ADU Powerhouse

Smart use of SIP panels and building science principles in this ADU shows people it is easy to cut energy bills.

When it comes to energy usage, smaller can be better. That was certainly the case when it came to “Coradu,” this year’s Green Builder Home of the Year in the General Homes category.

HOTY-2024-logoFProject Stats

Name: Coradu, Seattle, Washington

Builder: Dan Allison, Abode Builders 

Architect/Designer: John Novak, Velocipede Architects 

Photographer: Chuan Photography

According to Dan Allison of Abode Home Builders (AHB), the Seattle-based project was designed to reuse the existing foundation of an abandoned structure, one that was on a tight building site—a place to live where a non-descript shed was. It wouldn’t be the first time AHB would take on such a job. 

“We already have two high-performance structural insulated panel (SIP) houses that use about a quarter of the energy used by conventionally built structures,” Allison notes. 

“Being able to quickly and efficiently build on a tight site in a wonderful residential neighborhood allowed us to show the neighbors what they didn’t know was possible.”

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The project was also inspired by a 700-square-foot detached accessory dwelling unit (DADU), which used more electricity than a 2,000-square-foot SIPs home on the same property. “It was insane to think that a building one-third the size used three times the energy,” Allison says.

In contrast, AHB’s new 900-square-foot building exceeds net zero and produces all of the owners’ energy needs, including for two electric vehicles (EVs), courtesy of features such as solar panels, thick SIP walls and roofing (6-inch GPS and 10-inch GPS, respectively), all LED lighting, ENERGY STAR appliances, triple-pane windows (U-values of 15 and 17, depending on location within the house), and even cork siding.

“Now,” Allison remarks, “we have a super-efficient backyard cottage!”

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Non-VOC paints are used throughout the house to preserve healthy indoor air quality.

For an HVAC system, a radiant concrete slab is warmed with an air-to-water heat pump water heater. The water heater uses carbon dioxide as its refrigerant, which Allison notes is ozone friendly and doesn’t contribute to global warming. Both domestic water and heat are produced by the same unit.

“We couldn’t love this cottage any more than we do,” he says. “The inside temperature is so even you really can’t tell the heat is on. It’s quiet, and large window openings bring in tons of natural light.”

The structure is also sustainably designed. Non-VOC paints were used. Siding is thermally modified wood which is rot- and pest-resistant and never needs painting or staining. Even better, upon replacement, old product can be put into the city’s yard waste bins and made into compost or mulch, Allison adds.

The design is nature friendly in another way. AHB was able to reuse the existing foundation as well as protect a large tree that would have prevented new construction. “We didn’t need to touch a branch or a root,” Allison says. “We still met Seattle’s stringent earthquake requirements.”

A Japanese maple planted by the previous owner helps keep the house cool in the summer and bright in the winter, Allison adds. “We love the tree,” he says. “The bright green new leaves in spring, shade in summer, and fall colors all reinforce our affection.”

Coradu has become quite the conversation piece around the neighborhood. “The response was overwhelmingly positive to this project,” Allison says. “Telling someone that a home can be so efficient that there are no energy bills is mind-blowing to most passersby. Showing them how quickly it can be built is equally impressive.” 

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Key Components

Appliances: LG induction range, LG ENERGY STAR refrigerator, washing machine and dryer

Building Envelope: SIPs, Benjamin Obdyke hydrogap fully adhered WRB, 2-inch cork facade grade insulation

Cabinets, Shelves, Millwork: IKEA

Countertops: IKEA Caesarstone

Exterior Finishes: Unfinished 2-inch cork and Douglas fir windowsills

Flooring: Polished concrete

HVAC/Ducts: Fantech ERV

Insulation: SIPs, 2-inch exterior rigid cork, spray foam rim joist

Lighting: All LED

Paints and Stains: Home Depot non-VOC eggshell custom white (all trim, walls and ceiling)

Roof: Snap-Loc standing seam metal roof

Ventilation: Fantech ERV

Water Heating: Sanden/SANCO air-to-water heat pump

Windows, Skylights, Patio Doors: PLYGEM triple pane