Home is Zero Energy, Zero Water and Zero Sewer

This living laboratory is a home performance triple-play.

According to Bundle Design Studio architect Dan Welch, the Birch Case Study House was “developed as our business card.” Welch now lives in the home with his wife in the Birchwood neighborhood of Bellingham, in northwest Washington state. Welch had designed or consulted on several high-performance homes since leaving a traditional commercial architecture firm to start Bundle Design in 2014, but this was the first home he actually built.

Welch and builder Chris Tretwold of Tretwold Construction built the home to the high energy performance standards of the U.S. Department of Energy’s Zero Energy Ready Home (ZERH) program. All DOE ZERHs must be certified to Energy Star Certified Homes Version 3.0 or Version 3.1 standards and the U.S. Environmental Protection Agency (EPA)’s Indoor airPLUS program. Each ZERH home also meets the hot water distribution requirements of the EPA’s WaterSense program and the insulation requirements of the 2015 International Energy Conservation Code. In addition, homes are required to have solar electric panels installed or have the conduit and electrical panel space in place for their future installation.

In the case of Birch House, solar panels produce more power than the highly efficient home uses in a year, while rain water provides all of the home’s water supply and waste water is disposed on site. The home carries a Home Energy Rating System (HERS) Index rating of minus 9 when photovoltaics are used and a 36 when they are not.

Welch designed the 2,000-square-foot, two-story home with a shed roof. The entire asymmetrical roof faces south at a 7/12 pitch to maximize the roof space available for the 9.28-kW solar photovoltaic array. Together with the highly insulated building envelope and passive solar heating elements, the solar panels help the house achieve net zero energy bills over the course of the year. A web-based monitoring system tracks solar power production and energy usage.

Water Warrior

Birch House is completely net-zero water with no connection to the City of Bellingham municipal water and sewer system. Water is collected, stored and treated onsite. To reduce demand, the house is equipped with ultra-water-efficient appliances and plumbing fixtures and composting toilets.

Water for all uses, potable and non-potable, comes from rain water catchment off the metal roof and solar panels. The rain water is stored in two 5,000-gallon tanks located on the north side of the house among the existing trees. Water that is used for indoor uses is drawn from these cisterns, and filtered and treated with ultraviolet light prior to use.

The house has a “green roof” of plants installed on a lower-level roof over the first-floor solarium. Downspouts from this green roof carry overflow downhill through woodchip-lined trenches to two native-plant rain gardens. Water from the showers, tubs, sinks, clothes washer and dishwasher is treated in a small septic tank. An indoor planter was designed and constructed in the solarium to treat this grey water prior to the septic tank.

Although the building department would not approve the use of grey water in this planter, the planter was constructed and planted, but is watered with approved potable water. In winter, the water from the septic tank is distributed in two small infiltration beds. In summer the grey water from the septic tank is instead routed to irrigate 30 fruit trees that line the driveway. Beyond the food garden and the grey water irrigation at the orchard, no irrigation is used to maintain landscape plantings.

Welch notes the decision to achieve net-zero water use was initially not prompted by a desire to save water but rather to highlight how much community energy is used to convey water to the house and back to the sewer treatment plant. This energy is not accounted for in most zero energy projects.

On Friendly Terms

The two-story home is designed with a solarium along the south-facing side that has nearly floor-to-ceiling windows and a thermal mass floor consisting of dirt, stone and concrete pavers to absorb beneficial solar heat. A few operable windows face east and west for cross ventilation.

To minimize heat loss, only two of the home’s windows face north. The shed roof allowed for the construction of a loft above the second-floor bedrooms. Aside from the solarium and the loft, the first and second floors have identical floor plans, with the assumption that, as needs change over time, the house could be converted from a three-bedroom, two-bathroom home to two one-bedroom, one-bath units.
In constructing the home, Welch wanted to create a “builder friendly” assembly that met all the high-performance requirements of DOE ZERH while using construction techniques that most builders are familiar with. Prior to permit and construction, Bundle held a peer review session to evaluate the constructability of the project. The peer review included the builder, designers, structural engineers, contractors, plumbers and solar installers.

