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Old 12-19-2020, 09:30 PM   #81 (permalink)
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Originally Posted by Piotrsko View Post
The reason manufacturing provides a shiny surface is: the manufacturing process makes the material this way. The manufacturer do not provide additional finishing as that would add unnecessary costs.
So you're telling me that adding foil to a plastic sheet is cheaper than making a white plastic sheet?

Even if that were true, there would be expensive white insulation for high performance applications. Just like you can get aerogel (bulk) insulation where you don't have the space to use cheaper stuff but still need to meet required performance targets.

Fun fact, I almost did aerogel insulation under my headliner. Might still do it since I have to drop it soon.


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Old 12-21-2020, 10:23 AM   #82 (permalink)
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No I didn't say foil was cheaper than plastic, I said shiny was a by product of manufacturing.
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Old 12-21-2020, 06:16 PM   #83 (permalink)
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Originally Posted by oldtamiyaphile View Post
I already mentioned that. It does however confirm the whole premise of the thread that chrome wrap will make your car cooler, even if it's already white. It also debunks the theory that 'anything chrome left in the sun will get hot', if it's getting so hot, why isn't that transferring to the sheet metal?

There's also observational data that suggests chrome is cooler than white. My chrome roof stays wet all day in the sun on a hot day. That never happened with a white van.

The maths also supports chrome being cooler than white.

Chrome: out of 100W of IR energy 3W gets through
White: out of 100W of IR energy 10W gets through

Of the 3W that gets past our chrome surface 3% gets dissipated or 0.09W
Of the 10W that gets through the white surface 10% gets dissipated or 0.1W

Leaving us with a net heat gain of 2.91W for Chrome vs 9.9W for White.

It's a bit like leaving a window open at night because the more mosquitos you let in the more will get out.

Given enough time (probably longer than a days worth of sunlight) I suppose it's possible for emmisivity to catch up and build up heat, but it's really not relevant to the discussion. I'm not chroming the underside of the roof, so the majority of the heat is reflected by the top side, but painted underside can still emit the tiny bit of heat that gets through.

There's no way heat can be trapped by emmisivity if the coating is only on the top side (again exactly like building insulation).

I've been burned by hot tools left in the sun, we all know that. But I left a nice shiney chrome wrench outside on a 100 degree day, and it never got hotter than my white painted control. It's say most of the time the offending scorching hot tool is silver rather than chrome, and might have a textured surface (like all my shifting spanners which seem to be the main culprit). There's also the fact that if you leave it on burning hot driveway, it will also be burning hot, I know my driveway can easily reach 160* plus and it's not even a dark colour.

If you are going to do the math you need to do the entire equation not just part of it.

The question of whether a chrome or painted bumper gets hotter in the sun is a classic undergrad heat transfer problem.

Emissivity is the ability for an object to release "emit" radiant heat. The lower the emissivity, the more difficult it is for heat to leave the surface in radiant form. This why chrome tools in your toolbox and car door handles get too hot to touch when exposed to the sun.

Most paints emit in the .90 range which is very high. Chrome has an emissiviy or "E" value of .05. It will take more time for a chrome bumper to get as hot as one painted black but the low emissivity of chrome traps the heat making it much hotter. The low E value of chrome prevents the absorbed heat from escaping making the surface hot. This is why black chrome solar panels provide hotter water than panels painted flat black. Black chrome will take a little longer to get hot, but once it does, the low E selective surface traps heat in the absorber which in turn transfers it through conduction into the water passage

Why is chrome hot and white not?
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Old 03-05-2021, 05:41 PM   #84 (permalink)
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I was reminded by this:

Here's is a case where the base is a rubberized paint of variable thickness, buffed to a high gloss and then topcoated with a thin reflective coating. It could be 8-10 coats of Plastidip. I wonder what the insulative value would be.

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Old 07-29-2021, 05:19 PM   #85 (permalink)
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I am wondering if putting one of those mylar emergency blankets on the roof and attaching it with spray window cleaner.

Car is getting hot in the summer

I also have a 56L water tank in the car, wondering if i could use that to cool something down.
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Old 07-29-2021, 10:44 PM   #86 (permalink)
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Originally Posted by teoman View Post
I also have a 56L water tank in the car, wondering if i could use that to cool something down.
In a place with high air humidity levels, a swamp-cooler is not so effective, yet I see it on many trucks in my country.
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Old 07-30-2021, 12:37 AM   #87 (permalink)
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Doesn't aluminum foil reflective some 99% of the light and heat?

Shiny surfaces will emit less radiant heat but shiny doesn't affect heat emitted through convection.

I've painted my house's roof white and that helps. A relative of mine once covered his roof in an aluminum foil type product that was very reflective. He said it did wonders to keep his house cool. I'm thinking of doing the same some day.

I'd like to do it to my cars, but that would make a lot of people mad at me, I'm sure.
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Old 07-30-2021, 07:00 AM   #88 (permalink)
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Scientific American magazine (Aug 2021) has a short piece about a new nanomaterial mixture, now as paint, that lets surfaces release more heat than they absorb. This technique has been used for experimental cooling devices, though not in commercial use yet.
This coating absorbs just 1.9 percent of sunlight compared with 10 to 20 percent for conventional white or “heat-reflective” paints, says Purdue University mechanical engineer Xiulin Ruan, co-author of a study on the substance in ACS Applied Materials & Interfaces.
In the past 10 years researchers have found greater success with multilayered coatings that incorporate tiny particles of varying sizes, some on the nanoscale, to reflect many wavelengths of light. Teams at Stanford University and the University of Colorado Boulder have shown that such materials can cool a surface to below the ambient temperature.
The reflectivity of shiny aluminium is reported by Reynolds as 88%

But this chart from The IESNA Lighting Handbook:
Mirrored and optical coated glass 80-99 (mirroring is silver on polished glass)
Metallized and optical coated plastic 75-97 (generally vacuum deposited aluminum)
Processed anodized and optical coated aluminum 75-95 (what passes for chrome on cars now)
Polished aluminum 60-70
Chromium 60-65
Stainless steel 55-65
And each material has slightly different emissivity.
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Old 07-30-2021, 09:06 PM   #89 (permalink)
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Originally Posted by Isaac Zachary View Post
A relative of mine once covered his roof in an aluminum foil type product that was very reflective.
Quite usual in my country, but it's more often done as a makeshift repair to a leaky roof.

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