Buzz
A cool roof is engineered to reflect more sunlight than traditional roofing, helping to lower the temperature. Image Credit: olaser/Getty ImagesKey Points to Remember
- Cool roofs reflect sunlight more efficiently and absorb less heat compared to conventional roofs, keeping temperatures over 50 degrees Fahrenheit lower, which contributes to both comfort and energy savings in hot climates.
- There are various ways to create a cool roof, using materials like modified bitumen, single-ply membranes, specialized coatings, and even green roofs with vegetation. The best option for your roof depends on its type and your budget.
- Implementing cool roofing techniques can reduce the need for air conditioning, cut down on energy expenses, and mitigate the urban heat island effect, making them an eco-friendly choice for both new constructions and retrofits.
On a scorching, sunny day, the temperature on the surface of a traditional black roof can exceed 150°F (65°C). When you think about that intense heat on multiple rooftops, it's no surprise that the air in a city filled with dark, heat-absorbing roofs can often be warmer than the air in nearby areas.
Even in suburban and rural areas, the heat trapped by a traditional roof on a sunny day raises indoor temperatures, leading to higher air conditioning demands, increased energy costs, and discomfort for those without air conditioning. Thankfully, the solution to this excess heat – a cool roof – is relatively simple, affordable, and easier to implement than ever before.
A cool roof is a roof designed to keep a lower surface temperature under direct sunlight, often more than 50°F (28°C) cooler than a conventional roof. This type of roof reflects more sunlight and releases more heat compared to a dark roof. While the term 'cool roof' is sometimes used to refer to any roof that is cooler than a standard one, green certification programs such as LEED, Energy Star, and the Department of Energy Building Technology Program determine a cool roof's qualifications based on the material's ability to reflect sunlight (solar reflectance) and release absorbed heat (emissivity). The higher the values for solar reflectance and emissivity (on a scale from 0 to 1), the cooler the roof is. 0 represents pure black, and 1 represents pure white.
Looking to save money, reduce energy use, and help the planet by transforming your old, hot roof into a cool one? Keep reading to discover 10 ways to cool your roof!
10. Install Modified Bitumen
Almost any type of roof can be swapped for a cool roof. Additionally, certain roofs can be upgraded to cool roofs by applying specialized coatings or surface materials. In certain regions, cool roofs are mandatory for all new commercial buildings.
Cool roof regulations are determined by the roof's slope, with stricter rules for low-sloped roofs—those that rise less than 2 feet (0.6 meters) in height for every 12 horizontal feet (3.7 meters)—compared to steep-sloped roofs, which rise more than 2 feet (0.6 meters) per 12 horizontal feet (3.7 meters). To select the best roofing materials for your home or building, first measure the slope of your roof, then choose a cool roof product that fits your slope, budget, and design preferences.
One of the most common cool roofing options for flat or low-sloped roofs is a modified bitumen roof. Modified bitumen roofing consists of sheets of plasticized or rubberized asphalt, reinforced with fabric and attached to the roof deck using hot asphalt or a cool adhesive. Traditional modified bitumen roofs are typically black or gray with low solar reflectance, meaning they absorb rather than reflect sunlight. However, applying a white coating to the surface at the factory can increase solar reflectance to meet cool roof standards.
9. Choose a Single-ply Membrane
Another popular cool roofing material for low-sloped roofs is a single-ply membrane. These are prefabricated plastic or vinyl sheets that feature solar-reflective coatings or materials. The sheets are unrolled onto the roof deck and secured using a combination of chemical adhesives, mechanical fasteners, or ballast like gravel or pavers. Unlike other roofing options, single-ply membranes do not require additional coatings or surfacing, as the cool roof properties are built into the material itself.
There are various types of single-ply membranes, but the three most common are polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM), and thermoplastic polyolefin (TPO). Not all single-ply membranes are suitable for cool roof applications.
8. Increase Layers
Built-up roofing refers to the classic tar-and-gravel system. It is a popular roofing choice for flat and low-sloped roofs due to its affordability, ease of installation, and simple repairs. This roofing style involves layering a base sheet of asphalt or bitumen with fabric and adding a protective layer of gravel, mineral granules, or other aggregate rocks on top.
Historically, the asphalt and surface gravel used in built-up roofs were black or dark gray, absorbing more sunlight and heat than they could release. However, there are several methods to make a built-up roof cooler. One way is by replacing the surface gravel with white gravel or reflective marble chips to improve solar reflectance. Another option is applying a mineral surfaced cap sheet, which contains reflective granules or a coating that enhances the roof’s reflectivity. Additionally, special coatings or pigments can be applied to an existing asphalt surface to convert it into a cool roof. This usually involves applying a reflective coat followed by an emissive coat to release trapped heat.
The cost of installing a cool built-up roof, or any cool roofing system, is typically comparable to that of a conventional roof. While some cool roof options may be slightly more expensive, the additional cost is often offset by energy savings over time.
