1 around 50 percent of sunlight is visible light

2 around 50 percent of sunlight is heat

3 visible light is a frequency of light NOT absorbed by the atmosphere

4 some HEAT (infra red) from the sun IS absorbed by the atmosphere directly (greenhouse gasses which makes the atmosphere warmer).

5 HEAT from the sun also warms the surface of the planet and this HEAT is then radiated back into the atmosphere again being absorbed by greenhouse gasses.

6 visible light is poorly absorbed by the atmosphere, some of it gets reflected back by the atmosphere.

7 the visible light that doesn’t get reflected back by the atmosphere is either reflected by the surface (eg a snow field) or absorbed by the surface (an asphalt carpark/ road). this absorbed light heats the dark surface. this heat is then radiated into the atmosphere and is absorbed by greenhouses gasses makiig the atmosphere warmer.

8 by painting your roof white and other surface you reflect around 50 percent of the visible energy from the sun, a significant amount of this is reflected back into space WITHOUT heating the atmosphere.

9 by painting enough roofs you make buildings cooler and cool the atmosphere. air conditionig works easier to cool houses because they are cooler, perhaps you wouldn’t even need air conditioning?

if you are thinking of using white paint to cool your roof you will need a special white insulative paint that both reflects the visible frequencies of light and stops infra red frequencies from heating your roof.

the main thing to understand is that white paint reflects visible frequencies that would otherwise be transformed into heat (infra red) that is absorbed so readily by the atmosphere. stuff painted with white paint would still get warm but a significan percent of the suns energy would be reflected back into space without heating the atmosphere.

The incoming solar radiation in United States is about 3 to 5% UV (wavelength of 200 to 350 nm), 43% visible (350 to 700 nm) and 52% near infra-red (700 to 2500 nm). Over 99% of the incoming solar radiation is in the range of 250 to 2500 nm. When one is referring to solar reflectance and solar absorptance of materials, it refers to an average value of those properties over the range of 250 to 2500 nm.

When the light is absorbed by a surface it makes the surface hot and the surface emits that energy back to the environment. The characteristics emitted radiation has a wavelength of 10,000 nm. When one is referring to thermal reflectance and thermal absorptance of materials, it refers to an average value of those properties over the range of 8000 to 20000 nm. A low-E material is a surface that has low thermal emittance (= thermal absorptance) and a high thermal reflectance. In principal, solar reflectance and thermal emittance are independent of each other.

The reason that metals are shinny is that it reflects light specularly.

In conclusion: A white surface stays cooler under the sun than a shinny metal surface with the same solar reflectance. The reason is that a white non-metallic surface has a high thermal emittance (it emits the thermal radiation freely) and a metallic surface has a low thermal emittance. To emit the thermal radiation, the temperature of the metallic surface will rise to a level that it can emit the absorbed solar radiation. The lower the thermal emittance, the higher the surface temperature.

As I recall, the low emissivity is more a surface property of metals and is not necessarily restricted to shiny metallic surfaces. I think that the manufacturers of solar collectors have been selling collectors with low-E surfaces for years and these surfaces are not shiny.

I’ve pretty much forgotten all of the heat transfer that I learned in college, but, as I recall, surface properties are dependent on wavelength. Metallic surfaces are highly emissive at the wavelength of incident solar radiation but have low emissivity at the wavelengths associated with heat radiated at perhaps 140°F.

I don’t know if the shiny wrench will take longer to heat up, but I’m pretty certain that over the longer time periods experienced by a roof over a single day, the differences between a conventional (i.e. dark and absorptive) roof and low-E roof are pretty insignificant.

You are correct – as soon as you skin comes into contact with the wrench, conductivity takes over and the surface properties are no longer a factor. And yes, because of the low emissivity, heat will be stored longer, which in the case of low-E roofs, means that more of the heat will transferred into the conditioned space.

Doesn’t the shiny wrench have low emissivity because it is shiny? The more an object approximates a black body the higher the emissivity.

I guess I’d like to see some data. My common sense (which has been know to be uncommonly wrong at times) tells me that because the shiny wrench reflects heat away it will take longer to warm to hot.

Given a low emissivity (along with high mass) will mean that heat will be stored longer after the heat source is withdrawn.

The rapid transfer of stored heat from the wrench to your hand would have to do with conductance. I’m unaware that shiny/black has any relationship to rate of conductivity.

It’s not too confusing. There are indeed two problems – global warming and cooling commercial buildings in the summer. Cool roofs help to address both problems. Metallic roofs help with global warming but don’t keep buildings cooler.

There are several viable ways of applying a cool roof to a building. Ordinary paint is not adequate. Instead, there a number of white roof coatings that work quite well.

A fair amount of monitoring has been done on buildings with cool roofs. Much of the monitoring included measurements taken before and after the cool roof was installed. Reductions in peak demand for electricity and electrical energy usage were repeatedly demonstrated. I refer you to LBNL and the Florida Solar Energy Center for information. You might also try Oak Ridge National Laboratory. They have done interesting work on reflective roof coatings. Most of the work at FSEC was performed on residential structures, I think, but the results are applicable to come types of commercial buildings.

