Should I paint my roof white?

Ducky's Dad

Explorer
How that isn't significant I guess I don't understand...
It is significant, but as pointed out by another poster, there are a lot of variables. And if you leave a black truck and a white truck out in the desert sun all day, the black truck will heat up faster, but by mid afternoon they will both be at about the same temperature inside, assuming same vehicle, same glass, same interior, same sun orientation, etc. White is better, but not so much that you will notice in certain conditions. All of my trucks are white, always will be, partially because they see a lot of time in the desert and partially because they are a lot more visible to other drivers at night. Also easier to spot from the air if you are stranded in East BF. And a lot easier to repaint if you bend the metal.
 

4x4junkie

Explorer
I still don't see how a vehicle that is being heated by the sun to 140° is going to get anywhere near as hot inside as one being cooked to over 190°... Sure, the amount of solar energy coming in through windows may be similar, but the amount of energy being reflected by the white roof & sides is probably double that of what's entering the windows, if not more (the roof would be facing the sun more directly as well).

I guess I'll have to get a couple thermometers and put one inside each vehicle to see how much difference there is. I know for sure after 3-4 hours that truck gets way cookin' inside lol. Fortunately the A/C on it works incredibly well also.
 

Ducky's Dad

Explorer
If you leave two of anything in a given set of conditions, they will eventually stabilize to those conditions. In this case, they will stabilize at the same temperature if left in the sun long enough. Look up "thermodynamic equilibrium" for more detailed discussion. Heat is transmitted in three ways: conduction, convection, and radiation. The difference in color affects mainly heating through radiation (from the sun), with minimal differences in heat gain or loss through conduction or convection (although there are some differences). Ever notice that many heat sinks on electronic equipment are flat black? That's because a flat black sink will release heat more efficiently than will a polished aluminum surface, and getting the heat out of the equipment is the objective. Same principle applies the the black truck: it heats up faster and releases the heat faster, so (at least theoretically) it will cool off faster after the sun goes down. I previously mentioned that I used to carry a digital thermometer in my truck (to see if the A/C was working properly and to remind myself how miserable I was in the desert). On an early September day in Ripley CA, with ambient temp around 118F, the temperature in my white truck with white camper shell and light tan interior and tinted windows was 185F. I didn't have a black truck handy to measure, but this was late afternoon and both trucks would have stabilized their temps by that time of day. White will usually be a bit cooler than black, except in those kinds of conditions, but sometimes not enough to make a practical difference. As another poster noted, it's all physics.
 

4x4junkie

Explorer
If you leave two of anything in a given set of conditions, they will eventually stabilize to those conditions. In this case, they will stabilize at the same temperature if left in the sun long enough. Look up "thermodynamic equilibrium" for more detailed discussion. Heat is transmitted in three ways: conduction, convection, and radiation. The difference in color affects mainly heating through radiation (from the sun), with minimal differences in heat gain or loss through conduction or convection (although there are some differences). Ever notice that many heat sinks on electronic equipment are flat black? That's because a flat black sink will release heat more efficiently than will a polished aluminum surface, and getting the heat out of the equipment is the objective. Same principle applies the the black truck: it heats up faster and releases the heat faster, so (at least theoretically) it will cool off faster after the sun goes down.
Obviously not if the black truck is sitting at 190° and the white, 140°...
A heat sink on a piece of equipment does the exact opposite of a black vehicle sitting in the sun... A heat sink expels heat generated by some internal source out into it's surrounding. OTOH, a black truck with the sun beating down on it absorbs heat from an external source (the sun). I don't think the two are comparable... What I see in reality doesn't support it either. Whatever additional ability a black object has in dissipating heat energy obviously doesn't make up for it's vastly increased ability to absorb the heat of solar radiation.

Again, I'll try a couple thermometers in each vehicle next time the opportunity arises, but I fully expect the white vehicle to stay much cooler inside throughout the day (I'm not saying it'll be tolerable inside, but it'll be cooler nonetheless)
 

4x4junkie

Explorer
^^

Good link.

I didn't read the full report yet (will do so when I have time), but indeed I remember hearing something a few years ago about how the EPA wanted to ban black cars "because they caused excessive pollution" lol. They said black paint soaks up so much solar heat that it makes the A/C work harder and therefore consumes more fuel and therefore spews more pollutants and "greenhouse gasses" out into the atmosphere. I s'pose there's some truth in it, though whether it's enough to make any real difference I'm not so sure of (cars are typically in motion whenever the engine is running, the air passing over the car's surface is gonna have a considerable effect on how much solar heat is retained on it's surfaces. At a quick glance it looked like that report was studying cars that were sitting still).
 

Ducky's Dad

Explorer
Click the PDF to read the whole report.
Well, alrighty then. First let me state the anything put out by the California Energy Commission is immediately suspect. You need to consider their agenda and their tactics in evaluating anything they state as fact. These are the guys who at the time of this report wanted to ban dark colors on cars in California. These are the same people who have now successfully banned the sale of most automotive battery chargers that are transformer-based, forcing us to buy microprocessor-controlled high tech chargers at high prices, generally at low amperage resulting in longer charge times and no jumpstart capability in most new consumer chargers. Yet they state in one of their reports that the cost of charging batteries in CA will go down as a result of this mandate. I can get the citation if it is important to anyone.

Re the paint color analysis, I have seen this report in the past and it is flawed in many ways. First example is line 1 of the Executive Summary: "Air-conditioning in cars and small trucks lowers fuel economy by an estimated 22%." Well, sure, but the US DOE also states that driving at highway speed with your windows down uses more fuel than rolling up the windows and using the A/C. So a more accurate statement would be that hot weather lowers fuel economy by X%. You can turn on the A/C or you can roll down the windows. Choose your poison.

