Re: [Phys-l] paint your roof?

[Brian Whatcott <betwys1@sbcglobal.net>]

Woolf, Lawrence wrote:
> The membrane has high emissivity at the IR wavelengths relevant to 
> where objects near room temperature emit and high reflectivity over 
> most of the solar spectrum, which is about 250-2500nm (TiO2 absorbs in 
> the UV).
> The high reflectivity is associated with multiple scattering due to TiO2 particles in a polymer matrix, all of which are non-absorbing between about 400nm and 2500nm.
>
> The high emissivity is associated with the high absorptivity (via Kirchoff's Law) of both the polymer and TiO2, since both the polymer and TiO2 absorb in the longer wave IR near 10 microns.
>
> A roof painted white and a roof painted black will have similar emissivities, so will emissively cool the roof comparably.
>
> An object that is reflective at visual wavelengths is not necessarily reflective at IR wavelengths. ...
>   
> Larry Woolf
It took a while for me to get back into this high solar reflectivity  - 
high thermal emissivity
or differential behavior across the spectrum. So I looked it up.  I was 
surprised to see that this trick is an easy one. Almost anything painted 
white seems to have this property in general.    And remembering the 
comments about colder climes reminded me that the favored material for 
solar collectors had (has?) just the opposite behavior: high solar 
absorption, but low thermal emission It's THIS trick that's the harder 
one: a material with the desired behavior is black chrome plate. A 
smooth surface layer thin compared to a wavelength at IR and a nickel 
backing coat. Molybdate helps blacken the chrome, apparently.

Come to think of it - in this climate there's probably more need to 
store cold air from under a cool roof piped into some rock bed in the 
wee small hours, in order to circulate the cool air during the daytime.

Brian W