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Old 01-31-2018, 08:28 PM   #31 (permalink)
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White radiates infrared and reflects visible light, were chrome's infrared emmisivity is low. I've never seen anyone in the desert wearing chrome cloths, but they do sell shinny aluminized emergency blankets to keep warm.

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Old 02-01-2018, 12:28 AM   #32 (permalink)
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As arcosine said, visible reflectivity is only part of the equation. In addition to visible light you also have to consider infrared and ultraviolet. Even the reflectivity of the whole light spectrum is only half the equation. The reflectivity of an object determines how quickly it absorbs heat. The other half is how quickly an object can radiate heat. This is called Emissivity. The scale is 0 to 1 with a larger number representing faster heat radiation. Chrome has an emissivity of about 0.05 to 0.1 while paint is about 0.90 to 0.95.

So chrome heats up slower but holds more heat because it cannot radiate very quickly. A real world example is a chromed metal bumper vs a painted metal bumper. Under the blazing afternoon sun the chrome bumper will be hotter to the touch than the painted bumper.
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Old 02-02-2018, 04:58 AM   #33 (permalink)
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Presented without comment:





Revlectivity & Emissivity - Zelltech Insulation Heat barrier - Aluminium Heat Radiant Barrier
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Old 02-02-2018, 05:48 AM   #34 (permalink)
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^^ The only comment to the above is that the IR radiation has to penetrate the roofing to get to the radiant barrier, after which it gets reflected and returns the same way. In effect, it goes through the roofing material twice, leaving some of its energy (heat) in it each time. This can make the roofing warmer than without the barrier, and the higher temperature can shorten its lifetime.

This is why it is better to reflect heat closer to the source.
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Old 02-02-2018, 07:13 AM   #35 (permalink)
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Quote:
Originally Posted by arcosine View Post
White radiates infrared and reflects visible light, were chrome's infrared emmisivity is low. I've never seen anyone in the desert wearing chrome cloths, but they do sell shinny aluminized emergency blankets to keep warm.
The emergency blankets are for reflecting infrared radiation from the body and keeping it close to the body. That's why when you use one, you're supposed to be nude!
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Old 02-04-2018, 10:16 PM   #36 (permalink)
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Quote:
Originally Posted by Piwoslaw View Post
^^ The only comment to the above is that the IR radiation has to penetrate the roofing to get to the radiant barrier, after which it gets reflected and returns the same way. In effect, it goes through the roofing material twice, leaving some of its energy (heat) in it each time. This can make the roofing warmer than without the barrier, and the higher temperature can shorten its lifetime.

This is why it is better to reflect heat closer to the source.
No wonder I have seen many houses with bright aluminium foil over the roof instead of under it. But anyway, when those reflective materials were introduced here, they were also meant to provide some degree of watertightness when applied under the roof.
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Old 07-27-2018, 07:49 PM   #37 (permalink)
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I'm dragging this thread back up because I just removed my antenna and now have a nice, smooth roof to wrap with something more reflective than the silver paint.

I need to decide between white vinyl or chrome vinyl. The above posts about emissivity vs. reflectivity make some good points, but no one has mentioned the fact that emissivity and reflectivity sum to 1 for any given wavelength. So, in the spectrum where that paint has an emissivity of .90-.95, it has an abysmal reflectivity of .05-.10. Add to that that we're talking about the same material in two different colors, not paint vs. metal, and I'm not sure having a good emissivity rating in any particular part of the spectrum is as important. Plus, there will be silver paint underneath, which has a fairly balanced .57 reflectivity/.43 emissivity in visible spectrum.

As near as I can tell, even at sea level the bulk of solar radiation is in a visible spectrum, not IR, where chrome should outperform white. Has anyone else experimented with these wraps? Which should I go with?
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Old 07-28-2018, 06:28 AM   #38 (permalink)
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Do a test in the sun with aluminum foil over on hand and and white paper over the other to so see that the white is better.

Then do white paper with aluminum foil underneath.
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Old 07-28-2018, 06:55 AM   #39 (permalink)
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Quote:
Originally Posted by oldtamiyaphile View Post
Chrome is much more reflective than white, that's why you can see yourself it it.

The performance of white sits about half way between chrome and light colours (red, light blue, light green).

There's a reason why radiant heat insulation is always chrome (or gold), and why they use mirrors to reflect light onto molten salt reactors and not white panels.

The seat buckle feels hot because metal transfers heat faster than the plastic surround even though both are at the same temperature. Same principle hot coal walkers use - high temps- inefficient heat transfer.

My chrome roof now stays wet after a morning shower for the whole day, I've never seen that before.
Quick correction, it is actually the thermal capacity of the material that changes the feeling of initial touch and not conducivity.
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Old 07-28-2018, 10:38 AM   #40 (permalink)
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When you do your experiments just remember that infrared thermometers are not accurate on reflective surfaces. From Omega:

"In general, the higher the emissivity of an object, the easier it is to obtain an accurate temperature measurement using infrared. Objects with very low emissivities (below 0.2) can be difficult applications. Some polished, shiny metallic surfaces, such as aluminum, are so reflective in the infrared that accurate temperature measurements are not always possible."

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