The color of a clay or concrete tile roof makes a huge difference in solar reflectance and absorption. It's time to harness this wasted energy.
Reading through some technical specs for tile roofs yesterday, I noticed that there's a big difference in the solar reflectance of clay/concrete tile, depending on the 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. That's 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.
But 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.
Active Solar: A Better Approach
From the few sources I could find on the topic, a couple of themes emerge. First, the real potential of dark roofs is to use them as an ACTIVE system, not a passive one. With a clay tile roof, their cold climate installation guide (available as PDF Here) includes several details for venting the roof to avoid ice dams and trapped moisture. But the end game of these designs is to discharge the "unwanted" hot air through some form of ridge vent, as shown.
What if, instead of discharging that heated air, it were to be captured and mechanically vented into living spaces? The pitfalls, to my understanding, would be related to improper installation of such a system, inadvertently causing poor roof ventilation that results in ice dams and other problems. But with a simple thermostatic control tied to a vent, it seems to me, you could have a fairly low-tech solution. Take a look at this somewhat amateurish step-by-step guide from wikiHow. Their building science is a bit crude, but the concept is good. For example, they suggest simply adding a duct at the ridge peak attached to a thermostatic control.
It's time for the roofing industry to start talking not just about "cool" roofs, but also "hot" roofs. Otherwise, we're wasting a lot of beneficial hot air.
If you've done a hot roof system of any type, please drop me a note.—M. PowerDefinition of Solar Reflectance HERE
- Boral Tile brochure showing dark colors HERE