Do Reflective Roofs in the North Make Sense?
Cool roofs reflect the sun’s rays and reduce the air conditioning load on buildings. Membranes for commercial roofs often have reflectivities of 75 to 80 percent, or 0.75 to 0.80 SR, which means only 20 to 25 percent of the sun’s energy is absorbed into the roof. For many areas, cool roofing is a clear win and these membranes save money whenever air conditioning is used. For climate zones 1, 2, and 3, the savings can be very significant.
So, do cool roofs have a benefit further north where building heating costs are much higher? Could darker, less reflective membranes absorb heat during the winter and help lower building heating costs? In this first of a three-part blog series, we’ll look at the arguments.
Do non-reflective roofs help heat northern buildings in the winter?
- There is no evidence this is the case. During the winter, the sun’s angle is fairly low and absorption isn’t significant.
- Winter days are short, so the amount of sunlight hitting a roof is far less than during the summer.
- Winters have far more cloudy days than the summer; so again, not much solar heat reaches the roof.
- Buildings generally are not heated using electricity. Natural gas or oil is typically used, and both are lower cost.
- Target Corporation has for approximately two decades installed, almost exclusively, white PVC membranes on their stores. Fenner, DiPietro, and Graveline did a case study and noted that the roofs represented about 75% of the building envelope of these big box buildings. The study stated that Target has experienced net energy savings from the use of cool roofs in all but the most extreme climates. In fact, on the few “non-cool” dark roofs in northern climates, Target did not see any measurable reduction of energy consumption during heating seasons that could be attributed to heat gain via the roof.
Do reflective roofs help northern buildings in the summer?
- The benefit of reflective roofs in reducing the load on air conditioning systems is well established and not in dispute. Non-reflective roofs get very hot, often above 190°F, and that heat finds its way into the building.
- Days in northern cities are longer and the sun is high in the sky. If the roof is not reflective, this adds a large load to air conditioning systems.
- Buildings are cooled using electricity – the most expensive source of power.
- Utility companies price electricity to commercial customers using peak demand levels. This means if a building has very high demand in the summer, it affects charges all year ’round. For a larger discussion on this subject, read our demand charges blog post.
What about the number of heating days versus cooling days?
- The number of heating versus cooling days is not the metric to look at. A far better way is to look at air conditioning costs versus heating costs. If heating costs are larger, then it can make sense to evaluate non-reflective roof membranes.
- The best evaluation is done by examining heating costs, cooling costs, and total annual electric charges. The electric charges can be dominated by summertime air conditioning demand – and cool roofs are a key way to reduce those. Check out our demand charges blog post for more details.
Do reflective roofs stay reflective?
- Reflective roof membranes voluntarily conform to standards set by Energy Star®. Even though these are voluntary, they are often required by various local code agencies or they might actually be legislated.
- The Energy Star® cool roofing rating requires an initial solar reflectance of ≥ 0.65 and a three-year aged value of ≥ 0.50. The testing program is administered by the Cool Roof Rating Council (CRRC). Many of today’s reflective membranes exceed the standard.
- Reflective, smooth membranes do stay surprisingly clean as materials, such as TPO, which gets cleaned fairly effectively by the rain. If not, it can be worth discussing a cleaning program with a local roofing professional.
- About 50 percent of the sun’s rays are heat energy – this has a longer wavelength than visible light. It goes through thin layers of dust and grime and is still reflected by the membrane.
In Part 2 of this blog series, we take a look at the issue with moisture in low-slope roofs, something that’s been noted since well before reflective membranes made their debut.