comfort in the car.

I can touch the inside of my black roof from my driver seat through a hole where the sun visor used to connect before I deleted it. I have tested repeatedly the temp difference between how it feels when I first step in on a sunny day and how it feels when I have been driving. It's often the difference between scorching hot and cool to the touch.

I don't doubt your test can find differences in temps correlted to color. I have seen the tests that show higher temps inside a black car than inside a white one after a few minutes, even after a few minutes of subsequent AC use to bring temps down. But such tests assumes we get into a 130* car and just sit there with the windows and doors closed for 10 minutes sweating while the AC slowly brings the temp to 85*. Who does that? No, we open the windows to let the hottest air out and start driving. The ambient temp comes down fast as hot cabin air escapes and the metal temps on the exterior of the car also drop (how fast?), regardless of color.

I think light colored interior is where it is at, though. Anyone who has driven a car with black leather seats knows they do not cool off, like ever.

[Ecky]

I prefer a dark dashboard, because a light colored one can be blindingly bright in some conditions. Everything else can be light though.

[Shaneajanderson]

Originally Posted by California98Civic

Tests are always welcome around here, but this one might not tell you much about what you would actually experience in your vehicle: the greenhouse effect from all the glass and closed interior as well as the cooling effect on the metal of hurtling down the roadway at X mph are not accounted for in this planned test and they will effect actual heat and comfort in the car.

I can touch the inside of my black roof from my driver seat through a hole where the sun visor used to connect before I deleted it. I have tested repeatedly the temp difference between how it feels when I first step in on a sunny day and how it feels when I have been driving. It's often the difference between scorching hot and cool to the touch.

I don't doubt your test can find differences in temps correlted to color. I have seen the tests that show higher temps inside a black car than inside a white one after a few minutes, even after a few minutes of subsequent AC use to bring temps down. But such tests assumes we get into a 130* car and just sit there with the windows and doors closed for 10 minutes sweating while the AC slowly brings the temp to 85*. Who does that? No, we open the windows to let the hottest air out and start driving. The ambient temp comes down fast as hot cabin air escapes and the metal temps on the exterior of the car also drop (how fast?), regardless of color.

I think light colored interior is where it is at, though. Anyone who has driven a car with black leather seats knows they do not cool off, like ever.

Good points. I was mainly thinking this would be a good way to start testing for cheap. Ie. if I found significant differences here then I could build a box with a window in each color, etc. But if there was no significant difference on even just a flat panel there would be no need to invest for time and money into the effort.

So then another thought I had: what effect does window tint have? afterall it blocks most light from getting through, but being black does it reflect that light, or just absorb it and turn it to heat? (Full disclosure: I have done 0 research on this thought yet, just popped into my head.)

[Shaneajanderson]

Originally Posted by Ecky

Buy a $5 IR gun on Amazon and go around a parking lot one day, pointing it at people's cars.

This is an idea. No way to control for exposure time though, unless I sit and watch them for a while. Maybe with a large enough sample size that wouldn't matter.

I like medium grey dashes, seems to be the best middle ground. Also flat finished, I've had a couple cars with satin dashboards and they were terrible with low angle sunlight.

[hayden55]

Originally Posted by Jez77

I remember reading an article on different paint colours on cars.
Essentially there was no difference in the maximum internal temp any car reached regardless of the colour, the only difference was the amount of time it took to get to that temp and there was only about 10min in it.

I've always read the exact opposite.

"The researchers developed a thermal model that predicted the air conditioning capacity (rate of heat removal) required to cool each vehicle to a comfortable final temperature of 25°C (77°F) within 30 minutes. (This is an industry standard for vehicle air conditioner performance.) Based on the experimental measurements, the analysis predicted that the capacity required to cool the cabin air in the silver car is 13% less than that required in the black car."

"Using a white or silver paint (solar reflectance = 0.60) instead of a black paint would raise fuel economy by 0.44 mpg (2.0 percent). It would also decrease carbon dioxide emissions by 1.9%, and reduce other automotive emissions by about 1%."

https://eta.lbl.gov/news/article/110...duce-emissions

I know the Prius has a 4500w HVAC so that would equate to about 292.5 watts saved for that 30 minutes then level off. That's pretty significant given its smaller hybrid battery and how much using the hvac kills the battery around town when the engine is off at stop signs or coasting.

I remember reading a more detailed report in 2016 on window tints, and vehicle colors but I couldn't find that one.

I second the IR gun in a parking lot. You could easily measure roof surface temp and cabin temp with them. I'm gonna do that one day when I'm roaming around campus with my IR gun.

I'd say when you get some measurements buy a can of white plastidip and have at it.

[California98Civic]

Originally Posted by Shaneajanderson

... So then another thought I had: what effect does window tint have? afterall it blocks most light from getting through, but being black does it reflect that light, or just absorb it and turn it to heat? (Full disclosure: I have done 0 research on this thought yet, just popped into my head.)

I have very high quality (and expensive) ceramic and metallic heat rejecting tint on my car. I can feel very significant and Marge difference if I roll the window down versus rolling it up from the Sun on my skin. That's not science of course, but it confirms some of what testers have shown.