Message Board Thread - "Copper roof issue"

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Copper roof issue roofman 5/8/2006
Last night was a great night for roof scans. The sky was crystal clear and near zero wind. I finished with the roof scan I needed to do and went to another job site to snap some images of some copper roofs.

The boss wanted to demonstrate to the architects the effect "super cooling" can have on construction materials. More specifically the copper roofs, as morning dew dripping off these roofs most mornings has become an anoyance to the building occupants.

My problem is this, what are the emmisivity values for copper? Particularly for exterior copper panels in their three states = mill finish (shiny), black oxide and patina green.

The images I took are perplexing. The mill finish panels appear to be reflecting deep space (the sky) at the camera and giving a reading <-4 and the older roof with a black oxide is giving a temp of ~31 degrees. The mill finish I can understand. It probably has a emmisivity of 0.05 and reflectance of .95! But the black copper oxide panels are bugging me. The air temp was 58.6 degrees and RH of 56% yet they are giving a temp of 31?

Am I getting an emmisivty from the panels and some reflectance from space too, causing them to appear to be so cold? "super cooling" of roof membranes typically will be 5 to 10 degrees below air temp but below freezing?

I'll try attaching an image here. the building on the left has the new copper panels and the building on the right has panels that have been exposed for about a year.

any thoughts?
 
Re:Copper roof issue wildbill 6/5/2006
Roofman,

Remember that any angle of incidence greater than 45 degrees from perpendicular yields more reflection than emission. Judging the angles from the perspective in your image, you are probably more than 45 degrees. You are observing some emission but mostly reflection of the ambient source, which is the cold sky.

You need to be on or slighty above the roof level to properly measure these surfaces, taking care to measure background temperatures. I would take the time to accurately determine the exact emissivity of each surface then take your readings while maintaining near perpendicular angles. Remember, you will see your body if you are directily perpendicular to the surface of a highly reflective surface.

Here are some copper emissivity values from a camera manufacturer.

Polished, annealed: 0.01-0.02
Commercial burnished: 0.07
Oxidized: 0.65
Oxidized to black: 0.88

I question the "super cooling" statement by the boss in your question. When the copper is new and shiny, significant heat is reflected and not transmitted through the copper. This is good. As the copper oxidizes you can see from the emissivity values above that they significantly lose that reflective capacity and become a heat transmitter only slightly better than black asphalt!

You can confirm this by imaging the interior surfaces of the copper during a sunny day when the sun is bearing down on the roof. Again, perform an emissivity test of the surface and check background temperatures. Use black tape if you must. You will surprise the boss, and the architects!

 
Re:Copper roof issue LaBella 8/10/2006
Here's another number to throw into the mix: The nigttime sky temperature of -18 degrees Celcius. I agree, reflectivity may be the culprit here.



 


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