Reflective roof paint? - Fiberglass RV


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Old 06-21-2007, 01:50 PM   #1
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The walls of my trailer are polishing up fairly well but the roof of course has more oxidation. I was thinking of painting it. Since the trailer is ivory (more white than ivory, really) it would work to have the roof white or close to white, and I was thinking of using a reflective paint that would help keep the interior cooler. Has anyone tried this?

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Old 06-22-2007, 12:42 AM   #2
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The walls of my trailer are polishing up fairly well but the roof of course has more oxidation. I was thinking of painting it. Since the trailer is ivory (more white than ivory, really) it would work to have the roof white or close to white, and I was thinking of using a reflective paint that would help keep the interior cooler. Has anyone tried this?
What you're looking to do is find a paint that has a low heat absorbtion and "emissivity." (It turns out that a surface's ability to absorb and give off heat are closely related; through luck of the draw physicists just call the whole thing "emissivity." That's why windows with a low emissivity value are called "Low-E" windows.)

Now, the curious thing about different colors of paint is that they often don't reflect heat the way you expect them to. You can even find white paints that absorb more heat than black ones! So color is not a good guide. What you need is a table that tells you how much heat a paint color absorbs.

I keyed in the words "emissivity Krylon paint" into Google and found this site at the top of the list. Emissivity values are kind of like a percentage figure, with numbers that are always between zero and one; for keeping heat out what you're looking for is a paint with a lower E-value, like the "Chromatone Stabilized Silver Finish (Alumnatone)" with an E-Value of 0.26. If, on the other hand, you don't want to paint your egg's roof to resemble Flash Gordon's Spaceship, you'll see that most of the non-metalic colored paints, even black and white, are right around 0.95 emissivity. Color choice doesn't make a big difference here. (Cadmium Yellow is the sole paint color that wasn't; e=0.33, almost as good as Alumnatone. So if you want a really cheery, egg-yolk-like egg, you're in luck.)

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Old 06-22-2007, 12:54 AM   #3
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I notice very little, if any difference when I went from a cream/light tan color on my 13 to bright white on the upper half.

If you are doing it for strictly cosmetic reasons.. that's OK, but.. who is gonna look up there?

I have only seen the roof of my 17 once, when I took it to a large bay car wash and could climb up on the platform to scrub the roof. Yep, it's a roof alright....
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Old 06-22-2007, 04:23 PM   #4
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I'd still tend to go by color. I recall one of my friends noting as he walked his dog around his neighborhood that the light-colored cars still had dew on them and the darker cars were already dry. I also noticed that older vans with the small "roof" over the sliding door mechanisms showed a definite correlation between light color and more underside (condensation) rust.

There's also the hand test, in which one walks around on a mild day touching various paint jobs and rating them by pain.
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Old 06-23-2007, 06:35 AM   #5
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Hi Bobbie,

I do not know if this stuff is a scam or not, never used it, no axe to grind one way or the other. Claim is it started at NASA. If you google insulating paint many listings come up, this is just one:

http://www.insuladd.com/insulating-p...tory/index.cfm

Might not wax well if at all??
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Old 06-23-2007, 10:54 AM   #6
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That's the kind of thing I was thinking of. Polishing the top is a problem, anyway, so not polishing would not really be a problem. Just curious as to whether anyone has tried it.

I used a ground glass non-slip additive for my walkway paint, and it had the unfortunate side-effect of reflecting heat onto my tomato plants (not good in Fresno) and causing them to burn. I know some of these insulating additives are ceramic, similar to glass. It seems like it would work, and the one you pointed out says 40% heat reflection and only 10% less heat loss (some insulating paints will also reduce cooling which might be just as big a problem.)

Bobbie
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Old 06-23-2007, 06:21 PM   #7
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The best color for keeping a trailer cool is appliance white.
A higher emissivity value is actually better, as this is the rate that the surface radiates heat away from the surface. Metallic surfaces have very poor emissivity, while a polished painted surface will have a higher emissivity.

An easy test is to on a hot day put you hand on an aluminum Airstream, and white panted car or truck sitting in the direct sun. Even though the polished aluminum surface will have a higher reflective coeficent, the white painted metal will be cooler, as it will in addition to reflecting the sunlight, it will radiate away excess heat.
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Old 06-23-2007, 06:36 PM   #8
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Higher emissivity is good for letting something hot cool down by radiation, but it also means a higher rate of heat gain when exposed to a radiant source, such as the sun. You really need changeable emissivity...

I would be more interested in the inside surface of those metal vehicles, and the air inside. Much of the information you get by touching a surface is about the ability of the surface to conduct heat to your hand, rather than about the actual temperature of the surface. For example, if a metal object and ceramic object are left in a room, not in sunlight, long enough that they are both clearly at the room's temperature, and that temperature is very cold or very hot, the metallic object will feel like it it at a more extreme temperature (colder in the cold room; hotter in the hot room) than the ceramic one.
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