The researchers are overly excited about the real world effects of lighter colors vs darker colors. On page 6 of the report they state: "Use of cool-colored paint can reduce the soak temperature by a few degrees (3-5°F)." That is only soak temperature, and using my prior example, measured by me with my own digital thermometer on my own white truck parked in the sun in Ripley, with the cab interior at 185F, a 3F reduction would only be a difference of 1.6% in interior temp of a static vehicle, probably below what is known in psychology circles as JND (Just Noticeable Difference). It should also be noted that the authors of the report use Celsius when it suits their purposes and Fahrenheit when it suits other purposes. Up to this point on page 6, virtually all data was presented metric/Celsius, but when they wanted the reader to think there is a large impact to using light paint vs dark paint, they switch to Fahrenheit. Note that 3F difference between 185F and 188F is only about 1.7C. 3F seems like a bigger difference to most readers than 1.7C, so they went with Fahrenheit to shade the data without falsifying it.

See pages 8-9: "The roof of the black car was up to 25°C warmer than the silver surface. While soaking the cabin air temperature differences peaked around 5-6°C." Back to Celsius again, but 25C is about 45F, consistent with 4x4Junkie's 50F measured differential. However their cabin air temp delta of 5-6C stated here is inconsistent with the 3-5F delta stated on page 6. So, which is it? What you can take from this information is that there is a difference in sheet metal temps on dark cars vs light cars, and that translates into a difference in cabin air temps, but the difference in cabin air temps is relatively small compared to the difference in sheet metal temps.

See page 10/46: "Air conditioning has little effect on roof surface temperature, indicating that the conductive heat
flow through the lined ceiling is small compared to the roof's solar heat gain.
" (Emphasis mine.) This supports my contention that, although white (or in this case silver) will be cooler than black, it really does not make that much difference in interior temperatures. See also page 19/46, Table 2: "thermal emittance, 0.83 (black), 0.79 (silver)." Not a heck of a lot of difference in the emittance between light and dark roofs, is there?

So, getting back to the OP's question: Sure you can paint the roof of your blue Suburban white. It will help a little, but don't expect a huge improvement.
 
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jlocster

Explorer
So, getting back to the OP's question: Sure you can paint the roof of your blue Suburban white. It will help a little, but don't expect a huge improvement.
Agreed...and that's been my personal observation with my Montero. I think a more effective solution to reducing in car temps is just to keep your windows shaded when possible.
 

fog cutter

Adventurer
Does your rig have a roofrack tray ?
A solid floor makes a great reduction to thermal absorbtion......
this is one of the things i'm considering (along with inside coatings & insulation). anyone suggest a good product to use for the solid deck portions? Starboard or some synthetic/ variation thereof?

thanks
 

rayra

Expedition Leader
sheet aluminum, I would think. ~~$15 / sq'. About 1/10" / 2mm thick. Thicker if you can afford it. Waterproof, durable, corrosion resistant. Drill an array of perimeter holes to facilitate tie downs. As simple as it gets. The earlier post in this topic about the the false roof (as well as the fabric awning solutions) has me thinking about it. I'm also working up ideas for room mount solar that would also require a tray to protect the panel from rooftop gear storage. And generally protect the roof from storing things on it. A solid sheet does everything at once. I can use the perimeter rails of my Z71 rack and the hardware from the 3 factor crosspieces to suspend an aluminum sheet floor. The risers of the rack frame would hold that sheet about an inch clear of the factory roof at the crown, 2"+ around the perimeter. There's my air gap / ventilation of heated generated by sun exposure. And my big metal roof is mostly in the shade. Add the clear heat-blocking film to my windows and there goes a good portion of the heat from solar insolation.


eta - been discussing some of the same sun / heat issues with pappawheely in his vehicle project thread -
http://www.expeditionportal.com/forum/threads/128106-Project-quot-Autonomous-quot-F-350

he's trying the white paint with glass bubbles in it, for insulation properties. Looking forward to his results. He's also going with mechanical ventilation. And we also talked about a 'false roof' albeit not in those terms. I was talking about a fabric layer akin to the pic I flashed earlier. He and I also talked about a sort of trampoline surface akin to the deck of a Hobie Catamaran. And now I'm thinking of trying something similar. Sewing a panel with heavier-duty 500D or 1000D woven nylon. Reinforced and grommeted edges and lace the perimeter to my roof rack such that the fabric is suspended tight within it. That will block the sun from about 80% of my roof. Leave it in place whenever the roof rack isn't needed. Which is about 95% of the time.
 
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fog cutter

Adventurer
(TRAMPOLINE or FABRIC) ... Leave it in place whenever the roof rack isn't needed. Which is about 95% of the time.
do you think this would set up harmonic vibration or whatever the hum is like from guy wires in the wind?

i like the aluminum sheets with necessary apertures - and maybe even powdercoat them - ready for it? - White!!!

could even put some of the 3M grit tape here and there to slip-stop. or vise visa.
 

thairish

Observer
SPINOFF: A black car heats up faster than an identical white car. (From Pilot 2)
CONFIRMED
A fan wrote in and asked a follow up question: "Does the color of a car affect the way it heats up?". The MythBusters used two identical cars, one black the other white and left them both out in the summer heat with thermometers in both. By mid-afternoon the black car had heated up to a temperature of 135 °F while the white car topped off at 126 °F, almost 10 degrees cooler.

http://mythbustersresults.com/episode38
 

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