Responding to ‘Ballast Cooler Than You Think’

The recent article by Annie McCarren called ‘Ballast Cooler Than You Think’ published in Environmental Design and Construction provides an overview of the “cool” properties of thermally massive ballasted roofs.  We largely agree that ballasted roofs can be a good roofing option under the right circumstances.  There are, however, a few inaccuracies in the article that are worth dispelling because they recur frequently in discussions of cool roofing concepts.

The extra thermal mass of a ballasted roof can make a lot of sense, particularly in climates that are hot in the daytime and cool at night.  Some codes and standards do allow ballasted roofs to count as exceptions to cool requirements, though the threshold weight required for the tradeoff is pretty high and will probably not be a cost-effective substitute for reflective roofs. For example, California’s Title 24 building code allows stone ballasted roofs weighing 17 pounds per square foot or a roof with pavers weighing 25 pounds per square foot to qualify for the exemption, which is heavier than most ballast rock on the market. Ballasted roofs may also increase nighttime cooling energy demand in areas with hot days and warm nights.

There are a few inaccuracies to address, however.

First, reflecting sunlight is not the same thing as reducing heat flow into the building (with insulation) or delaying heat flow into the building (with thermal mass). Increasing roof reflectance rejects solar energy as light, cooling the building and the outside air. Adding insulation to the roof reduces heat flow into the building, while raising the roof surface temperature and warming the outside air.  Adding thermal mass to the roof makes it slower to warm during the day and slower to cool at night. Ideally, a designer would opt for a high-performance roof system that optimizes the solar reflectance, thermal resistance, and thermal mass of the roof to cost-effectively gain the benefits of all three approaches.

The next few issues arise when the author says:  “A cool roof can be a great choice for buildings in warm, southern climates where the number of cooling degree days exceeds the number of heating degree days. The term ‘cooling degree days’ refers to the number of days that a building would need to run an air conditioning unit versus a heating unit to stay at a set temperature.”

One problem here is that the author neglects to mention that cool roofs are a viable roofing option in northern climates.  While cool roofs certainly make a great choice in warm southern climates, they can also provide net energy benefits in places where energy usage for heating exceeds cooling energy consumption. There are plenty of peer-reviewed studies to support the notion that cool roofs can provide net energy savings in northern climates. Perhaps more importantly, the well-established and rapidly-growing marketplace for cool roofing in northern regions (covering billions of square feet of roof space) demonstrates that the author is presenting an overly narrow view of the energy benefits that cool roofs are providing to building owners across the country.  The case for cool roofs is even stronger when the health benefits of cooler cities (such as cleaner air and lowered heat health risks) are factored in.

A second problem with the author’s statement is the misuse of heating and cooling degree days as a means to determine if reflective roofing makes sense from an energy savings perspective.  The article incorrectly defines heating and cooling degree days. Heating/cooling degree days gauge the need for heating or cooling that is driven by the difference between outdoor and indoor air temperature. For example, annual cooling degrees days (base, or threshold temperature of 65°F) would be determined by calculating for every hour of the year the outside air temperature and subtracting the base temperature (in this case, 65°F), summing all positive values, and dividing this total by 24 (hours in a day). But because cooling degree days and heating degree days are based on outdoor air temperature and do not account for the sun’s ability to heat buildings, they paint a misleading picture. To illustrate this point, consider a cool sunny day during which the outdoor temperature never exceeds 65°F (meaning zero cooling degree days). The building may still require air conditioning to remove its solar heat gain. In fact, cool roof savings/penalties are not proportional to cooling/heating degree days. Rather, they scale with the solar energy that strikes the roof during hours when the building is cooled/heated.