Page 60 - Green Builder Magazine January 2016 Digital Edition

Building

Innovative Solutions for High-Performance Homes

Dark Roofs: An Untapped
Solar Resource?

The color of a metal or clay or concrete tile roof makes a huge difference in solar

reflectance and absorption. It’s time to harness this wasted energy.

IBY MATT POWER, EDITOR-IN-CHIEF
F YOU’VE EVER read through the technical specs for roofing
products, you’ve probably noticed that there’s a big difference
in the solar reflectance of clay or concrete tile, depending
on its color. Boral’s Ebony blend, for example, has a solar
reflectance of just 5.1, while its Barley Stone scores 60. And
that same Ebony product has a solar absorption of 94.9. Thatʼs a lot
of energy being absorbed, and gradually released.

Which begs the question, with dark roof coverings, where does
all that unreflected energy go? The answer is pretty obvious: it’s
absorbed by the tiles and radiated more slowly in all directions after
that. In the U.S., most research on this process has focused on how
much of the energy gets through into living spaces. Also, there’s a
load of research (and marketing push) out there about reflective
surfaces and coatings for hot climates—but almost nothing about
using the heat captured by dark roofing to augment heating systems
in cold climates.

Measuring Materials CREDIT: BORAL

Solar Reflectance is the fraction of the incident solar energy which is reflected Solar Potential? Dark roof tiles, such as these Ebony concrete tiles
by the surface in question. It is usually correlated with the solar reflectance, from Australian manufacturer Boral, may represent an untapped
but the two quantities are not necessarily equal. For example, a white coating solar resource.
with a solar reflectance of 0.8 typically has a visible reflectance of about 0.9.
Solar Reflectance Index (SRI) is a measure of the roof’s ability to reject solar ACTIVE SOLAR: A BETTER APPROACH
heat, as shown by a small temperature rise. It is defined so that a standard
black (reflectance 0.05, emittance 0.9) is 0 and a standard white (reflectance The real potential of dark roofs is to use them as active, not
0.8, emittance 0.9) is 100. Materials with the highest SRI values are the coolest passive systems. A typical cold climate installation guide for clay
choices for roofing. Due to the way SRI is defined, particularly hot materials tile roofs includes several details for venting the roof to avoid
can even take slightly negative values, and particularly cool materials can ice dams and trapped moisture. The end game of these designs
even exceed 100. is to discharge the “unwanted” hot air through some form of
ridge vent.
58 GREEN BUILDER January/February 2016
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