Building an Energy Efficient Home Addition

A homeowner makes sure his retrofit of his 70-year-old home addition saves energy.

Building an addition to your home is the last thing you'd want to do if you're trying to reduce your energy bills. But that's exactly what Jeff and Sherri Wilson did as part of the Deep Energy Retrofit of their 70-year-old home.

The Wilsons simply needed more space. With two home offices and a family of four, their 1,000-square-foot Cape Cod was starting to get a bit cramped. Even after reconfiguring and reorganizing the space, they realized that the home would only be sustainable for the family if they expanded it slightly.


Jeff's Energy Efficient Home Addition

"An addition is the very last resort," Jeff Wilson says. "You really don't want to overbuild. Then you end up pumping energy into a structure that you rarely use. You end up having to clean a bigger space."

The key is to build the addition strategically. Jeff applied the best practices of modern green-building technology and began with a plan to replace the existing garage/office — a two-story appendage to their home that was the "leakiest, worst part of the house." In fact, when an energy rater came out to assess the home, Jeff nixed the garage from the overall home rating because it was so bad. "It was dangerous," he says. "It had to come down."

Jeff and Sherri had used the ramshackle space above the garage as a home office, with its four-foot Alice-in-Wonderland sized door (the sloped roof would allow no larger entry), rotted wood and floorboards supported only by the rafter beams. "I used to warn the kids not to jump up and down in Poppy's office," Jeff says about the space's overall instability. "The siding was really holding the back of the building together."

Expanding the Footprint

Step one was demolition. Then, a new driveway and insulated concrete foundation was laid for the garage and office addition. The old structure's footprint was 23 feet deep by 9 feet wide. The Wilsons would increase the width by seven significant feet, creating a loft space upstairs to serve as a roomy office and later be converted into an apartment.

The old concrete slab foundation had heaved and cracked, so Jeff replaced it with a new "floating" concrete slab and insulated four inches of foam board underneath to prevent heat from passing from the ground into the foundation. "The concrete slab and foundation can wick a lot of heat out of a space in a hurry if they're not properly insulated," Jeff explains.

Meanwhile, he raised the roof angle of the entire back of the house (old and new) to create a flatter surface to support solar panels. This would allow for a 13-foot ceiling on the addition's second level — no more ducking through the door.

The engineered lumber used to build the structure is sustainably harvested, meaning it comes from small-diameter, rapidly regenerating trees. It uses safe resins (no formaldehyde), so it's greener, straighter, stronger and easier to work with, Jeff says. Plus, it has a 50-year warranty.

The walls were insulated inside and out: inside with Spray-It Green foam, and outside with standard house wrap. The home siding is SmartSide, which is ready-to-paint engineered wood siding treated with nontoxic resins. "SmartSide is a great replacement for fiber-cement siding," Jeff says of typical siding that creates a lot of harmful silica dust when cut. "It's lighter, less brittle and stronger than fiber cement."

Jeff splurged on the $2,000 garage door, but it's "super-insulated," he says, adding that he wanted one with a high R value. The Clopay custom door Jeff purchased through Home Depot is the highest rated he could find at R-19. Other garage doors were R-4 to R-6.

Jeff estimates the total price tag on the home addition was $40,000 to $50,000.

Before You Build More

Before you add on to your home, Jeff encourages making the absolute best use of the space you have. Can you reconfigure the home by knocking out a wall to open up a space? Can you re-assign rooms to more purposeful spaces, such as turning a formal living room into an office or media/family room? If these changes aren't enough, will a simple bump-out provide enough room to accommodate your needs? If the answer is no, then plan an addition that takes up only the space you need.

Avoid over-building. Bigger spaces use more energy — and they require more of your personal energy for upkeep. How much space do you really need? "If you build it small, it won't be as expensive," Jeff points out.

Check building codes. Before you break ground, talk with your local building code office about regulations and filing permits.

Do it right the first time. Invest as much as you can in this new addition. It's better to put more features into a smaller space to make it as efficient as possible. "Build to the highest standards you can now," Jeff advises.

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