Cool roofing has become one of the most significant green technologies, preventing heat absorption by reflecting the sun’s heat and then emitting its radiation back to the atmosphere. Green roofs and white roofs allow for a more comfortable and controller environment, where as “black,” or non-efficient rooftops, contribute to excessive heat issues. Some, however, say that cool roofs aren’t so great. We asked our Executive Director, Aurora Sharrard, just how cool are cool roofs?
Well, it depends.
Admittedly, the cool roofs discussion is very back and forth. However, the veracity of cool roofs is a technically sound and understood implementation, which is why people install them. There is a lot of science behind why we recommend people install cool roofs (of which white roofs are the coolest).
BuildingGreen did a nice summary of this in June 2014, offering the following summary:
- “In colder climates or in urban conditions where condensation or glare might be an issue, greater expertise and attention to detail might be required of architects, engineers, roofing designers, and contractors in choosing and detailing cool roofs (or choosing another roofing option).
- Solar panels—or a vegetated (green) roof—might be an even more sustainable roofing option, but they typically cost more. One of LBNL’s most recent reports, featuring an economic life-cycle assessment of black, white, and green roofs, found that white roofs offered the best value, with a 50-year net savings of $25/m2 ($2.40/ft2). With cool roofs offering an economically comparable and usually more efficient alternative, they’re a good fit for codes.
- They’re also usually in the owner’s best interest. “I do not need to have a cool roof on every building everywhere,” says Levinson. “But the science suggests that in warm, sunny places, it is good way to reduce solar heat gain and reduce energy use”—not to mention a way to achieve substantial savings on time-of-use electricity charges and demand charge savings.
- “There really is an energy sweet spot where you can do a roof with R-24 insulation instead of R-30 if you use a reflective membrane, adds Martin Grohman. “Why argue against that? If I am taking the side of the property owner who wants the most cost-effective solution, then I can deliver that with a cool roof.”
Plenty of evidence supporting white roofs over black already exists. Just like climate change, it’s only a handful of studies that distract from the primary research. Most times white roofs make sense.
The science and implementation of roofs in general are always evolving; there are a lot of issues, no matter what type of roof is being installed. The main point of contention is that in colder climates, the heating penalty for having a white roof (not necessarily a cool roof) outweighs the cooling benefit. Many important other details are often overlooked (or not even mentioned), mainly the positive benefit on heat island effect, which has an even larger impact on many buildings than a single roofing decision does on one. The Lawrence Berkeley National Laboratory (LBNL) and its Heat Island Group does a good job of detailing this point. In my opinion, they are THE resource on the science of cool roofs.
“Evolution of Cool-Roof Standards in the U.S.” offers:
- “Roofs that have high solar reflectance and high thermal emittance stay cool in the sun. A roof with lower thermal emittance, but exceptionally high solar reflectance, can also stay cool in the sun. Substituting a cool roof for a non-cool roof decreases cooling electricity use, cooling power demand and cooling equipment capacity requirements, while slightly increasing heating energy consumption. Cool roofs can also lower the citywide ambient air temperature in summer, slowing ozone formation and increasing human comfort.
- Provisions for cool roofs in energy efficiency standards can promote the building and climate-appropriate use of cool roofing technologies. Cool-roof requirements are designed to reduce building energy use, while energy-neutral cool-roof credits permit the use of less energy-efficient components (e.g. larger windows) in a building that has the energy-saving cool roofs. Both types of measures can reduce the life-cycle cost of a building (initial cost plus lifetime energy cost).
- Since 1999, several widely used building energy efficiency standards – including ASHRAE 90.1, ASHRAE 90.2, the International Energy Conservation Code and California’s Title 24 – have adopted cool-roof credits or requirements. This chapter of “Evolution of Cool-Roof Standards in the US” reviews the technical development of cool-roof provisions in the ASHRAE 90.1, ASHRAE 90.2 and California Title 24 Standards and discusses the treatment of cool roofs in other standards and energy efficiency programmes. The techniques used to develop the ASHRAE and Title 24 cool-roof provisions can be used as models to address cool roofs in building energy efficiency standards worldwide.”
One evaluation by researchers at LBNL and Nanyang Technological University recommends that every U.S. city and state, as far north as Chicago or Boston, require white roofs for new construction and end-of-life roof replacements of flat-roof commercial buildings.
So, the simple answer is: It depends. BUT the answer is also that unless you are in a REALLY cold climate, white roofs make more sense – they just also require considered design JUST like green buildings.
In case you’re interested, here is some other cool science around cool roofs:
- Long-term performance of high albedo roof coatings
- Measurement of albedo and analysis of its influence the surface temperature of building roof materials
- Peak power and cooling energy savings of high albedo roofs
- Effects of soiling and cleaning on the reflectance and solar heat gain of a light-colored roofing membrane
- Modeling impacts of roof reflectivity, integrated photovoltaic panels and green roof systems on sensible heat flux into the urban environment