Three ways to make your roof more energy-efficient
Not everyone can have a living roof, but most people can have a “green” roof fairly easily. And the great news is that improving the energy efficiency of your roof doesn’t have to be costly. Small improvements can deliver a big difference, not only to your home’s energy usage, but also to the size of your energy bill.
1) Cool your roof
"Cool" roofs are lighter in color than traditional black asphalt or dark wood shingles and save energy by reflecting light and heat away rather than absorbing them. This is known as "the albedo effect," and many studies have documented significant energy savings from simply lightening the color of a roof.
- The problem with dark roofs: The temperature outside may be 95 degrees, which is plenty hot, but your dark roof is conducting a much higher temperature down toward you and your family, making the rooms below hotter and forcing the air conditioner to work harder, as Barbara Collins points out in her answer to the Ask A Pro question "NYC has a 'cool roof' program. Is there a white paint or coating to make my roof cooler?" In New York City, she notes, a dark-colored asphalt roof can reach 150 to 175 degrees on a hot summer day. This not only increases the energy needs of your home, it also contributes to the heat island effect in urban areas.
- The benefits of cool roofs: A cool roof, on the other hand, can be 50 to 60 degrees cooler than a conventional dark-colored roof, reducing the cooling load on your home, saving energy and reducing utility costs. By decreasing the solar gain and heat retention of your home, a white roof also increases its comfort and reduces the heat island effect to which your home’s dark roof contributes.
- How to cool your roof: The easiest route is to slap a cool roof coating onto an existing roof. Check out these recommendations from David Edwards, PhD, of EarthBound Homes, for coating options. If you’re already re-roofing, or building a new residence, consider replacing shingles or asphalt with lighter-colored versions. If you have a flat-top roof, Edwards suggests DuraLast’s sheets of fiber-reinforced white PVC membranes, in his Q&A "I'm reroofing a typical sloped roof—what materials should I be considering?" A quick search of the Cool Roof Rating Council’s Rated Products Directory will provide you with performance data about various roofing products. Products are rated for their reflectiveness: the higher the number, the more reflective the product is, and the cooler your roof will be.
2) Insulate from below
Insulation is one of the first things people think of when they set about making their homes more energy-efficient.
According to the Rocky Mountain Institute (RMI), the attic is usually the top priority, because installing insulation there is easy and provides immediate benefits. When insulating the attic, it’s important not to forget about the roof. Unfortunately, deciding on the best insulation isn’t always so straightforward, but our experts have advice on everything from the materials you should consider to how they should be installed.
- The problem: Roofs are your heating and cooling system's biggest nightmare. A poorly insulated roof will require more air conditioning in the summer and more heating in the winter, and will make your attic uncomfortable for use as a bedroom or study. Even if you do have insulation, it may not be enough. Check out Steve Saunders's comprehensive answer to the question "Is three inches of batt insulation enough for my attic?" for more information.
- The benefits of insulation: A well-insulated roof, on the other hand, can almost negate the need for either mechanical system and will drastically reduce your heating and cooling needs (along with your electricity bill and your home’s emissions). Mick Dalrymple, in his answer to the question "What type of roof should I choose in a hot climate?", describes how his roof’s insulation and weatherization coatings make air conditioning unecessary until outdoor temperatures reach over 100 degrees in Scottsdale, Arizona.
- How to properly insulate your roof: The material you choose will depend heavily on the type of house you have and the climate you live in. Our experts have advice on everything from the more high-tech solutions such as home foam insulation (even a cogent answer to the open cell vs. closed cell foam debate) and cellulose insulation to simpler options such as fiberglass or cotton batting. Those with open beams (especially cathedral ceilings) in cooler climates should also consider a radiant barrier—simple reflective film that wraps around beams and reflects heat rather than absorbing it. No matter what material you choose, leaving an air space between the insulation and the roofing material is generally a good idea. Although in very dry climates, and with some metal roofing, it may not be necessary, an air space will typically reduce the possibility of moisture getting trapped in either the structure or the insulation and causing mildew or mold to grow. Read Mike Binder's answer to the question "Should I leave an air space between my attic insulation and the wood roofing?" for more details.
3) Re-roof sustainably
Whether you’re putting a roof on a new home, or re-roofing an existing residence, it’s a golden opportunity to do things as sustainably as possible.
But with new, supposedly green roofing materials coming to market every month, it’s hard to separate the great from the greenwashed. In her answer to the question "What are the sustainable options for re-roofing our 30-year-old home?", Cynthia Phakos advises that anyone looking to re-roof their house start not by reading about the coolest new roofing materials, but by understanding their home’s specific conditions, particularly climate.
- The problem: You want to choose a roof that lasts a long time, improves your home’s efficiency and comfort, and reduces the heat island effect. That ideal roofing material will shift depending on where you live and the sort of house you have, but our experts agree that wood shingles are probably not going to cut it no matter where you live, primarily because they degrade rather quickly and it’s not sustainable to replace your roof every 15 years.
- The benefits of sustainable roofing materials: Simply choosing the right material could result in as much as a 30 percent decrease in your home’s energy needs. Whether that ends up being a metal roof (which is generally more expensive, but does deliver meaningful energy savings), tile roof, asphalt shingles, concrete or even wood, all of our experts recommend reflective coatings to make your roof cool. Elastomeric rooftop coatings not only reflect heat, and thereby save energy, but they can also extend the lifetime of your roof.
- How to sustainably re-roof your home: Any decent roofer should be able to advise you on the best roofing material for your house type and climate, but it’s a good idea to have a grasp of the basics before talking to a professional. “If you live near the coast, you'll want to select roofing that will withstand the salt spray and have no wind uplift,” Phakos writes. Once you’ve got a solid understanding of the conditions facing your roof, you’ll need to consider both roofing materials and sheathing. If you’re taking the roof down to the rafters, particularly if you live in a cooler climate, consider installing a sheathing with a radiant barrier—a laminated sheet of aluminum that will reflect heat into your home instead of absorbing it.