Dana1-

Thanks for the explanation of why metal roofs can get so hot underneath. I was always a bit puzzled by why a fairly reflective surface could get so hot.

Aluminum Commercial sheet 0.09
Aluminum Foil 0.04
Aluminum Commercial Sheet 0.09
Aluminum Heavily Oxidized 0.2 - 0.31
Aluminum Highly Polished 0.039 - 0.057
Aluminum Rough 0.07
Aluminum Anodized 0.77


The last line is the one to use to evaluate aluminum's potential (20x better might change the figures, eh?). It's very good but not the best possible coating out there.

Posted By jonr on 13 Sep 2011 09:42 AM
Aluminum Commercial sheet 0.09
Aluminum Foil 0.04
Aluminum Commercial Sheet 0.09
Aluminum Heavily Oxidized 0.2 - 0.31
Aluminum Highly Polished 0.039 - 0.057
Aluminum Rough 0.07
Aluminum Anodized 0.77


The last line is the one to use to evaluate aluminum's potential (20x better might change the figures, eh?). It's very good but not the best possible coating out there.

Hum ... ok, for the 'peanut gallery' or the 'remedial class,' what are these numbers please?

Those are the emissivity numbers for different aluminum types/conditions/finishes. The emissivity of anodized finishes will vary with the type of anodizing- brite iridite has a very different emissivity (0.11) than black anodizing (0.88) but also has a very high solar reflectivity whereas black anodizing is very aborptive, not reflective. Yellow chromate and blue/red/green anodized finishes are also in the 0.80s, but clear anodizing on a aluminum would be consistent with the 0.77 number.

Using a reflectance of 0.90 and an emissivity of 0.77 brings the SRI up to 113 with an equilbribum temp of 39.8 C (104F) under ASTM E1980 conditions using the LBL calculator. But it's not clear that with an emissivity that high it would retain a solar reflectivity quite that high (but it might), since it would be somewhat absorptive in the deep infra-red.