Cool Roofing in Northern Climates: It’s Not a Black and White Decision
The logic seems so black and white: white roofs are cool and black roofs are warm, therefore a white roof is better in a hot climate and a black roof is better in a cold climate. However, the logic begins to break down as one considers the application of cool roofs in the less obvious conditions of northern climates where the right answer becomes increasingly “gray.”
The energy efficiency benefits of cool roofs are dependent on a number of variables including temperature fluctuations of the climate, sunniness/cloudiness, internal heat loads within the building, number of stories of the building, etc.
The erosion of cool roof benefits in northern climates occurs when the heating penalty of a cool roof in the winter is significant and negates or exceeds the cooling energy savings in the summer. One seemingly consistent finding in many studies is that the savings, if not negative in the northern climates, are generally very small. Such outcomes highlight the need to be very selective in the use of cool roof membranes to achieve even a thin margin of energy savings in moderately cold northern climates.
When other costs are considered, such as periodic roof cleaning to maintain the effectiveness of a reflective roof surface, the meager energy savings (if any) for a particular application may be quickly consumed. Recent studies suggest the cost of cleaning a cool roof can be as much as 5 cents per square foot. The economic justification of a cool roof becomes much more difficult to achieve the deeper a building is positioned in a northern climate.
Regardless of whether or not a white roof can be economically justified in a particular northern climate building application, the question is moot if moisture vapor problems occur. While all roofs are sensitive to indoor relative humidity and air-leakage, white roofs show a greater sensitivity.
Compared to a traditional black roof, the amount of accumulated moisture during the winter can be two to three times greater in a cool roof construction. These implications suggest use of a white roof in a northern climate may require additional moisture control considerations such as improved air-leakage control and indoor relative humidity control.
A roof is more than just the top layer membrane, whether it is black or white. The roof assembly or roofing system is complex and made up of many parts and the top layer membrane is just ONE of those parts. Further, the decision of what to include in the roof assembly is based on many factors including building usage, occupants, location, orientation, climate, service life, and value.