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Cool Roofs, and Why You Need One

white flat roof

In my last article I talked about was about a new roofing material that is designed to keep your roof and the interior if the house cooler than the temperature outside. Now, I’d like to take a quick second to explain exactly what a Cool Roof is and why it’s relevant for not just us in Phoenix, Arizona but throughout the country.

What is a Cool Roof?

A cool roof is, simply put, a roof that does a better job of keeping your house and interior of the roof cooler than a standard shingle roof. To qualify as a cool roof, it must redirect its heat – technically speaking, reflects solar radiation.

It’s kind of like when you wear black vs wear white on a hot day here in Arizona. If you wear a black shirt, you’re going to be pretty hot. If you wear something white instead, you will stay much cooler.

This is because white reflects more of the hot Southwestern sun than black does. As a result, it absorbs less heat – and keeps you cooler even as the sun beats down on you. This same concept is also why there are so many white cars in Arizona. With that being said, Cool Roof’s come in many different colors – not just black and white – as there are a variety of cool roofing materials to choose from).

Why Get A Cool Roof?  Benefits of a Cool Roof

The primary benefit of getting a cool roof is that it will keep the inside of your home much cooler in hot, sunny weather. The surface of a black roof can easily get up to 160F on a hot sunny day, and much of that heat is absorbed right through the roof and into the home below.

Thanks to cool roofing’s high solar reflectance and thermal emittance, however – a good portion of that heat is bounced right back up into the air, keeping your home significantly cooler.

This is great for comfort, but also has the added benefit of reducing your electric or cooling bill. Since your AC is working less, you get a smaller bill at the end of every month – sometimes by up to 20 to 30%!

That’s some significant savings, especially when you consider how much we run our AC here in the sizzling Arizona heat. Imagine saving 25% of your cooling bill every month?

Additionally, since the cool roof absorbs much less solar energy then other roofing materials, the roofing material itself will be much cooler, and thus will be less affected by the effects of UV rays and heat over its lifespan. That means less wear and less maintenance over the roof’s lifespan – which in turn translates to a much longer lifespan and less need to replace your roof.

As many high-end roofs already last 20-30 years with proper maintenance, cool roofing can make that upwards of 40 years provided the right materials, proper installation and effective maintenance are used. Opt for a cool metal roof? That can last more than 50 years.

How Cool Roofing Works

a cool roof

Solar Reflectance

Solar Reflectance (also referred to as Albedo in scientific circles), is the how much of the solar energy that hits the roof, is reflected right off it and back into the atmosphere.  solar energy that is reflected. Cool roofs have high solar reflectance compared to regular roofs – kind of like how a piece of shiny metal reflects more light than a piece of shiny plastic.

Thermal Emittance

Thermal Emittance is a measure of how much solar energy is radiated off the roof, in the form of heat. As no cool roof can reflect all the solar energy that hits it, some of that energy is still absorbed into the roofing materials. The higher the thermal emittance, the more of that energy/heat is radiated right back out into the air and atmosphere, instead of being absorbed into the attic and house or building below.

With the right materials, paint and coatings, a good cool roofing material can have a  thermal emittance of up to 90%.

When looking for a cool roof, you’re looking for materials with a high solar reflectance and a high thermal emittance.

Here’s a practical look at how this plays out:

When looking for a cool roof, you’re looking for materials with a high solar reflectance and a high thermal emittance. While the manufacturers will usually show these ratings on their materials, you shouldn’t need to worry about them as a consumer. Instead, if you’re in the the market for a Cool Roof, you are mostly concerned with the CRRC rating. This standard set by the Cool Roof Rating Council (CRRC) will show you an objective rating of a cool roof’s effectiveness so you can compare roofs against each other and find one you trust.

Cool Roof Design

What Are Cool Roofs Made Of?

Cool roofs are traditionally made of metal and come in white or some other light color. These days, they come in all different materials and colors, with asphalt shingles – indistinguishable from regular shingles – becoming more and more popular. They often come in slate or brown colors for those who don’t a very light-colored roof. Plastic cool roofing is also becoming more popular as technology advances, offering a more durable roof than asphalt shingles without the weight or cost of metal.

Styles

Many of the reasons that you’ll want to upgrade your roof is not just because it shouldn’t leak when it rains. There’s also a perfectly rational reason that a cool roof can be energy saving for the temperature inside your home. How is this possible? The science behind cool roofs design has been seen in several ways in recent years.

  • Low slopes roofs – This is where you’ll find roofs covered with single-ply membranes, modified bitumen, or spray polyurethane coatings. These work well on roofs that are low lying or flat and are relatively easy to apply.
  • Steel sloped roofs – Much like every other home that has a pitched roof where shingles and overlapping panels are used, the materials will vary greatly. There are asphalt shingles, fiberglass infused slabs, and wooden shakes that all are used to cover pitched roofs.
  • Low and steep-sloped roofs – This is where you’ll often see steel panels placed on these roof surfaces. These panels may be reflective with a natural finish or have an oven-baked finish. These panels can also have granules that are added to each sheet so that the surface is textured.
  • Cool Roofs VS Green Roofs

    Now with the benefits of adding cool roofs, we need to remember that these materials used aren’t always energy efficient. For them to become Green roof rated, the whole idea of cool roofs comes down to science Vs standard materials. As we can see from this handy guide for energy-efficient roof design.

    A green roof uses more efficiency to maximize how a roof can preserve the heat inside a home, or prevent heat from entering. This choice often sounds complicated but it has a lot to do with reflecting away heat. This is achieved by using a roof surface as a reflector rather than a magnet. Many high pitched roofs that have asphalt shingles are fine for insulation but if they are dark colors, they attract heat.

    installing a green roof
    But by using lighter colored shingles, the light is reflected off of them and prevents heat from being absorbed. The science behind this is rather simple but often complicated how energy is collected. Darker roofing materials will readily absorb the sun’s energy over lighter colors. In AZ, the amount of sun that homes receive in the day can be very generous and often brutal.

    The cost of air conditioning to keep a home cool inside is wasted if your roof acts like a magnet for storing heat. This is why it’s recommended to redirect that heat that may otherwise end-up inside your home. When it reappears as a secondary heating effect heating your home, this is from poor choices in Green roofing materials. It doesn’t end at that point just because of the color.

    Green roofing materials can allow your home to retain heat longer in the winter if the insulation that goes into the material itself. This is where single-ply materials help seal in the heat using layers of plastic, rubber, or modified bitumen. Another excellent choice is a spray polyurethane foam that is an excellent insulation for retaining heat and resisting heat. Be sure to ask us more about how you can turn your cool roof into a greener energy efficient one.

    Drawbacks to Cool Roofs?

    A cool roof is almost always a good idea, especially here in Arizona. People in colder climates, however, may want to consider a cool roof more carefully – as they keep the home from heating up as efficiently in the cooler months.

    Cool roofs can also have more problems with mold and algae growth than regular roofs, especially in humid climates. Because they do not get as hot regular roof, they can collect more condensation and create a more hospitable climate for mold and mildew, which like things cool and wet.

    For this reason, many cool roofing materials come with mold-resistant coatings and treatments to ward off mold or algae growth.

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    Juan del Leon is the owner of Estrella Roofing Company in Phoenix, Arizona. He has severed Arizona since 1997 with repair and installation services. Throughout the years he has gained knowledge of of the industry and utilizes his website as a place to blog about his experiences and share his expertise.

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