What Is A Cool Roof — And How Do I Get One?
There’s a term going around that you may have heard: heat island effect. The phrase refers to the disparity between temperatures in cities and those of the surrounding rural areas.
According to the Environmental Protection Agency (EPA), cities with 1 million or more people can be up to 5.4 degrees Fahrenheit warmer than their surroundings. At night, the difference is even more star: up to 22 degrees Fahrenheit warmer!
“Heat islands can affect communities,” the EPA writes, “by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water pollution.”
So what can we city dwellers do to deal with the heat island effect?
The answer is a term you may not have heard too much about: cool roofs.
Cool Roofs Defined
Cool roofs aren’t necessarily your hipster neighbor’s turquoise adobe tiles that catch your eye as you drive by. (However, it is true that a chic roof can also be very cool.)
No, what we’re talking about here are roofs — including turquoise adobe tiles! — that control the temperature of buildings and entire neighborhoods.
How? With innovative technology that reflects the sun’s heat and prevents heat absorption within the building itself.
Ultimately, this combination of heat reflection and heat-absorption prevention creates a more comfortable indoor environment. This in turn leads naturally to lower energy bills and a chilling effect on heat islands overall.
There are other proven benefits of cool roofs, including:
* Reduced maintenance costs
* Reduced air pollutant emissions
* Improved air quality
* Reduced energy use
* Increased utility rebates, if and where available
How Do Cool Roofs Work?
When it comes to calculating just how cool a roof is, there are two major factors that we must consider: solar reflectance and thermal emittance.
Solar reflectivity is measured on a scale from 0% to 100%. The difference between a cool roof and conventional roofing materials is overwhelming. Many cool roofs reflect more than 65% of solar energy away from the building. Conventional materials, on the other hand, only reflect about 5% to 15% away.
Thermal emittance is a roof’s capability to release absorbed heat. The higher a roof’s thermal emittance, the less heat is retained within the roof itself and the building below. Like solar reflectivity, thermal emittance is also rated on a scale from 0% to 100%. Cool roofs have been found to emit a drastically larger amount of heat than conventional roofing materials.
When combined, these two values provide an overall Solar Reflectance Index (SRI). This number provides a representation of how cool your roof is. SRI is an extremely important statistic. It helps determine how well a particular roofing material and that material’s color work on rooftops.
Types of Cool Roofs
Just like the buildings they top, cool roofs come in all different shapes and sizes. Deciphering which techniques and materials are most efficient depends on the type of building and, more importantly, the slope of the building’s roof. Techniques used in low-sloped roofs (roofs with slight inclines) are typically associated with commercial and industrial buildings.
Here are three common techniques for cool roofs:
1. A common technique for low-sloped roofs is creating a coated roof, which is when you coat just about any roof in a paint-like finish. This not only increases SRI but also improves the roof’s durability. Coating can be done to just about any properly prepared conventional roofing surface, and it’s not just white paint; it’s pigmented in a variety of materials and colors in order to perform unique cooling technologies, including ceramic nano technology.
2. Foam roofs have been very popular for decades, and they work great for insulating a building’s interior while also having a very high thermal emittance. The foam is usually made from two different liquid chemicals that combine to form a very lightweight, flexible roofing material that can go on most rooftops while reducing environmental externalities. This foam must have a coating on it in order to perform as a waterproof system. It isn’t a popular system for Oregon, though, due to our short window of summer.
3. Built-up roofing systems (BUR Systems) are a more complex type of low-sloped cool roofing material, and that’s because these roofs require multiple layers. These include a base sheet, a varying form of fiberglass reinforcement layer, and also a granulated protective top surface layer. This type of roof can be made from a variety of materials. One method is to put reflective materials into hot tar to increase solar reflectivity, such as reflective ceramic granulated cap sheets. Built-up roofs are sometimes referred to as tar and gravel roofs. They are converted to cool roofs by using reflective marble chips or gray slag rather than dark gravel. A modified bitumen (mod-bit) is a roofing system that is similar to a BUR system in that it uses asphalt to contain both cold and warm temperatures. These roofs also tend to be more elastic than the typical BUR systems due to the blend of rubber and asphalt.
If your low-sloped roof seems to need an extensive amount of repairs, then a single-ply membrane may be a good cool roof option. These are flexible, plastic polymer sheets, like polyvinyl chloride (PVC) material, and they make for a very high-quality cool roof. For the most part, these roofs are white, but several color options are available to accommodate aesthetic preferences.
Steep-sloped roofs are more common for residential homes, and the main materials used for steep-sloped cool roofs include metal roofing, tiles, asphalt shingles, and shakes. However, there are over 3,000 Energy Star-rated cool roofing materials that can be used for roofs with steep slopes.
Asphalt shingles are probably the most common form of rooftop. We’re all probably accustomed to seeing them everywhere. However, the problem with asphalt shingles is that their SRI is very low. Even white shingles only have a 30% solar reflectivity. (Other colors are much worse.) The reason why we see so many roofs with asphalt shingles: They’re cheap and easy to install.
In recent years, more and more people have been making the shift to metal roofs. These are extremely efficient — with a solar reflectivity of over 70%! Metal roofs are also very durable, lightweight, and can withstand even the toughest weather.
Other Examples of Reflective Coatings
There are many different kinds of reflective coatings, including white roof coatings, which have a solar reflectivity of 70% to 80%. One is silicon coating, which can also withstand ponding water, unlike latex coatings.
Aluminum roof coatings are also very efficient and can exceed a 70% solar reflectivity with some of the more premium brands. (Even better, indoor temperatures can also be reduced as much as 20 degrees Fahrenheit!)
Tiles, such as your chic neighbor’s flashy turquoise adobe tiles, can also be extremely efficient. They have an SRI of over 50% and a thermal emittance of around 86%. Tiles are predominantly made of clay, but concrete tiles have also become popular in recent years given that they are a little more durable in more extreme climates.
Pacific West Roofing
There is no denying that heat islands are a tangible part of urban living. But that doesn’t mean we can’t do anything about this complex environmental externality. Cool roofs may just be the answer.
A traditional dark roof can reach temperatures of 150 degrees Fahrenheit or higher when exposed to the summer sun. However, a cool roof under similar weather conditions can stay more than 50 degrees Fahrenheit cooler.
Cool roofs can extend the life of your roof and reduce thermal shock. It also helps un-air-conditioned rooms, such as garages, stay cooler. Finally, communities that make a concerted effort to convert to cool roofs can improve their town’s air quality.
Cool roofs are ideal for warm or hot climates. For those of us in the Pacific Northwest, we also have to consider our cold and damp climate. The moisture from condensation may result in eventual material degradation. It’s possible that cool roofs are more susceptible to accumulating moisture than similarly designed dark roofs. Portland is much more successful with darker roofs due to our location on the planet’s 45th parallel because we heat more months than we cool.
If you live in the Portland, Oregon, area, Pacific West Roofing can help guide you on your path to your own cool roof.
Questions? Pacific West Roofing has the answer. Get in touch today!
