10 Ways to Cool Your Roof

On a hot, sunny day, the temperature on the surface of a traditional black roof

On a hot, sunny day, the temperature on the surface of a traditional black roof can be more than 150 degrees Fahrenheit (65 degrees Celsius). When you imagine that sweltering rooftop next to many others, it's no surprise that the air in a large city filled with dark, heat-absorbing rooftops can often be hotter than the air in surrounding areas.

Even in rural and suburban neighborhoods, the heat absorbed by a traditional roof on a sunny day raises temperatures inside the house, contributing to higher air conditioning requirements, steeper energy costs and discomfort for those without air conditioning. Fortunately, the remedy for all this extra heat — a cool roof — is relatively low-tech, low-cost and increasingly easy to find.

A cool roof is a roof designed to maintain a lower surface temperature in bright sunshine, more than 50 degrees F (28 degrees C) cooler than a traditional roof. The surface of a cool roof reflects more sunlight and releases more heat than a so-called hot or dark roof. While the term "cool roof" is sometimes used to describe any roofing surface that is cooler than a standard rooftop, green certification programs such as LEED, Energy Star and the Department of Energy Building Technology Program base their respective cool-roof credentials on a roofing material's ability to reflect the sun (solar reflectance) and release absorbed heat (emissivity). The higher the solar reflectance and emissivity (in decimals on a scale from 0 to 1), the cooler the roof. 0 would represent truest black and 1 would represent truest white.

Ready to save money, conserve energy and cool the planet by turning your old, hot roof into a cool roof? Read on to discover 10 ways to cool your roof!

Just about any kind of roof can be replaced with a cool roof. And some roofs can be converted to cool roofs by adding special coatings or surface materials. In some states and municipalities, cool roofs are required for all new commercial construction.

Cool roof requirements are based on the roof's slope, with more stringent requirements for low-sloped roofs — those that rise less than 2 feet (0.6 meters) in height for every 12 horizontal feet (3.7 meters) — than for steep-sloped roofs, which rise more than 2 feet (0.6 meters) in height every 12 horizontal feet (3.7 meters). To determine which roofing materials are best for your home or building, first determine the slope of your roof, then select the cool roof product that best suits your slope, budget and architectural style.

One of the most common cool roof options for a flat or low-sloped roof is a modified bitumen roof. Modified bitumen roofing is a membrane made up of sheets of plasticized or rubberized asphalt held together with rolled reinforcing fabric and fastened to the roof deck with hot asphalt or cool adhesive. Traditional modified bitumen roofs are black or gray with a low solar reflectance, meaning that they absorb, rather than reflect, most of the sun's light. But simply applying a white coating over the surface of the material at the factory can raise the solar reflectance to acceptable cool roof standards.

Another type of cool roofing material used for low-sloping roofs is a single-ply membrane. Single-ply membranes are prefabricated plastic or vinyl sheets containing solar reflective coatings or materials. The sheets are rolled onto the roof deck, then attached to the structure with some combination of chemical adhesives, mechanical fasteners or ballast, such as gravel or pavers. Single-ply membranes don't require any additional coatings or surfacing — the cool roof properties are integrated within the product itself.

There are several types of single-ply membrane, but the three most common are polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM) and thermoplastic polyolefin (TPO). Not all single-ply membranes are cool roof products.

Built-up roofing is the term for the standard tar-and-gravel roof. Because it's inexpensive, easy to apply and easy to repair, it's among the most commonly used roofing systems for flat and low-sloped roofs. Built-up roofing involves layering a base sheet of asphalt or bitumen with fabric, followed by a protective layer of gravel, mineral granules or some sort of aggregate rock.

Traditionally, the asphalt and surface gravel in a built-up roof have been black or dark gray, trapping much more light and heat than they release, but there are several ways to make a built-up roof cooler. One is to replace the surface layer with white gravel or reflective marble chips to increase its solar reflectance. Another is to apply something called a mineral surfaced cap sheet, which incorporates reflective granules or a reflective coating into the roofing material itself. Finally, special coatings or pigments can sometimes be added to an existing asphalt surface to convert a standard roof to a cool roof. This typically requires one coat of a reflective material and another of an emissive material that releases heat.

The average cost to install a cool built-up roofing system, or any cool roof system, is comparable to that of a traditional roof. While some cool roof products are slightly more expensive than traditional roofs, the extra cost is often recouped through energy savings.