It’s not likely that anyone will install shiny metal roof on a building just to reduce global warming and, in general, shiny metal roofs are rare. On the other hand, synthetic coatings loaded with aluminum particles are very common and are inadequate for cool roofs. I have observed that the exposed surfaces aluminum particles oxidize very quickly. I’m guessing that this significantly reduces their reflectivity, while emissivity stays very low. Not good.

Of course if you insulate the handle of the Crescent wrench, it won’t get as hot. You have not only insulated the handle – you have also increased the emissivity of the surface. The point is this – the uninsulated Crescent wrench gets hot because the surface of the wrench has low emissivity, in spite of the high reflectivity. If the surface of the Crescent wrench had high emissivity, the wrench would stay much cooler.

First there is the problem of global warming which is producing climate change. Part of that warming is due to the fact that the Earth now bounces less light (heat) back out into space because we have removed a lot of the stuff that used to reflect light (snow). Joe’s original post was about replacing some of that lost reflectance via white/light colored roofs and pavements.

Then there’s the problem of undesired solar heating of buildings. A summer problem. It might well be that simply painting a roof white has little value in keeping the interior cooler in the summer. I find no reason to believe that painting an existing flat roof white or installing/replacing light color shingles on a pitched roof isn’t going to bounce away some of the heat striking it.

I can see that attaching a metal roof, even a shiny one, directly to the structure with no air circulation or insulation might leave the building still hot. But I don’t think anyone is suggesting that we follow that practice. Metal such as aluminum is an excellent conductor of heat and will readily transfer any solar gain to whatever substance with which it is in contact.

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I think there’s a problem with your Crescent wrench analogy. While the surface is shiny, there’s lots of mass and metal is an excellent conductor of heat. If the wrench had a small piece of wood attached the handle for you to grasp could still store as much heat but it would not be passed quickly to your hand by flesh/metal contact. That’s what insulation does.

Good point about bouncing light and heat waves out of the atmosphere to reduce global warming. It’s possible that low-E reflective roofs do that as well as high-E reflective roofs. There, however, a number of economic reasons for high-E reflective roofs.

But first, let me try to explain why low-E reflective roofs get as hot as low-reflectivity roofs. In spite of the fact that the reflectivity is high, the low emissivity traps heat and causes the material to warm. Consider, for example a reflective crescent wrench left out in the sun on a hot day – the handle gets hot, really hot. I have been on low-E reflective roofs that are every bit as hot as low reflectivity (dark) conventional roofs.

Why are low-E, high-reflectivity roofs bad? In the summer, the low emissivity negates the value of the reflectivity to the building owner. From the point of view of commercial building owners, electric utilities, and all of the entities involved in managing electrical transmissions systems, the most important aspects of cool roofs are peak-load reduction in cooling and reduction in peak demand for electricity. Any early-day reduction in cooling loads (if any) would be small and have little economic benefit for building owners and electric utilities.

Other factors that favor high-E reflective roofs are: (1) the typical location of air-distribution systems in commercial buildings; and (2) the fact that rooftop, packaged HVAC units with air-cooled condensing are mounted on rooftops. Air-cooled condensing is negatively impacted by higher air temperatures, and, at least in California, about 75% of the commercial building stock has roof-mounted, packaged HVAC units with air-cooled condensing.

The suitability of low-E roofing materials was debated endlessly when the original research on cool roofs was being performed at LBNL and the Florida Solar Energy Center. It was concluded, by virtually everyone involved except for the manufacturers of low-E aluminum-based roof coatings, that low-E roofing materials were not acceptable for cool roofs.

Yes, building heat loads can be reduced by increasing insulation. However, you cannot ignore the immense existing stock of commercial and residential structures, where, in many cases, a cool roof is less expensive than adding insulation, particularly if the building needs re-roofing. Put most simply, if you’re going to put a roof on a building, it might as well be a cool roof.

Emissivity and reflectivity are surface conditions so, in my experience, low-E high reflectivity roofs heat up as fast as dark roofs. I never checked monitoring data to verify this assumption, but I have been on a number of roofs with metallic surfaces in the summer in California and I can tell you that they get darn hot long before noon.

I need another conceptual tool before that makes sense to me.

I can understand that over time reflective roofs might manage to absorb enough energy to raise their temperatures to that of dark roofs. But that still leaves room for earlier in the heating day coolness. And it would argue for a net decrease in heat passed through to the building below.

Of course the building heat issue can be solved via either insulation or ample airflow under the roof. The real issue here is about bouncing some heat back out of our atmosphere.

Low emissivity roofs are not cool roofs. In spite of their high reflectivity, they get almost as hot as dark-colored roofs. That is, there are essentially no cool-roof benefits for aluminum-based roofing products.

On a different subject, the paving solution proposed (15 or more years ago) by researchers at LBNL was the use of white aggregate in the paving mix. As the asphalt is abraded off of the surface by traffic, the white aggregate is exposed, increasing the albedo of the pavement. I don’t know if this theoretical approach has ever been tested on an actual roadway.