There’s a term going around, you may have heard it: heat island effect. A heat island refers to the disparity between the temperatures in cities and surrounding rural areas, and researchers have concluded that an average American city can be anywhere up to 5.4°F warmer than surrounding areas during the day and a staggering 22°F warmer at night!
These higher temperatures equate to higher monthly energy costs for the average homeowner in densely populated areas, and one way homeowners in heat islands are combating these energy costs is by switching to cool roofs.
Cool roofs aren’t necessarily your hipster neighbor’s turquoise adobe tiles that always catch your eye as your drive by (although a rather chic roof is many times also cool). A cool roof can take all different kinds of appearances—including turquoise adobe tiles. What makes cool roofs such an innovative technology is that they control the temperatures of buildings and entire neighborhoods by reflecting the sun’s heat and preventing heat absorption within the building, ultimately creating a more comfortable indoor environment, lower energy bills and overall more chilled heat islands.
Some of the other proven benefits that go hand-in-hand with cool roofs include reduced maintenance costs, reduced air pollutant emissions, improved air quality, reduced energy use and cost and utility rebates.
How Cool Roofs Work
When it comes to calculating just how cool your roof is, there are two major factors that you have to consider: solar reflectance and thermal emittance.
Solar reflectivity is measured on a scale from 0%-100%, and the differences between a cool roof and conventional roofing materials are overwhelming in that many cool roofs reflect more than 65% of solar energy away from a building, while the conventional materials reflect about 5-15%.
Thermal emittance is a roof’s capability to release absorbed heat, so the higher a roof’s thermal emittance the less amount of heat is held within the roof and building. This is also rated on a scale from 0%-100%, and cool roofs have also proven to emit a drastically larger amount of heat than conventional roofing materials.
It’s the combination of both your roof’s solar reflectance and thermal emittance percentages that will give you the Solar Reflectance Index (SRI) percentage value of the roof, which ultimately represents how cool your roof is. SRI is an extremely important statistic that helps determine how well a particular roofing material, and that material’s color, works on rooftops.
Types of Cool Roofs
Cool roofs come in all different shapes and sizes, just like buildings. So it really depends on what type of building, and more importantly the slope of the building’s roof, to decipher which techniques and materials are most efficient.
The techniques that are used in low-sloped roofs, which are roofs with slight inclines, typically are associated with commercial and industrial buildings and include the following:
- 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 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.
- 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 any rooftop while reducing environmental externalities. This foam has to have a coating on it in order to perform as a waterproof system.
- 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 protective surface layer. This type of roof can be made from a variety of materials, and one method is to put reflective materials into hot tar to increase solar reflectivity, such as granulated cap sheets
- A Modified Bitumen, or 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.
- If your low-sloped roof seems to need some extensive amount of repairs then a single-ply membrane may be a good cool roof option. These are flexible, plastic polymer sheets like a polyvinyl chloride (PVC) material and they can 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 interests.
- 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 that we are all accustomed to seeing everywhere, but the problem with asphalt shingles is that their SRI is very low. Even white shingles only have a 30% solar reflectivity (and other colors are much worse). The reason why we see so many roofs with asphalt shingles is because they are cheap and easy to install.
- In recent years more and more people have been making the shift to metal roofs, and 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 that have a solar reflectivity of 70-80%. One is silicon coating, which can also withstand ponding water.
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°F!
Tiles, like 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 predominately 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.
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 be just the answer to making your heat island beach much more comfortable.
Do you have any questions about cool roofs? Feel free to contact us and let us know.