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

I did some searching and didn't find much. I wanted to know what difference the particular color of a car makes. Obviously white is the best option for repelling heat, but my geo is a really light red color, and my curiosity was whether a particular color was better than any other (assuming the same...lightness?).

Also during my searching I found someone had brought up painting a car with white elastomeric roof coating as an idea to repel heat, and I wondered if anyone has heard of this.

I usually come up with the weird ideas.

You probably won't do better for heat rejection than putting this on your car's roof.

https://www.metrorestyling.com/3M-Chrome-s/366819.htm

Yes, but it's a first gen metro. If I put that on my roof I'd get run over by someone I blinded within a week.

That stuff is pricey too! I can get five gallons of the white roof stuff for ~$80 and it would cover my car and my camper (which needs to be done anyway.)

There are other options for paint out there. There is stuff that has additives to reflect more heat. I doubt its worth the extra cash looking into it for an automotive application. However, it is interesting stuff.

A quick google search found this company:

https://www.ultimatecoatings.net/

https://static.wixstatic.com/media/e...176a2~mv2.webphttps://static.wixstatic.com/media/e...d49ac~mv2.webp

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.

I chrome-wrapped my roof at the end of last summer. The problem with red paint is that it absorbs the higher-frequency (and higher-energy) end of the visible spectrum; that's why on interstate signage, the red part always fades long before the blue does. So, if you want a color other than white or silver, something toward the blue end would probably be best.

I'm not sure how much the chrome wrap is actually doing, but if I can find someone else with a silver Prius this summer it might be fun to leave both of them in the same parking lot and monitor the interior temperatures.

Wouldn't blue absorb the red end of the spectrum (including infrared, which equals heat). I understand what you're saying about absorbing the blue end of the spectrum, I deal with UV heating of materials at my job (we use a lot of black canvas and black painted aluminum in our products. The aluminum can reach 200F under a UV light in 5 minutes.)

Maybe the infrared from the sun isn't as large a factor as all the high energy stuff though, I'm not sure (again why I want to do that test.) As it is I may end up just painting the car white myself. After all, it was $350 and already looks ugly.

Sound deadeners can have some insulating quality from the heat. That's something I would like to do on my Jeep before this summer.

Well, and because it is loud in my Jeep...

When sitting still, heat rejection would mean alot, I guess. But once the car is rolling the aflow of air over the car will cool the roof regardless of color. The benefit of paint therefore is minimal. On your camper, tho... well you hang out in that more often with it stationary, so... makes more sense there.

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.

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

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

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. ;)

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

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.)

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.

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.

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.

a while ago on the Prius I found the numbers for the oem windshield heat rejection tint and added the numbers of my 3m Crystalline to it.
Windshield: OEM + 70% 3m Crystalline = 56% VLT, 79% TSER.
Drivers Window: OEM + 40% 3m Crystalline = 25% VLT, 78% TSER.
Big difference in HVAC needed, you can feel the difference on a sunny day if you roll down the rear window in the sun and pass your hand back and forth. I can usually roll around town with very minimal HVAC use most of the time none up to about 30 minutes.
Running around town on short 10-15 minute trips with the AC set on 71 Auto with the vents screaming vs not using the hvac is the difference between 39 mpg and 55+ mpg for those short trips.

https://ecomodder.com/forum/showthre...d-34665-6.html

post #57

That's probably correct.
The article I read the cars where parked with windows up for the test. Although all cars reached the same temp the dark cars got there quicker so logically the ac would need to work harder in dark cars to combat the temp rise.

In the article I read they were testing identical cars.
My black work van is particularly bad as there is no headlining in the back part of the van so you get a lot of radiant heat straight off the steel into the cabin. It's not a nice place to be on a 40deg Ozzy summer.
Yet I use to own a black fastback that was much cooler.

I painted my Suburban's roof white. The factory paint was completely baked off and it was starting to surface rust. I wanted to keep it from getting worse, so I did quick surface prep, primed, and painted. It has helped it some, but it would have been just as effective to put some insulation between the headliner and the roof. In fact, when I finally get around to repainting the entire vehicle I may do just that and return the roof to the body color.

https://ecomodder.com/forum/showthre...oof-35655.html

Most colours I've found sit in the 50*C range in the sun. Black sits in the 70+ range. My (not very shiney) grey trailer was the hottest colour I ever tested at 90+ and hotter than my black Jeep parked along side.

White is the coolest other than chrome- a chrome roof will stay wet all day after a morning shower and is at least 5*C cooler than white.

Only buy quality, genuine branded wrap. eBay wrap will crack and peel after a couple of years.

Tint's been tested too:

https://ecomodder.com/forum/showthre...ted-35441.html

8*C cooler on interior surfaces.

Those differences are quite substantial

A good article done by RACQ in OZ on temps in cars.
Shows only a 3 deg difference between black and white and not much difference in rate of temp rise either which surprised me.

https://www.racq.com.au/~/media/pdf/...rvey_2009.ashx

Seconded.
pro: removable (if you put it on thick enough)
con: cost 1.5x regular paint.

Trees are Forest Green for a reason. On a red car I'd go with https://en.wikipedia.org/wiki/Mountbatten_pink, the manliest pink. That's (#997A8D) in hexidecimal RGB.

http://1.bp.blogspot.com/-t7np81atTN...pg%2Blarge.jpg
http://1.bp.blogspot.com/-t7np81atTN...pg%2Blarge.jpg

That's because most of the heat comes in through the windows of a typical car.