Although Welch was an experienced designer and Tretwold was an experienced builder, neither had worked on a high-performance building before. “We met constantly to review the construction documents, brain storm applications, oversee product installs, and check completed construction for deficiencies,” Welch says.

Building Blocks

Welch and Tretwold used 2-by-6 wood-framed walls, with only two inches of exterior mineral wool rigid insulation to reduce changes in window flashing details. The wall cavities were filled with R-23 mineral wool batts for a total wall assembly of about R-29. They used a liquid-applied sealant to seal seams in the plywood and flash around the windows, then covered the entire sheathed walls with a roller-applied silyl ether terminated product that provides a durable, seamless, elastomeric weatherproofing membrane over the exterior sheathing. Windows were sized and placed to keep rough openings within the framing layout. Over this, the builder installed 1-by-4 fir battens to provide a rain screen and air gap behind the exterior siding, which included metal panels and cedar planks.

All of the roof’s insulation was installed above the 5/8-inch plywood roof decking. Seams in the plywood were sealed with seam filler, then three 3-inch layers (R-60) of polyiso rigid insulation were stacked on the decking and covered with 30-minute roofing felt, 2-by-4 battens (for ventilation), half-inch oriented strand board (OSB), and a continuous ice and water shield. The roof was then covered with standing seam roofing that was Energy Star Cool Roof certified.

Welch used a common Northwest foundation type, a framed floor over a vented crawlspace, selected because the site is primarily clay and has sub-surface water issues that would be difficult to control with a slab-on-grade floor. The floor was constructed from salvaged 2-inch-by-15-inch joists spaced 24 inches on center. The floor joists were filled with two layers of R-30 mineral wool batts for R-60 total. A ¾-inch plywood subfloor was installed that was glued in at all edges with construction adhesive and sealed on top of the floor with a putty knife-applied seam filler.

A layer of ½-inch Expanded Polystyrene (EPS) rigid foam was laid over the plywood and glued to the wall studs to provide a thermal break around a two-inch suspended concrete floor slab. Before pouring the floor slab, the builder laid down tubing for radiant floor heat. The concrete for the slab was reinforced with a product consisting of tiny pieces of twisted steel micro-rebar rather than standard rebar or wire mesh; the product is reported to distribute loads for less cracking. The floor assembly had a total insulation value of R-58 (when calculated with framing reduction).

For the home’s air barrier, Welch used liquid-applied sealants to air seal the sheathing for floors, walls and roofs. This approach paid off. Results from three blower door tests (conducted at dry-in, just prior to wall cavity insulation, and at completion of the project) showed final blower door results of 0.4 air changes per hour at 50 Pascals pressure difference, one-third lower than the maximum air leakage rate allowed by Passive House. Balanced ventilation was a necessity in such a tight home. Welch installed a heat recovery ventilator (HRV) with a Minimum Efficiency Reporting Value (MERV) 13 filter that runs continuously to bring in fresh air while exhausting stale air.

‘Combi’ Culture

Bundle Design worked with Washington State University (WSU) to field test a unique heating system, an air-to-water CO2 heat pump that was installed to supply floor heat and meet domestic hot water uses. As a result, the Birch Case Study House was the first in North America to use the CO2 heat pump as a “combi” system.

In addition to collaborating with WSU, Welch has partnered with the local non-profit Sustainable Connections on workshops, with the Northwest Energy Efficiency Alliance (NEEA) to do electrical, water and humidity monitoring, with the Northwest Clean Air Agency (NWCAA) on a video, and with the Northwest Eco Building Guild on its Code Innovation Database. Welch’s unique Birch Case Study House has also captured the attention of numerous local and national media outlets.

Welch knows that affordability is an important consideration and he will use lessons learned on this project to evaluate ways to be cost effective in future projects.

The architect notes that Bundle Design’s business is steadily growing as the culture continues to quickly learn the importance of energy efficiency. However, it is when customers realize that energy-efficient design also provides the byproducts of comfort and quality that real change happens. “Every builder and client we have worked with using high-performance building strategies has become an advocate,” Welch says. “Most are astonished at the drastic difference they experience between living in a conventional house and their new, well-designed, high-performance house.”