R Keller wrote:As background, remember that heat is tranferred in three ways: radiation (think sun's rays), convection, and conduction. When we're talking about solar gain on a trailer, radiated heat from the sun that is absorbed by the surface (skin) is then transferred via conduction to lower layers. The cooler the surface stays, the less the heat gain on the system. Insulation will also help keep the interior cooler (or less hot) longer by slowing the rate of heat transfer through the roof/wall.
When looking at how hot any given material gets in the sun, there are two main factors to consider. The first is the reflectivity of the material. The second is how quickly it can radiate heat off of its surface back into the air; this factor is called emissivity. Both reflectivity and emissivity are based on the surface properties of the material.
Speaking about the first point, reflectivity, aluminum (or any metal) will reflect a lot of the solar energy radiating on it. And the shinier the metal, the more it will reflect. But a white surface will also reflect to a large degree, usually more than the metal. Solar reflectance of a white surface can get as high as 0.85, meaning it reflects back 85% of the solar radiation. This is about the same as highly polished aluminum. Mill finish aluminum has a value more like 0.6, depending on the oxidation level. Also, generally, the darker the color of a material, the less reflective it will be. This is why, given the same material, the darker color will get hotter (however, there are exceptions, including some new paints that have ceramic particles embedded and are almost as reflective in darker colors as in lighter.).
So, the higher the reflectivity, the longer a material will take to heat up when exposed to a given amount of solar radiation. But that's only half the story.
The second factor is the emissivity of the material. The higher this number is, the faster the material can radiate heat that is has absorbed back into the atmosphere (different wavelength than the incoming solar radiation). Most materials have an emissivity of around 0.8 to 0.9, meaning they emit 80% to 90% of their heat. Bare metals (including aluminum) are an exception, with low emissivities that range between 0.05 to 0.6 depending on the type of metal, and its surface finish and condition. And the shinier the metal, the lower the emissivity. Highly polished aluminum has an emissivity of about 0.05. Even highly oxidized aluminum's emissivity is only about 0.25. So, even though they're reflective, metal surfaces tend to get hot since they're unable to emit away even the modest amount of solar energy they do absorb (hence "Cat on a Hot Tin Roof" or cat on a hot trailer roof!).
The ideal material, if one wants a surface that doesn't get hot in the sun, would be one that has a high reflectivity and high emissivity.
White paint does the job nicely. This is why bright white elastomeric coatings are increasingly being used for "cool roofs" that significantly reduce heat gain and hence cooling bills for buildings.
As it turns out though, a coating on aluminum, whether it is anodized, or a clear coat sprayed on, can increase the emissivity of the aluminum tremendously. Clear anodized aluminum has an emissivity of 0.76-0.84 (depending on the thickness of anodization). A clear coat would be similar, depending on thickness. Of course, white painted aluminum could have even higher emissivity (and would have a higher reflectivity). Bare mill-finish or polished aluminum, though, is going to get really hot in the direct sun.
So what kind of difference does it make in reality? The Lawrence Berkeley National Laboratory did a study on roof materials (
http://eetd.lbl.gov/CoolRoofs/) and found the following average temperature gains above ambient temperatures in full sun and no wind (a 40°F gain on a 100°F degree day would equal 140°F surface temperature):
Bright white smooth materials: 15°F
Rough white surface: 35°F
Very light (pastel) colors: 15 to 55°F
Bare aluminum: 48°F
Medium gray (reflectance half way between white and black): 52°F
New, bare galvanized steel: 55°F
Intense but not very light colors (green, red, blue, etc.): 79 to 83°F
Black materials: 90°F
So on a day with an air temperature of 90°F, you could expect a clean white trailer roof (Filon, painted aluminum, painted wood, etc.) in the direct sun to reach a surface temperature of 105°F. A non-coated, non-anodized aluminum roof would reach a surface temperature of 138°F, and a black roof would reach a surface temperature of 180°F!
The lower the heat load from the skin, the less your cooling needs are in the trailer and the more effective your insulation will be.
Rik
by Guy » Wed Jan 12, 2005 12:29 am 
But that's not my main insulation concern. I'm trying to figure out how or IF all this radiant heat reflection will help keep a teardrop trailer warmer inside. Is it just the sealed air bubbles that help keep a teardrop warm, even more than the "radiant barrier"? Do humans product radiant heat? I dunno, but I doubt it.