Cool roof coatings are best applied to low-sloped roofs in good condition. They are divided into two categories: cementitious coatings, which contain concrete or ceramic particles, and elastomeric coatings, which contain added polymers that make them less brittle and more adhesive to building surfaces.

Both cementitious and elastomeric coatings improve the solar reflectance of the roof surface. The primary difference is that elastomeric coatings act as a watertight membrane, while cementitious coatings must be applied over roofing materials that have already been thoroughly waterproofed.

Cool roof coatings are sprayed, rolled, or brushed onto an existing flat or low-sloping rooftop to create a bright white, paint-like reflective surface. These coatings are more commonly used in re-roofing projects than in new construction.

For the steep-sloped roofs found on most U.S. homes, standard asphalt shingles can be replaced with cool asphalt shingles containing specially coated reflective granules to help them better reflect sunlight. Shingles made from wood, polymer and metal can also be coated to improve their solar reflectance.

Shingles are generally not as reflective as any of the cool roof options available for low-sloping roofs, but their familiar appearance and traditional style make them a popular choice for residential construction. A cool shingle with white granules costs about the same as one with the old, standard dark asphalt granules, while a shingle with its specially coated granules colored to match the desired color of the roof can cost a little more.

Unpainted metal roofs are naturally reflective (high solar reflectance), but they tend to hold heat in (low emissivity), so they are generally a better choice for steep roofs than for flat or low-sloping roofs, which receive more sunlight throughout the course of a day. A factory-applied cool roof coating can improve the heat releasing properties of a metal roof, enough so that some coated metal roofs are able to qualify as cool roofs.

A standard unpainted metal roof can be a cost-effective solution for some steep-sloped applications, since it is already reflective and requires no additional technologies or materials to achieve cool roof status.

While they aren't as widely used as shingles, both traditional and "cool-colored" tiles can be good choices for cooling a steep-sloped roof. Some traditional tiles have naturally reflective properties, depending on their color and the type of the clay they contain. Cool-colored tiles are created with pigments that reflect solar energy in the infrared spectrum.

Cool-colored tiles are typically produced in earth tones such as brown, green and terracotta. Many qualify as Energy Star materials for steep-sloped roofs. The cost of these cool-colored tiles is comparable to the cost of traditional roofing tiles.

If you'd rather enjoy the benefits of a cool roof without the addition of new surface materials or coatings, consider a roof mist coolingsystem. Also called an evaporative cooling system, a roof misting system lowers surface temperatures by spraying an extremely small amount of water across the roof, allowing the water to cool the roof as it evaporates. Think of it as akin to the way sweating cools your body.

At its most basic, a roof misting system can be little more than a garden hose set to the mist setting and aimed across the roof. (The same method will also quickly and effectively cool your stone patio or wooden deck on a hot day.) More sophisticated roof mist cooling systems typically feature a misting pump with a built-in thermostat, timer and 360-degree misting nozzles that are strategically placed so the water hits every part of the roof. These systems are available for both industrial and residential use.

While a rooftop garden isn't technically a cool roof, it achieves many of the same benefits as qualified cool roof products and reduces stormwater runoff to boot.

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop, with varying levels of materials in between. For example, they may or may not contain an irrigation systems. Green roofs shade the buildings below from direct sunlight, and reduce both surface temperatures and surrounding air temperatures through evaporation and transpiration, the processes by which plants remove moisture from the soil and emit it through their leaves. Rooftop gardens are so effective, they can lower a city's ambient temperatures by up to 5 degrees Fahrenheit (minus 15 degrees Celsius), according to the U.S. Environmental Protection Agency (EPA).

With costs ranging from $10 to $24 per square foot for materials and installation, green roofs are more expensive than other cool roof options. But it's hard to put a price on the innumerable benefits they provide in addition to roof-cooling, such as beautification, improved air quality and noise reduction.

Ballasted roofing systems are roofs in which a membrane is laid on the roof deck, then held in place by the weight of concrete pavers or natural stones. These stones also provide an insulating barrier against solar heat, much like an adobe building in the desert or an old stone farmhouse that stays cool inside, thanks to its thick stone walls. Ballasted roofing systems are relatively inexpensive, and ballasted areas may be combined with rooftop gardens to reduce the overall cost per square foot of a landscaped roof.

While ballasted roofing systems don't meet current Energy Star guidelines for cool roofing options, the EPA recognizes their effectiveness in energy savings. They also do meet the cool roof standards in Chicago, which has been one of the earliest adopters of rooftop gardens and other green roofing technologies.

Originally Published: Feb 14, 2011

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