Chances are that you design and build your houses with a vented attic. This is the most preferred (and affordable) method of construction for most builders, and if you’re in the right climate, it’s perfectly fine. But a growing number of researchers say many builders aren’t doing a good enough job to make the space energy-efficient.

I recently discovered how important the attic space is to the overall energy efficiency of a house when I had an assessment done on my small, c. 1975 three-story townhouse in Hyattsville, Md.

My house has a three-year-old SEER 14 HVAC system, insulated replacement vinyl windows, and fairly decent (preexisting) fiberglass insulation. With three months to go in 2009, my electric bill is already $1,655.90. If history is any guide, by the end of the year I will have paid $1,865. And this is with a relatively mild Washington summer.

It’s been a mystery that my electricity bill continued to rise even though I have taken measures to help reduce it. A price hike by my utility provider, Potomac Electric Power Co., is one part of the explanation. But recently I found another: My unsealed and uninsulated attic was literally sucking conditioned air out of the house and money right out of my wallet.

This is partly the reason I’m uncomfortable in my house, why my house is dusty (despite my fastidious efforts), and why my electricity bill for one person was way out of control, says Dan Robinette, a comfort advisor at Minnick's, a Laurel, Md.-based heating and air-conditioning firm that participates in Maryland’s Home Performance with Energy Star program.

“Most attics are under-insulated and poorly sealed,” says Robinette, who did the assessment of my house. “This is a big problem when it comes to the comfort of the home. These two things affect everything from the temperature in the upper levels, to the air quality, to the number of times your A/C is running in a day. So with a little time and materials and you can get that attic sealed up, insulated, and be on your way to a happy home.”

Robinette’s recommendation? Seal the attic and have 13 inches inches of cellulose blown into the space, which is what I did.

“Air sealing and insulation have a large impact [on a house],” says Amber Wood, program manager for energy efficiency at the NAHB Research Center in Upper Marlboro, Md. “Before anything, you have to seal all penetrations into the attic such as electric boxes, ceiling fans, knee walls, and attic openings, but you have to make sure the soffit vent can ventilate or it can lead to moisture problems.”

Sealing the attic and blowing in insulation—either cellulose or fiberglass—establish the house’s thermal envelope, help prevent air leakage, and help maintain the temperature of the conditioned space below.

“In cold weather, warm air is continually rising,” Denver-based insulation manufacturer Johns Manville says on its Website. “Leaks into the attic allow the expensive, heated air to escape into the attic, while at the same time drawing in cold air to displace it from the basement or other exterior leaks. This continuous air movement makes the home feel drafty and raises energy bills. By sealing attic air leaks, you plug the escape route of rising air and effectively stop the chimney effect.”

Other common sources of attic leaks that Johns Manville says should be checked include: areas between floor joists and behind kneewalls, the attic hatch, wiring holes, plumbing vents, recessed lights, and the furnace flue.

The sealing and insulation of my attic came in at a shade under $1,400. Robinette says—and other consultants agree—that this is typically what it costs. (During my assessment, he also discovered that my house has severely leaky ducts and proposed AeroSeal treatment to have them sealed from the inside out, which he estimated would cost another $1,900.)

André Desjarlais, who manages the building envelopes program at Oak Ridge National Laboratory in Oak Ridge, Tenn., says Robinette made the right call on my attic. In fact, Desjarlais says people don’t realize just how important the treatment of the attic space is in the overall energy efficiency of a home.

“There are lots of stats,” Desjarlais says. “But let’s take the average home with 2.3 people. Half of the energy bill for that home is used to heat and cool it. Half of the remaining [portion] is due to energy losses through the attic.”

It would be logical to assume that under-insulated attics is a problem largely for older homes, and in some ways this would be an accurate assumption. Older homes tend to be leaky and poorly insulated, and some construction techniques from yesterday may not be appropriate for how homes are built today. “Yes, the older [the homes] get, the worse the problem is,” Robinette says.

But Mike Barcik, senior research engineer and director of technical services at the Southface Energy Institute in Atlanta, says attic inadequacies aren’t only relegated to old homes; many new homes suffer from the same ailments. “I would say 90 percent of all existing homes need some improvement and are probably under-built, including those built two years ago,” Barcik says. “I might even up that to 98 percent.”

The problem with new homes, Barcik continues, is that building inspectors are largely focused on safety, and so a home's energy usage gets overlooked. “The energy code is a fantastic code if it’s enforced, but it’s not enforced in many states,” he explains. “We have done blower tests to prove it.”

Neglecting the attic seems unthinkable given that it’s relatively cheap to address during construction of the house. Wood says, for example, that a builder may pay about $100 to $500 more in material to seal and insulate an attic. “It’s not that expensive,” she says. “They can do it either with foam and fiberglass, or they can use caulk. Caulking takes more time, but it’s inexpensive.” So the tradeoff is a $500 upfront cost compared to the $1,400 I paid to retrofit my house? That sure sounds like an investment that would be worth considering for new-home buyers and their builders.

Nigel Maynard is senior editor for products at BUILDER magazine.

Learn more about markets featured in this article: Washington, DC, Atlanta, GA.

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