Albedo and Cool roofs

Sunday, December 23, 2007

"Albedo includes the visible, infrared and ultraviolet wavelengths on a scale of 0 to 1. An albedo value of 0.0 indicates that the surface absorbs all solar radiation, and a 1.0 albedo value represents total reflectivity. EPA ENERGY STAR specifies an albedo of 0.65 or higher for low-slope roof applications and 0.25 for sloped roofs."

"Most roofing industry experts agree that a cool roof is one the exhibits a combination of high reflectivity and high emissivity."

"LEED version 2.2, released in October 2005, is the first national specification to use a relatively new measure of reporting a cool roof's properties. LEED 2.2 sustainable site credit 7.2 states that to receive one point, building owners should use a roof with a Solar Reflective Index (SRI) of 78 over at least 75% of the roof surface for roofs with slopes less than 2:12. The new twist is SRI, a unit developed by scientists at Lawrence Berkeley National Laboratory. SRI incorporates reflectivity and emissivity properties into one, easy to read, standardized measure so that roof buyers won't have to scratch their heads and try to figure our if a high reflectivity and low emissivity is better or worse than a medium reflectivity and high emissivity."

"SRI is calculated with a complex formula spelled out in ASTM E1980 and is a scale of 1 to 100 that is a measure of a roof's combined thermal properties. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. But some hot roofs can have negative values, and some white thermoplastics and white roof coatings have scored as high as 104 to 100."

"SRI as a method for reporting cool roof data will probably take a little while to catch on. The Cool Roof Rating Council, an organization that verifies and labels cool roofing products has begun using the measure, while retaining reflectivity and emissivity measurements."

"The albedo of an object is the extent to which it diffusely reflects light, defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure indicative of a surface's or body's diffuse reflectivity. The word is derived from Latin albedo "whiteness, in turn from albus "white".

"The term albedo (Latin for white) is commonly used to applied to the overall average reflection coefficient of an object. For example, the albedo of the Earth is 0.39 (Kaufmann) and this affects the equilibrium temperature of the Earth. The greenhouse effect, by trapping infrared radiation, can lower the albedo of the earth and cause global warming."

"The albedo of an object will determine its visual brightness when viewed with reflected light. "

"In more technical treatments of albedo, such as that of de Pater and Lissauer, a distinction is made between "bond albedo" and "geometric albedo", the numbers quoted above being geometric albedos. The geometric albedo is defined as the amount of radiation relative to that from a flat Lambertian surface which is an ideal reflector at all wavelengths. The bond albedo is the total radiation reflected from an object compared to the total incident radiation from the Sun. The bond albedo for the Earth is given as 0.29 by de Pater and Lissauer, compared to their value of 0.37 for the geometrical albedo."

ECBC U-value adjustment:

Cool Roofs:

"Roofs with slopes less than 20 degrees shall have an initial solar reflectance of no less than 0.70 and an initial emittance no less than 0.75. Solar reflectance shall be determined in accordance with ASTM E903-96 and emittance shall be determined in accordance with ASTM E408-71 (RA 1996)."

There seems to be no adjustment to the U value for a high albedo surface, unlike that available in some countries.

"The Roof/Ceiling U-value for buildings that are mechanically cooled and have a roof covering with a high-albedo surface can be increased in accordance with this section provided the roof surface:

a) has a minimum total solar reflectance of 0.75 when tested in accordance with ASTM E 903, and

b) has a minimum infrared emittance of 0.75 when tested in accordance with ASTM E408."

Ref: http://eetd.lbl.gov/coolroof/

For databases on properties of cool roof materials.

Posted by Kaks at 1:51 PM  

1 comments:

We are Cool Earth Contracting & Coating.This seems to be a great site which offers Reflective Paint Austin, Commercial roof, Commercial roofing, Cool roof etc. and i would surely like to try their service...i had been relying on Cool Roofs earlier and they too offered good stuff. You will be glad you decided to work with them.

January 25, 2012 at 4:14 AM  

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