We get questions most frequently from architects regarding SRI (Solar Reflectance Index) which is defined by the USGBC as, “a measure of the constructed surface’s ability to stay cool in the sun by reflecting solar radiation and emitting thermal radiation”. Many municipalities have building regulations set in place with regard to SRI in hopes of promoting a more responsible, energy efficient community.
The trouble with SRI is that when you are considering a metal such as pure copper that “lives” and constantly changes, it’s not really eligible for an official SRI number. SRI is generally used in context with paints where the finish can stay consistent.
The following is an excerpt by Bill Swanson PE, LEED AP, Integrated Architecture from a piece he wrote about an individual study done by Paul Berdahl of the Lawrence Berkeley National Laboratory’s Heat Island Group.
The backstory is he was asked what the SRI for copper was, and for materials that patina over time if it is necessary to submit the original SRI value or the aged SRI value for SSc7.2 Heat Island Effect, Roof. Since he didn’t know the answer to those questions, he sent in a few samples of various ages of copper to be tested at the lab.
He writes, “The results for the samples are as you might expect; the copper has a high SRI when new but quickly loses reflectivity as it patinas with time. Strangely when it gets really old it starts to climb up in SRI value again, though not enough to put you in compliance with LEED’s requirements.”
We caution you to use the above figures at your own risk (this is in no way an ‘official’ USGBC accepted set of SRI values), but reason would dictate that most copper roofs would have the same characteristics. The question still remains about which value to use, and that is open to some interpretation.
Sometimes municipalities have some wiggle room and we can provide documentation that shows a copper SRI that reflects their requirements…at a given moment…but that’s probably a gray line.
We have SRI values at the ready for our solid, standard PVDF paint colors. Even our special patina finishes can comply. Once you have settled on a finish, just send along a list of requirements (SRI, emittance, Cool Roof) and we will match accordingly.
All of our special finishes can also be formulated with “Cool Roof” pigmentation to meet Energy Star ® Cool Roof application requirements for IR (inferred reflective) building envelope products, adding to the building’s energy efficiency.
If you have any particular questions with regard to SRI and Thermal Emittance, let us know. We’ll do our best to ensure you have the answers you need to properly plan for the requisites of your project.