[JSH]

Originally Posted by oldtamiyaphile

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.

"UNDERSTANDING EMISSIVITY
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 passages."

Why is chrome hot and white not?