7. Apply a Cool Coat
Cool roof coatings are most effective when applied to low-sloped roofs that are in good condition. These coatings are divided into two types: cementitious coatings, which are made with concrete or ceramic particles, and elastomeric coatings, which include added polymers that enhance their flexibility and adhesive properties to building surfaces.
Both cementitious and elastomeric coatings boost the solar reflectance of the roof surface. The key distinction is that elastomeric coatings serve as a waterproof membrane, while cementitious coatings require an underlying waterproofed roofing material for application.
Cool roof coatings are typically sprayed, rolled, or brushed onto an existing flat or low-sloped roof, creating a bright white reflective surface that resembles paint. These coatings are more frequently used in re-roofing projects than during new construction.
6. Choose the Ideal Shingle
For steep-sloped roofs, common on many U.S. homes, regular fiberglass asphalt shingles can be replaced with cool asphalt shingles that are designed to reflect more sunlight. Bouillante/Getty ImagesFor steep-sloped roofs, typically seen on most homes in the U.S., regular asphalt shingles can be swapped for cool asphalt shingles that are made with specially coated reflective granules, which enhance their ability to reflect sunlight. Shingles made of wood, polymer, and metal can also be coated to boost their solar reflectance.
Shingles are not as reflective as some of the cool roof options available for low-sloped roofs, but their traditional and familiar appearance makes them a common choice for residential buildings. Cool shingles with white granules are priced similarly to those with the standard dark asphalt granules, while those with specially coated granules in a custom color might cost slightly more.
5. Opt for Metal
Uncoated metal roofs have a natural reflective quality (high solar reflectance), but they tend to retain heat (low emissivity), making them more suitable for steep roofs than flat or low-sloped roofs, which get more direct sunlight throughout the day. Adding a cool roof coating at the factory can enhance the heat-releasing properties of a metal roof, making some coated metal roofs eligible to be considered cool roofs.
A basic unpainted metal roof can be a cost-effective solution for some steep-sloped applications, as it is naturally reflective and does not require additional materials or technology to achieve cool roof status.
4. Choose a Tile, (Almost) Any Tile
While not as commonly used as shingles, both traditional and 'cool-colored' tiles can be great options for cooling a steep-sloped roof. Some traditional tiles are naturally reflective, depending on their color and the clay used. Cool-colored tiles, on the other hand, are made with pigments designed to reflect solar energy in the infrared spectrum.
Cool-colored tiles are usually available in earth tones like brown, green, and terracotta. Many of these qualify as Energy Star materials for steep-sloped roofs. Their cost is generally similar to that of traditional roofing tiles.
3. Get a Little Misty-Eyed
If you'd like to enjoy the benefits of a cool roof without adding new surface materials or coatings, consider a roof mist cooling system. Also referred to as an evaporative cooling system, this technique cools the roof surface by releasing a fine mist of water that evaporates, lowering the roof's temperature. It works much like how your body cools itself through sweating.
At its simplest, a roof misting system can be as basic as a garden hose set to mist mode and directed over the roof. (This same setup is an effective way to cool down a stone patio or wooden deck on a hot day.) More advanced misting systems often include a misting pump with a thermostat, timer, and strategically placed nozzles that spray water evenly across the roof. These systems are available for both residential and industrial use.
2. Go Green (Literally!)
While a rooftop garden isn't considered a traditional cool roof, it provides many of the same benefits, such as reducing heat absorption, and also helps to manage stormwater runoff effectively.
A green roof, also known as a rooftop garden, consists of a layer of vegetation grown on a building's roof, with various materials in between. Some green roofs include irrigation systems, while others do not. These roofs provide shade to buildings, lowering surface temperatures and cooling surrounding air through evaporation and transpiration—the processes where plants release moisture from the soil into the air through their leaves. According to the U.S. Environmental Protection Agency (EPA), rooftop gardens can lower city temperatures by up to 5 degrees Fahrenheit (or about 15 degrees Celsius).
Green roofs, which cost between $10 and $24 per square foot for both materials and installation, are generally more expensive than other cool roofing options. However, their value goes beyond just cooling, offering additional benefits like aesthetic appeal, enhanced air quality, and noise reduction, making them a worthwhile investment.
1. Rock On!
Ballasted roofing systems feature a membrane laid over the roof deck, which is then secured by the weight of concrete pavers or natural stones. These stones serve as an insulating layer against solar heat, much like thick stone walls of an adobe building or a traditional farmhouse, which remain cool inside due to their dense walls. These systems are cost-effective and can be combined with rooftop gardens to reduce the overall cost of a landscaped roof.
Although ballasted roofing systems do not meet the current Energy Star criteria for cool roofs, the EPA acknowledges their role in energy savings. They also comply with cool roof standards in Chicago, which has been a pioneer in adopting green roofing technologies, including rooftop gardens.
