Welcome to the Energy Efficiency Myths series from Direct Energy! As many myths arise from incomplete knowledge, they can create seemingly possible answers that many people accept as fact. Each month, we will examine common misunderstandings about energy efficiency — energy savvywhether it’s in your home or about the energy industry — and deliver real facts behind the myth (and how they they might be costing you money).
Summer Cooling Myths — Part Two
Some myths grow out of over simplifications or generalizations of important and valid research. When that happens, consumers get easily misled by a snappy headline. The fact is not every type of structure shares the same performance or problems with a particular product or practice. In short, what’s true for a warehouse isn’t so for a suburban home.
White Roofs Increase the Earth’s Temperature
Back in 2011, researchers at Stanford University found that heat energy reflected by white (cool) roofs pumps more heat into pollution particulates (black carbon or soot). This increases the local air temperature and so reduces cloud cover, allowing more solar heat to warm the ground.
Not long after, a piece published in Huffington Post, “White Roofs, Green Myth?” also pointed out that white roofs in cooler climates tended to have problems with condensation.
Now, if you’re a green-minded homeowner, these two pieces of information might dissuade you from investing in a white or cool roof and go on enduring the cooling costs of a dark roof. After all, a lot of the credible sites feature stories about the “white roof myth.” It seems like a bad idea for your home and the planet, right?
The trouble is that these articles about white roofs aren’t exactly accurate with the facts.
The bulk of these articles (which date between 2009 and 2013) are not about structures like your house. First of all, the Stanford study concentrates on urban heat islands, which collect and concentrate heat so that dry, exposed urban surfaces, such as roofs and pavement, reach temperatures that are 50–90°F (27–50°C) hotter than the air. That means we’re talking about densely populated areas with lots of multi-story concrete buildings.
Second, condensation problems with white roofs mentioned in these articles usually involve flat or low-sloping commercial/industrial roof structures that rely on a single membrane roofing system over insulation. These are vastly different from common residential roofs that rely on shingled roofs with soffit ventilation where the roof deck does not come in contact with the attic insulation.
There’s also been more research about the overall impact on white or cool roofs in urban areas. One of the most recent studies from Notre Dame showed that cool roofs could lower roof temperatures by 14°F in downtown Chicago during peak demand hours in a typical August heat wave. By reducing the relative air temperature difference between the air over Chicago and the air over nearby Lake Michigan (which is 9°F cooler than downtown), this weakened the breezes blowing in from the lake to the shore that help cool shoreline neighborhoods. Researchers also discovered that cooling extended upwards as high as 1.5 miles into the lower atmosphere. While that sounds good, such cooling reduces regional air convection currents so that air would be more likely to stagnate near ground level. This runs counter to the increased convection air circulation that one would expect from the Stanford study, which predicts increased heating of the ground.
Does a White Roof Make Sense for My Home?
Since a white or cool roof primarily effects your air conditioning costs, then the answer here is it depends on where you live. Other things to consider include how well air-sealed and insulated your attic is, how well air-sealed and efficient your HVAC system is, and how well ventilated your attic is —particularly because things like air-sealing and insulation tend to be cheaper projects compared to installing a new roof. A good tool to help you evaluate the cost and benefits of a white roof for your home is the Oak Ridge National Laboratory Roof Saving Calculator.
If you live in the southern states, one roofing technique you might want to consider looking into is “above sheathing roof ventilation.” While more expensive than regular shingle roofing, this method uses a roof covering (such as tiles) fastened to battens nailed onto the roof sheathing. This creates a space underneath the roof covering for air to circulate, keeping the roof and the attic space cooler.