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The ‘cool roof’ paradox

There is no doubt about it, the effects of global warming are all too palpable, with longer and more intense heatwaves becoming the norm in summer. According to the European State of the Climate report, in 2022 Europe experienced its hottest summer on record and its second warmest year ever. This report also shows that Europe has been warming faster than any other continent in recent decades, with temperatures increasing at twice the global average rate. 

Buildings are, not surprisingly, struggling to keep a comfortable temperature for its inhabitants and contents. More people are resorting to air conditioning systems, which, in turn, generate more heat and more CO2 that are released into the atmosphere. According to a 2021 estimate from the French Agency for Environment and Energy Management (ADEME) 25% of French households and 40% of businesses currently have some kind of air conditioning equipment, with various degrees of efficacy.

In this context, the concept of ‘cool roof’ has been gaining momentum worldwide as the solution to mitigate the overheating of buildings, virtually becoming a synonym of supposed global effectiveness in terms of energy performance and sustainability.

But is this so?

Today, a cool roof is most commonly defined as a light colored roof designed to reflect sunlight, reducing in this way the amount of heat that permeates to the structure below, helping the building keep cool in the summer and consequently reducing the energy bills generated by air conditioning.

Opting for a cool roof – especially in warmer regions - would seem like a no-brainer. However, much of the information available out there creates confusion about its real benefits. Is it the best solution for the inhabitants of the building? Does it achieve substantial energy savings? Does it effectively mitigate the urban heat-island effect? Is it a sustainable solution in the long-term? Frequently, these concepts are mixed up as if they were one and the same.

The complete roof system must be considered

The cool roof concept is today linked almost exclusively to the (light) color of the roofing membrane, whether it’s the membrane itself that is light-colored or has been painted with a light-reflecting coating. However, a roof is actually a set of construction elements. It consists of different layers, each with a specific function.

In order for any roof to work properly and achieve the best thermal performance, each component must have a meaning and must work in synergy with each other. In short, we cannot consider only the top layer of the roof and not pay attention to the rest of the components.In order to truly achieve energy efficiency on any roof, a good thermal insulation layer is key, independent of the color of the roofing membrane.

A long-term view: does white remain white?

A subject not frequently talked about around the topic of cool roofs is what happens long term. First of all, the reflective quality of the roofing membrane (or coating) will not stay the same forever. All light-colored exposed membranes will get dirty and suffer an inevitable loss of reflectivity over time, which in the first 3 years alone can reach between 35 to 50%. 

Already during the installation itself, the membrane gets dirty when stepped on by roofers coming and going. In roofs with photovoltaic installations, foot traffic is even more intense and the membrane gets dirty faster.

Afterwards, during the roof’s service life, membranes get dirty due to normally occurring phenomena such as dust in suspension or blown by the wind, pollution, birds... Rain does not really have a cleaning effect and, in some cases, can make the membrane even dirtier. 

Experience also shows that very little roof cleaning is actually done. The reason for this is mostly based on costs: to keep a light-colored roofing membrane clean and with its reflecting qualities as on the day it was first installed would be infinitely higher than any energy savings gained from the initial choice of color.

So far, there is not a lot of information available on long-term values ​​for energy calculations. Until more research has been carried out, it’s good to remember that an aged, dirty, light-colored membrane is not much different in terms of reflectivity from an aged, dirty, dark membrane.

Why put white on your roof, then? What is the real impact of a light-colored roofing membrane on the comfort in the building?

On a hot summer day, surface temperatures on a light-colored roof are around 40 to 45°C, while they can reach up to 70 to 75°C on dark surfaces. Does this mean that by implementing a light-colored roofing membrane we automatically gain 30°C in comfort inside the building? Obviously not! Everything will depend on the roof design. If the roof system consists of a corrugated steel deck without any insulation, when the outside temperature reaches around 45°C we will see a 15°C or more drop in the temperature inside or around the building. If the roof is insulated at a minimum, the drop in temperature inside the building will be much lower.

With a more efficient insulation, the gain in interior comfort by switching to a light-colored membrane becomes virtually non-existent. With regards to their effect on the urban heat-island effect, light surfaces will contribute to lower the ambient heat outside the building, and inside the buildings to a lesser extent.

