1970s Home Goes Net Zero

A deep energy retrofit that outperforms most new homes

Intensive renovation has made this modest ranch house in the suburbs of Boulder, Colorado an example of what any green builder strives for - a net zero energy home. With an extreme insulation retrofit, a 6 kW photovoltaic array, and an evacuated tube solar hot water system, the build team expects this house to produce 130% of its own energy needs. A grid tied system allows the excess electricity to be “stored” until occasional winter cold snaps demand that the 9-kW modulating electric boiler fills in.
Motivated owners, a dedicated group of designers, consultants and builders and state renewable energy incentives, allowed the project to shoot for lofty goals. Carefully balancing the value of individual improvements by looking at them as parts of a bigger system was crucial. Energy use monitoring and computer modeling helped maximize overall efficiency of the home. A Web Energy Logger (WEL) was permanently installed to monitor performance and plan for future energy upgrades.
Starting from the ground up makes it relatively easy to insulate structures like the 700ft addition included in this project. Retrofitting existing buildings is not always as straightforward. Together, Eric Doub of Ecofutures Building, and owners John and Vicky Graham decided that wrapping the home with additional 2x4 framing and sprayed polyurethane foam would be the least intrusive method. Although some of the work replaced upgrades that were only 5 years old, and the cost was on par with a typical high end renovation, continually rising energy costs made John and Vicky’s choice to create their energy efficient dream home, a great long term investment.

Team:
Builder: Eric Doub, Ecofutures Building, Inc.
Architect/Designer: Andy Johnson, DAJ Design

Specs:
Location: Boulder, Colorado
Completed: 2006 - 2007
Bedrooms: 3
Bathrooms: 3
Cost ($/sf): $81/sq.ft.basement remodel, $250/sq.ft. main floor and addition (including renewable energy systems)
Size: 1000 sq.ft. basement, 1000 sq.ft. existing main floor, 700 sq.ft. addition

Existing:
Foundation type and R-value : concrete walls covered with 1” XPS on interior (R-5)
Wall construction and R-value: existing 2x4 framing filled with blown cellulose, 2x4 framing added to exterior and filled with Icynene spray foam (R-28 total)
Windows: Duxton foam-filled fiberglass window frames with 2 layers Heat Mirror, Krypton/Argon filled (R-7.1)
Roof construction and R-value: 8” open cell foam -(SPF) and 12” blown cellulose (r-70)
Garage: attached; thermal and air separation

Addition:
Foundation type and R-value: 6” ICF crawlspace (R-30)
Wall construction and R-value: 2'x6' frame, faced with 1 ½" resilient channel on interior creating a 7" wall cavity filled with Icynene spray-foam (R-27)
Windows: Duxton foam-filled fiberglass window frames with 2 layers Heat Mirror, Krypton/Argon filled (R-7.1)
Roof construction and R-value: SIP roof (R-42) with 3.5" SPF in dropped ceiling (R-50 total)

Energy specs:
Conditioning equipment:
- Operable windows and motorized skylight for ventilation and cooling
- Solar hot water - 180 evacuated tubes, 2 - 168 gallon, insulated storage tanks (R-80)
- Backup from on-demand 9-kW modulating electric boiler
- EPA-approved wood-burning fireplace
Water heating equipment: Same as space heating
Projected total electricity consumption: 6,300 kWh/yr
Projected total electricity production: 9,600 kWh/yr
Actual : June 2008 ~300kW surplus (sold back to utility company)

Water efficiency measures used in this project
- Dual flush toilet
- Xeriscaping
- Drip irrigation

Energy efficiency measures used in this project
- Natural daylighting
- CFLs throughout home
- Direct-from-solar-tank hot water dispenser to reduce energy demand in cooking
- Insulation far above recommended standards
- EnergyStar rated ceiling fan
- Existing Energy Star washer and dryer, plus clothesline
- Separate switch to eliminate phantom loads from electronics

Green materials and/or resource efficiency
- 90% (by weight) of construction waste either recycled or repurposed
- FSC-certified lumber for framing
- Recycled plastic decking
- Water-based floor finish
- Clay plaster
- Concrete and sorghum-based countertops
- Oriented-strand board subfloor

Indoor air quality measures
- Energy Recovery Ventilator (ERV)
- Linoleum tile with low-VOC adhesives
- Formaldehyde-free cabinets
- Low-and zero VOC paints and stains
- Natural wool carpet
- Natural clay plaster walls absorb odors, sounds, & indoor air pollutants

Alternate Energy Utilization
Photovoltaic power: Grid connected solar PV (6 kW)
Cost: $25,000
Solar water heating: Evacuated tube solar hot water system provides 90%–100% of domestic hot water as well as space heating using in-slab radiant tubes
Cost: $34,000

Green Building Product Sources:
OptiMiser, Web Energy Logger
American Clay Plaster

Lessons Learned:
Aside from the obvious utility cost savings, this remodel aims for the goal of passive survivability - the ability of a building to remain habitable when utility disruption occurs during extreme outside temperatures. The actual performance of the home falls slightly short of projections - something that builder Eric Doub believes could be tempered by the use of movable window insulation. “My recommendation is to have at least some cellular shades, if just to reduce convective heat transfer.” That’s getting pretty deep into the details of energy efficiency. Good Job Eric!