Cooling Your Home Efficiently

Passive strategies can help your home keep its cool during the hot summer months and cut down on air conditioning use.

In the days before mechanical cooling systems, architecture responded to the climate. In the humid Southeast, where cooling needs predominated, houses had big porches, deep eaves, high ceilings, and plenty of well-placed windows to encourage cross-ventilation. In the desert South­west, the thermal mass of earthen berms and adobe blocks helped slow heat transfer through the walls to keep interiors cool. But mechanical systems that could operate independently of the weather—keeping a home’s interior at a steady temperature—changed our architecture, and many of the features that were once relied on for achieving comfort without energy input were shunted to the wayside.

Fifty years later, we’re beyond the age of cheap energy and we’re again turning to traditional passive cooling strategies. Unlike mechanical air conditioning, passive cooling approaches take their cues from the area’s climate—see the “Find Your Passive Cooling Path” sidebar. What’s appropriate in Santa Fe, New Mexico, won’t likely be effective in Savannah, Georgia.

Preventing heat from entering your home’s roof, walls, and windows should be your first priority. Combat this by using reflective surfaces, high insulation levels, heat-blocking window films or shades, and appropriately sized roof overhangs. Shading with vegetation and structures, and if you’re building new, properly orienting your home, are also important.

Reflect It

According to the U.S. Department of Energy (DOE), dull, dark-colored exteriors can absorb 70% to 90% of the sun’s radiant energy. And your home’s roof could capture about 30% of this undesirable heat gain, depending on its pitch and orientation. Dark-colored roofs can reach temperatures of 150° or more in the summer.

White or light-colored roofing materials (“cool roofs”) reflect sunlight, staying 50°F cooler than their darker counterparts, and reduce the amount of heat absorbed and passed through to the attic or to living spaces below. Three terms come into play with a cool roof material:

• Solar reflectance (SR) is the fraction of sunlight reflected by a surface. Measured on a scale from 0 to 1, an SR of 0.45, for example, would represent a roofing material that reflects 45% of the sun’s light. The SR of a dark roofing material is usually between 0.05 and 0.20, while a light-colored roofing material may have an SR between 0.55 and 0.90.
• Thermal emittance (TE) measures how efficient a surface is at emitting thermal radiation. Like SR, it also uses a 0 to 1 scale. Nonmetallic surfaces typically are more efficient emitters, with TEs between 0.80 and 0.95. So even though a bare metal surface may reflect as much sunlight as a white surface, it will stay warmer in the sun because it has a lower TE.
• Solar reflectance index (SRI) is a metric derived from SR and TE values. SRI uses a scale from 0 to 100; the higher the SRI, the cooler the roof will be in the sun.

The Cool Roof Rating Council (CRRC) tests roof products, assigning them a performance label that shows the measured SR and TE values. However, just because a product is listed in the CRRC directory does not make it a “cool” roof. A roof can qualify as “cool” by meeting or exceeding the minimum SRI requirement or by meeting or exceeding the minimum SR and TE values. Soiling and weathering affect a roof’s surface properties over time, so values are also given for “three-year weathered conditions” to account for this.

While white roofs tend to be good reflectors, colored roofing materials can also be manufactured to reflect sunlight. Known as “cool dark-colored surfaces,” these materials might reflect 40% of the incoming sunlight as compared to a conventional dark-colored surface, which might only reflect 20% of incoming sunlight.

Cool roofs are strongly recommended for climate zones 1 through 3 in the United States (see map and other information in the DOE’s Guidelines for Selecting Cool Roofs at bit.ly/CoolRoofGuide), where they can help achieve the greatest savings in cooling costs, although some microclimates may also call for cool roofs. (Be aware that in colder, heating-dominated climates installing a cool roof may  increase  energy costs.)