Is a cool-roof sustainable?

The sustainability of a roofing membrane depends on several factors. One of them is the energy consumption required to manufacture it (its carbon footprint). Single-ply roofing membranes such as Elevate UltraPly TPO require less energy to be manufactured due to their low mass. In addition, its formulation is free of chlorine and halogens, and most important of all, it has a long life cycle.

The life cycle is related to the performance in use. An EPDM roofing membrane has a much longer service life than a light-colored membrane of the same thickness. This means that it will have to be replaced fewer times. If we take into account that re-roofing works generate more CO2 emissions, more waste and more costs, what is then the most sustainable membrane? The most reflective one, or the one that lasts the longest?

Continuing on the topic of the heat-island effect, recent studies carried out by Stanford University1 show that reflective roofs do not actually get rid of the heat problem, but displace it to neighboring surfaces. In an urban setting, the largest surfaces are not the roofs, but the streets and the building’s facades. Light-colored roofs can bounce the energy to large neighboring surfaces of glass curtain walls or concrete facades, and these surfaces have a greater capacity to accumulate heat and are not able to release it during night hours, actually worsening the situation.

In addition, tall buildings reflect the energy to the atmosphere in such a way that can disrupt rain cycles. White surfaces minimize the vertical movement of moisture to the atmosphere. This, in turn, reduces cloud coverage, resulting in reduced rain and an increase in drought-like conditions—the opposite result of the desired effect. A study from the Arizona State University published by the Proceedings of National Academy of Sciences (PNAS) confirmed that what might work well in one geographical area may not be optimal for another one. For example, reflective cool roofs might work well in California, but could reduce rainfall in Florida between 2 to 4 ml per day.

In short, to be truly sustainable, a roofing membrane needs to have a long life cycle, offer great performance and be suitable for solutions such as green roofs, which, besides keeping the roof cooler in summer than a white surface, and acting as an extra layer of insulation in winter, also provide benefits of oxygenation, evapotranspiration and others that are really useful to truly and sustainably mitigate the urban heat island effect.

Not a ‘one fits all’ solution

The usefulness of a cool roof should be calculated according to the climate, the geographic location, the type of building and the thermal insulation on the roof. Budget, of course, also plays an important part. A cool roof usually is an economical solution for renovations, but the cost long-term should be addressed, as well as the environmental cost.

“Going for a cool roof solution for renovating a poorly insulated building in a warm region such as the south of France can definitely be attractive,” says Jean-Luc Roudaut, Regional Prescription Manager for France at Holcim Solutions & Products EMEA. “However, in the north of France, where temperatures are cooler, questions start to arise: how much energy do I need to spend to air-condition the building in the summer vs. heating the building in winter? In winter, a reflective membrane or paint limits solar gain and can lead to an increase in heating needs, so the whole year needs to be considered, not just the summer”, he adds.

Before blindly going for a traditional cool roof approach, if you want to improve the comfort inside a building during hot periods, lowering the need for implementing cooling systems and, consequently, lowering energy bills, we advise taking into account the following points:

  • Do not see light-colored roofing membranes or reflective paints as the only solution. Widen your approach to the design of the entire building (facades, surroundings of the building, insulation, available budget, etc.) 
  • Consider the geographic location and the type of the building. Residential and industrial buildings have very specific needs.
  • How well insulated is the roof? Is it a renovation or a new build project?
  • Reflective roofs lose their reflectivity over time. Regular cleaning of the roof is crucial to minimize this loss of reflectivity.
  • If you use a reflecting coating, make sure no pollutants are released into the air or rainwater
  • The durability of the roofing system is one of the most important factors to be considered.

Elevate roofing systems offer a variety of solutions that can improve the thermal comfort inside buildings, and a cool roof is just one of them. Do not hesitate to contact one of our local representatives, who will be happy to discuss the option that best fits the requirements of your project.

*”Effects of Urban Surfaces and White Roofs on Global and Regional Climate”, Mark Z. Jacobson and John E. Ten Hoeve, Department of Civil and Environmental Engineering, Stanford University, Stanford, California