How to lower your energy bill with a roof overhang

This article was originally published on Houzz on April 12, 2012, as "Roof Overhangs Project Lower Energy Costs," and is presented here with permission. Read the original article.

Summer is coming upon us swiftly. Soon, if you are in the Northwest U.S., you will be outside enjoying all the sun you can get. If you are in the Southeast, you will be inside with air conditioning, taking a break from heat and humidity.

But no matter your locale, there is one building feature that can aid in summer cooling and winter warming: deep roof overhangs. Depending on depth and placement, large overhangs (or even strategic awnings) can shade a house in the summer and still allow warm light in in the winter.

In general, a south-facing overhang will direct the sun's rays lower on the window, allowing less light in and preventing that light from heating up the house in the summer. This means less need for air conditioning, less energy used and less money spent.

A good overhang will also allow more winter light to penetrate a building than summer light, thereby allowing light and heat in during the cooler months, which again saves in energy and electricity costs.

Overhangs are most effective for south-facing elements (in the northern hemisphere) and at midday. If the building element bears more than about 30 degrees off true south, the effectiveness of an overhang decreases, says the U.S. Department of Energy.

Even if you aren't interested in saving money by reducing cooling costs, large roof overhangs can still provide shelter and help protect a house from bad weather. Oh, and they look good, too.

Photo credit Coates Design Architects Seattle; original photo on Houzz.

Large roof overhangs on a house by Coates Design Architects in Seattle do more than put a modernist stamp on the structure. They also protect it from the Pacific Northwest's driving winter rain and mitigate hot summer sun.

Graphic credit DOE Office of Energy Efficiency and Renewable Energy; original photo on Houzz.

This diagram from the U.S. Department of Energy shows how a roof overhang can prevent hot summer sun from getting in. Here, the summer sun's angles are directed at the ground.

But that same smartly oriented overhang can also allow maximum light during the winter, as shown, when you want that extra bit of heat.

Photo credit Warmington & North; original photo on Houzz.

Roof overhangs can be all sizes, and they need not be purely utilitarian. This one, on another Pacific Northwest house, provides all the benefits of passive cooling and weather protection, but it's the drama of the overhang that may matter most. This house would be aesthetically naked without this roof.

Photo credit One SEED Architecture + Interiors, original photo on Houzz.

A diagram by One Seed Architecture + Interiors in Vancouver, British Columbia, shows how roof overhangs block southern sun but how south-facing glazing allows full sunlight exposure.

In this house, the thermal block walls—exposed to the sun during the winter—also absorb that winter sunlight and distribute the heat throughout the day, helping to warm the house.

Photo credit The Impatient Gardener; original photo on Houzz.

Not all roof overhangs are designed solely for passive cooling. This one is a quirky architectural addition, and it also protects people at the door from wind, rain and snow. 

Besides providing passive cooling and shelter, as in this example, roof overhangs also save doors and windows from bad weather (saving you money), and protect a house's exterior and foundation from water runoff.

Photo credit FINNE Architects; original photo on Houzz.

The roof overhang on this house in Michigan's Upper Peninsula, by Finne Architects, protects the house from sun, yes, but also from the Midwest's heavy snow.

The Department of Energy has some region-specific recommendations for roof overhangs:

  • For cold climates, at base 65 degrees Fahrenheit, locate the shadow line at midwindow using the June 21 (summer solstice) sun angle.
  • For moderate climates, at base 65 degrees during days when heat is needed and base 75 degrees during days when air conditioning is needed, locate the shadow line at the windowsill using the summer solstice sun angle.
  • For hot climates, at base 75 degrees, locate the shadow line at the windowsill using the March 21 (vernal equinox) sun angle.

Photo credit John Maniscalco Architecture; original photo on Houzz.

A house by John Maniscalco Architecture, covered in snow, showcases a surprising green benefit to large, flat roofs with overhangs: Light from interior and exterior fixtures is reflected and spread by the underside of the roof. This means fewer light fixtures and less energy used.

Photo credit Beard + Riser Architects; original photo on Houzz.

This house in Mississippi by Beard + Riser Architects is oriented along an east-west axis to maximize solar control. The front of the house faces south and features a large overhang that folds over the edge of the roof to block direct summer sun. The translucent fiberglass panels along the porch overhang allow filtered northern light into the house.

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