Chrome wrap your roof!

[oldtamiyaphile]

Most campervan conversions use thin plywood and fabric, you can also get various stick on foils etc. I don't think it's really worth doing on a work van. I mainly did the wrap to reflect the heat of my solar panels away, because those things get seriously hot.

I plan on doing a full undertray and DPF back exhaust, by the time I get around to a cargo area headliner, the Cybertruk should be out.

[arcosine]

Chrome or anything metallic will get hot in the sun. White is the best color to keep things cool. Why, because of the spectral emissivity. Silver and chrome reflect and emit the same at visible light and infared light (heat). White paint with titanium oxide reflects light and has a high emittance in infrared, therefore needs a lower temperature to radiate the same power as silver or chrome. Just try it, put something chrome and something white in the sun. Ive had chrome wrenches in the sun that were too hot to pick up. The roof of my RV is painted white and is cool in the sun. The whole premise of putting chrome wrap on to keep cool in the sun is false, unless your car is black.

[oldtamiyaphile]

Your theory has already been debunked, but here it is again.

I don't have a white car so I took a panel I cut from a white car and added some chrome wrap, angled it towards the sun:



Measurements taken from the underside:

Optical:


IR:


So two degrees cooler than white. Now if my cars were white, I wouldn't bother, but bright green already shows a 10-15 degree improvement, and black and grey show up to 60 degrees.

We use chrome wrap to insulate buildings in hot conditions, car window foils are chrome etc. It really shouldn't be controversial. We actually WANT high reflectivity and low emmisivity:



If we can reflect 97% of the IR energy, we don't really have to worry about the emmisivity of the remaining 3%.

[JSH]

To do a proper experiment you need to measure the effects of chrome wrap vs white wrap of the same material and thickness.

Is the reduced heat transfer due to the reflectivity of the wrap, the fact that the wrap is vinyl which has different heat transfer properties than metal, or the added thickness of the wrap?

[oldtamiyaphile]

Bottom line is it doesn't matter. Chrome wrapping even a white car will make it cooler.

Thermodynamics and Reflective Insulation | Energy-Efficient Houses

Reflectivity + Emmisivity =1

Why would you sacrifice reflectivity to gain emmisivity?

Chrome is 97% reflective
White is 90% reflective

Reflecting maximum energy will always win.

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.

The emmisivity of white would need to be three time greater to match the performance of chrome, because it's simply admitting too much heat in the first place.

And that's before we get to the fact that low-E means the A/C is more effective once on the move.

Must be a reason why construction uses silver foil and not white.



I'm not exactly re-inventing the wheel.

Chrome or white are both good choices, chrome happens to out perform white and I think better suits the appearance of modern cars.

[Stubby79]

Paint it black.

[Piotrsko]

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.

[JSH]

Originally Posted by oldtamiyaphile

Chrome or white are both good choices, chrome happens to out perform white and I think better suits the appearance of modern cars.

Your experiment does not support that conclusion.

Your experiment shows that chrome wrap over white paint outperforms white paint alone.

[cRiPpLe_rOoStEr]

Originally Posted by oldtamiyaphile

I don't think it's really worth doing on a work van.

Unless it had a cargo body totally separated from the cabin, a full-lenght headliner is still worth. Even if the load is not so sensitive to heat, a somewhat extreme temperature inside the load compartment might become a PITA while unloading.

[oldtamiyaphile]

Originally Posted by JSH

Your experiment does not support that conclusion.

Your experiment shows that chrome wrap over white paint outperforms white paint alone.

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.