Cool Roofs Not So Cool: Are Their Green Benefits a Myth?

Yesterday, Samir Ibrahim of the Huffington Post wrote about reflective, or "cool", roofs, which have not only been praised for reducing heat island effect, decreasing HVAC loads, and lowering energy bills, but have also been mandated in cities including New York and Seattle, awarded points in the LEED green building rating systems, and included in California's building code. However, as Ibrahim pointed out, the benefits of cool roofs, which are often considered a cheap green fix, may be based on flimsy science, which is being refuted by new studies.

The idea that reflective roofs are not actually beneficial to the environment is not new. A 2011 study by Stanford University researchers, Mark
 Jacobson and John
 Ten
Hoeve
 found that "a worldwide conversion to white roofs, accounting for their albedo effect only, was calculated to cool population-weighted temperatures by ~0.02 K but to warm the Earth overall by ~0.07 K", meaning that they actually contribute to climate change by increasing earth's temperatures. The researchers believe this is because the solar heat relected off cool roofs is actually absorbed by dark-colored, airborne particulates in the atmosphere, rather than being directly reflected back into space. This means that even if the cool roof is making a building easier to heat or cool and saving energy, it could be warming the planet. In a press release, Jacobson said, "There does not seem to be a benefit from investing in white roofs. The most important thing is to reduce emissions of the pollutants that contribute to global warming". In other words, the benefits of cool roofs are negligible compared to the possible negative effects on global warming.

Plus, cool roofs may actually increase energy use in some northern climates. A majority of cool roof-supporting studies were performed at Lawrence Berkeley Laboratories in sunny California, but not in cooler climates, where cool roofs aren't as effective. As Nate Berg pointed out in a 2009 Planetizen article, the Cool Roofs Rating Council (CRRC), obviously a major component of cool roofs, even stated, "heating costs may increase slightly for buildings located in cooler regions". Berg pointed out that in these northern climates, there are a much larger number of heating degree days than cooling degree days, which means that cool roofs increase energy use more than they decrease it in these buildings.

So, it's time local and state governments take a step back and reevaluate whether cool roofs are actually beneficial in their area - and whether they are good for the climate, even if they are saving energy on a small scale. Perhaps USGBC, the creator of the LEED rating systems, should commission some studies to determine whether high reflectance roofs actually deserve points toward certification in all regions.

What About Green Roofs and PV Panels?

As those who are familiar with LEED may know, green, vegetated roofs and roof-mounted solar panels are also supposed to decrease heat island effect, reduce building temperature fluctuations and HVAC needs, and reduce global warming. If cool roofs are possibly a myth, does this mean green roofs and photovoltaic (PV) panels are too?

The answer, simply, is no. Vegetated roofs and solar panels work differently than reflective roofs, because like a conventional dark-colored, they absorb solar heat. However, unlike a typical roof, they use the solar heat for photosynthesis (green roofs) or energy generation (solar PV), which means that it is "trapped" at the roof layer. This is different from conventional roofs, which transfer solar heat to the building's interior.

Plus, green roofs are carbon sinks that produce oxygen, and solar panels decrease the need for fossil fuels, so they both have added environmental benefits that cool roofs don't have - they just come at a higher price.

LEED V4 and Cool Roofs

In the current LEED for New Construction 2009 rating system, cool roofs can contribute 1 point to Heat Island Effect - Roof. This credit awards points for covering at least 75% of the roof with materials with a high solar reflectance index (SRI) value, or with 50% vegetated roof, or with a combination of both. For this credit, low sloped roofs must have a minimum of a 78 SRI, and steep-sloped roofs must have a minimum of a 29.

Despite doubts about cool roofs' benefits, they will still be included in LEED v4. Sustainable Sites Credit 5, Heat Island Reduction (worth 2 points) in v4 for New Construction is pretty similar to the Heat Island Effect credit in New Construction 2009. The major difference is that it combines roof and non-roof measures for reducing heat island effect. Option 1 for credit compliance allows projects to use either roof or non-roof measures, including green roofs and/or a "high reflectance roof".

To be considered high reflectance, a low-sloped roof must have an initial SRI of 82 and a 3-year aged SRI of 62, and a high-pitched roof must have an initial SRI of 39 and a 3-year aged SRI of 32.

It's important to note that while solar panels cannot contribute to these credits, the roof space that is covered with PV (or skylights or other equipment) is excluded from the roof calculation - meaning that it's easier to achieve this credit if the project has solar panels.

So, while USGBC has improved the definition of a high reflectance roof by increasing the length of its "effectiveness", the fact that the credit still includes cool roofs means that the organization clearly still believes in the environmental benefits of cool roofs for all climates. In my personal opinion, it seems that USGBC may want to reevaluate this credit (maybe for LEED v5), and at the very least create a regional exclusion, or make cool roofs a Regional Priority. It doesn't make sense that buildings in northern climates are installing high reflectance roofs just to earn a LEED point or two, when it's actually making their building more difficult to heat, and therefore, less energy efficient. While the climate science behind cool roofs in cold climates is a little less tangible, possible negative effects could also be avoided with a regional exclusion or Regional Priority credit.