I think in hot climates like OZ there's simply nothing can be done to keep the heat out. You may be able to slow the heat rise a little but after an hour there will be little difference. The sun simply has too much energy and short of making all the windows and panels 100mm thick your never going to keep it out.
Even the old school safari roof won't help a lot of the time as the sun is rarely directly overhead.

According to the article leaving windows down was the only effective method to keep heat down. It's the method I use even though it's a security and rain risk.

Heat re-radiates off the ground. :eek:

Spaz Stix Ultimate Mirror Chrome will make it shine like a mirror, if the surface it's sprayed on is mirror smooth. It shines on both sides so if sprayed on glass you get reflection both on the paint side and through the glass. It also works on plastics. It was developed to be sprayed on the inside of vacuum formed Lexan radio control car bodies. It will convert a CD jewel case lid into a styrene mirror.

For the brightest shine you'd want to apply this Spaz Stix paint over a bright white, high gloss paint.

Rust-Oleum makes a similar paint but it only reflects through what it's applied to. The paint side remains dull. It doesn't work on styrene because the carrier solvent used attacks that plastic. They also make a gold version. Not as shiny but it's the only paint I've found that shows a smooth metal finish through a transparent substrate. All other "gold" paints I've tried show a speckly 'metallic' finish even when on the paint side they appear to have a fairly uniform and shiny metal look.

Looking at the photo, where the thermometer reads IR radiation to determine heat, I became a bit confused whether: if the temp readout is higher would not meat that it is reflecting more heat than the one that shows less IR temperature... :)

There are two large factors that will influence the external temperature of the vehicle for a given surface coating, reflectivity and emissivity. In a nutshell, reflectivity is how much of the energy striking the surface is reflected back versus how much is absorbed. Emissivity has to do with the amount of energy that is re-radiated back off of the surface as it heats up. (Yes I understand this is a very simplified explanation and that is is actually more complicated.)

Even for darker paint colors, the overall reflectivity may be different due to reflectivity coefficients in non-visible spectrums. (E.g. cool roof shingles that aren't white.) The other complicating factor is that even if a material has high reflectivity if it has low emissivity it may reach a higher temperature because it can not shed heat as effectively. That is why some specialty coatings or polished metals are actually worse than a typical painted surface.

The other factor to consider is how much the temperature of the outside surface of a car impacts the temperature of the cabin. This is where insulation and/or ventilation comes into play.

This hits home with me, as my new (1999) 4Runner is black, which means I am at a reflectivity handicap. Don't really plan on painting this ones roof white. I have some peel and stick foam, which I plan to apply to the interior of the roof surface to slow the thermal transfer of heat into the cabin. I also plan to put thinsulate in between the headliner and the roof for the same purpose.

Being able to park the vehicle inside or under cover makes a huge difference obviously. A solar powered fan would be ideal for ventilation, but cracking the windows (assuming low chance of rain) can also be effective. Simple things like opening all the doors for a minute or two prior to getting in the vehicle can also make a huge difference in comfort and how quickly the AC can catch up.

I'm currently running an experiment with three identical 40' shipping containers sitting next to each other. I painted one with "mobile home" white coating. Stuff from Home Depot, total material cost about $100. The second unit I painted with the fancy NASA-spin off "ceramic" white paint. Material cost on this was around $800! The third unit was left uncoated as a control.

The white roofs were 36 degrees F cooler than the uncoated unit. This was measured on the exterior with a laser thermometer. The interior temperatures were 16 degrees cooler for the "mobile home" white and 18 degrees cooler for the NASA-spin off white.

Important to note: I did not coat the exterior walls. I think that would make a big difference... similar to the heat gain through the windows in a car. That said, back in the '70s I put chrome film on the windows. Much cooler. I later went to a very dark window tint and it seemed cooler.

By the way, I live in Louisiana and have been driving white trucks for 20 years.

So was the chrome on the windows better, or the dark black tint?

Are you going to try painting the sides too to see what difference it makes?

I remember the black window tint (window van) being cooler. It was much darker inside the van. However, I added spray foam insulation to the rest of the interior so that kills any apples-to-apples comparison.

As an aside, I also remember buying a solar powered fan that fit in the side window of a vehicle. The fan pulled out a little of the vehicle's hot air but the greenhouse effect of the car made hot air faster than the wimpy fan could suck it out!

FWIW, this was already discussed once on EM:
https://ecomodder.com/forum/showthre...tml#post308497

I can help you here.

I did paint the roof of my own car white.
As its an old car, i used white metal paint with anti rust properties too - usually ppl paint fences with that paint. :D

Before that car roof was british racing green. Every time i got in the car i feelt like i was in sauna. The heat accumulated by the roof severely radiatet inwards. It was like being 5 cm away from an electric 5kw heater on full blast. Not even an extra layer of heat reflecting insullation tucked between metal and rooflining helped. It was quire severe situation during summers.

Now this problem is solved. No more furnace effect from roof when entering the car. I cannot comment on specific numbers, or the total drop in temperature of the cabin as ive not measured it before and after.

What i can tell you exactly - before painting, roof lining was uncomfortably too warm to hold your hand on. Now its barely warm.

White color works miracles. My next car will be white due to AC economy and comfort considerations.