Cool roof ratings explained, for Phoenix homeowners

Why roofs get rated at all

On a July afternoon in Phoenix, a dark roof can run 150 F to 170 F while the air sits around 110 F. Some of that heat works its way down into the attic, the ducts and the rooms below. A roof surface that stays cooler sends less heat down and ages more slowly, since heat and UV are what break most roofing materials down over time.

Two shingles or two tiles can look nearly the same color and still behave differently in the sun. Ratings exist so you don't have to guess. They come from standardized tests on the product surface, which means you can compare one maker's tile or coating against another's using the same yardstick instead of brochure adjectives.

Solar reflectance: how much sun bounces off

Solar reflectance is the share of incoming sunlight a surface reflects instead of absorbing. It is written as a number between 0 and 1, or as a percentage. On that scale, a surface at 0.8 reflects about 80 percent of the sunlight that hits it, while one near 0.1 absorbs most of it and gets hot. Bright white coatings and membranes sit near the top of the scale, and dark asphalt shingles sit near the bottom.

Sunlight is more than what your eye sees. A large part of the sun's energy arrives as near-infrared light, which is invisible. That is why some tile and shingle colors use so-called cool pigments: they look like a mid-tone brown or gray but reflect more infrared than an ordinary pigment of the same shade. Color gives a hint, but the rating tells you what the surface actually does.

Thermal emittance: how fast it lets heat go

No roof reflects everything. The sunlight it absorbs turns into heat, and thermal emittance measures how readily the surface gives that heat back off as infrared radiation. It also runs from 0 to 1. A high number means the surface sheds heat quickly and cools down faster once the sun moves off it, which matters on summer evenings when the attic is still soaking up warmth.

Most roofing surfaces, including tile, shingles, coatings and painted metal, have fairly high emittance. The big exception is bare, shiny metal such as unpainted galvanized steel or aluminum. It can reflect plenty of sunlight yet hold on to the heat it absorbs, so it runs hotter than its shine suggests. That's one reason painted or coated panels usually beat bare metal in the desert, and why the finish on a metal roof matters as much as the panel.

SRI: one number that combines both

The Solar Reflectance Index rolls reflectance and emittance into a single score that predicts how hot a surface will get in the sun compared with two reference surfaces. A standard black surface is set at 0 and a standard white surface at 100. Very bright, high-emittance materials can score above 100, and very dark or low-emittance ones can fall below 0.

SRI is handy for quick comparisons, especially between different kinds of products. Its limit is that it's calculated under fixed assumptions about sun, wind and air temperature, not measured on your roof on a given day. Use it to rank options, not to forecast your power bill. Our earlier piece on what reflective coatings do for cooling bills explains why savings swing so much from house to house.

Where to look up the numbers

Two names show up on most data sheets. The Cool Roof Rating Council is an independent nonprofit that runs a rating program for roofing products. Manufacturers submit products, accredited labs test them, and the council publishes the results in a searchable list of rated products, showing initial values and values after three years of outdoor weathering. ENERGY STAR, the EPA program, has also published lists of reflective roof products that met its criteria. Its roof listings have changed over the years, so check what is current before leaning on an ENERGY STAR mention in an older data sheet.

A product with no published rating isn't necessarily hot. It may simply never have been submitted, which leaves you comparing it blind. When you look a product up, these habits keep the comparison honest:

• Search by exact product name and color, since ratings are listed per color
• Note both the initial and the three-year aged values
• Compare aged numbers between products, not initial ones
• Confirm the rated item matches what will actually go on the roof
• Keep a copy of the rating with your roof paperwork

Why the aged number matters more in the Valley

Initial ratings describe a clean, brand-new surface. Phoenix roofs don't stay that way. Dust settles daily, haboobs drop a layer of grit across whole neighborhoods, and the sun works on pigments and coatings every afternoon. White surfaces in particular lose some brightness as dirt builds up, and the three-year aged value captures that drop in a way the initial value can't.

Weathering at a test site still isn't the same as your roof downwind of a dirt lot or a busy freeway, so treat even the aged figure as a guide. Still, a product that keeps most of its reflectance after weathering will usually serve better here than one that starts higher and fades fast. On flat roofs, a gentle rinse during routine care brings back much of what dust takes, which matters even more on silicone-coated flat roofs, since silicone holds onto dirt.

Putting the ratings to work on tile, shingle and flat roofs

On flat foam and membrane roofs, the coating or membrane color decides almost everything, and white products rate very high. On tile, ratings help you compare colors inside the palette your HOA allows, and lighter or cool-pigment tiles score better. Tile also gets help a rating can't show: the air space between the tile and the deck carries heat away, which is part of why concrete tile roofs stay cooler underneath than their color alone suggests.

Shingles are where ratings vary most, because standard dark shingles absorb so much sun. If you're choosing shingles, ask for rated cool colors and compare aged SRI. Whatever the roof, ratings work alongside attic insulation and ventilation, not in place of them. For a straight answer on what fits your house and your HOA list, call (602) 878-3594 at any hour.

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