High-Priority Energy Efficiency
This home is headed to net-zero energy, with very little impact on its site.
Synopsis: Architect Garth Schwellenbach outlines the considerations made when building this house on the rocky Rhode Island shore. The house is designed with minimal site impact in mind, including advanced wastewater treatment, permeable paving, rainwater harvesting, and native plantings. It is also build for net-zero energy use with a full-perimeter air barrier, dense-pack cellulose and open-cell and closed-cell spray-foam insulation, high-performing windows and doors, efficient mechanicals, and a PV solar array. The article includes a detailed drawing of the wall framing as well as floor plans that show how the house and it’s interior rooms were sited to highlight surrounding views of the water.
Our clients always have some ideas for what they want in their home, but rarely do they have the clarity and determination of the homeowners who came to us with this stunning site on the coast of Rhode Island. They wanted a new home with little environmental impact, without sacrificing style, comfort, or livability. They wanted a home that would celebrate the views, and would stand up to the harsh coastal environment without endless maintenance demands. On the site was a house from the 1970s of questionable value beyond being well-sited to capture views and solar gain. The structure of the existing house wouldn’t support our new design, but we could reuse the foundation, which allowed us to avoid excavation and capture the embodied energy of the existing concrete. Working closely with the owners, who had refined aesthetic preferences from midcentury modern to traditional coastal style, we began designing the house, staying keenly focused on creating a super-insulated and airtight building assembly with high-performance windows and doors. We specified energy-efficient systems, including heating, cooling, and hot water; an ERV for ventilation; and LED lighting. At every turn, we kept four priorities in mind: low site impact, a low-maintenance exterior, getting to zero energy, and maximizing the views with an open plan and well-placed windows.
The house was built on the foundation of an existing home. The poured concrete foundation walls and footings were in good shape, but the existing basement slab was removed to allow for new interior footings, subslab insulation, and vapor and air barriers. Reusing the foundation reduced site impact on the environmentally sensitive shoreline lot, but required the new design to comply with the existing square 42-ft. by 42-ft. footprint. The north side of the house was extended with a 3-ft. cantilever, but otherwise held to the existing foundation plan and deck perimeter. We also employed these four critical strategies to reduce impact on the site.
Advanced wastewater treatment
A particularly challenging aspect of the project was the site constraints, driven primarily by the coastal location and the associated protections. These constraints were made more challenging by a high water table and ledge lurking just below the surface of the site. To minimize site impact, we relocated and redesigned the leaching field and installed an advanced wastewater treatment system. The treatment system needed to take into account both the soil type and high water table without being an eyesore. Civil engineer and landscape architect Brian Kuchar from Horsley Witten Group, Inc. designed a pressure-dosed GeoMat shallow leaching field, which is only 8 in. to 12 in. deep and located in the upper soil horizon. An AdvanTex AX20RT advanced treatment system and pump chamber, which is designed as a low-energy solution for small areas with high water tables, distributes the treated effluent to the leaching field. To better integrate the system into the overall site design, the leaching field is located within the existing landscaped driveway circle. The leaching field is planted with a flowering meadow, allowing for grass and plant uptake of nutrients, which provides the highest level of treatment and keeps it completely hidden
Because the site slopes down to the coast, it was important to control storm water to reduce any chance of runoff and erosion into the bay. Permeable grass pavers are used for the driveway to provide a durable erosion-resistant surface that reduces runoff by absorbing the water. In addition to improving the storm-water control, grass pavers provide a beautiful and durable living landscape where there is typically asphalt, concrete, or gravel. A central swale set into the downward slope directs surface water that is not absorbed toward a rain garden, where it can slowly percolate into the ground.
To reduce the runoff from the roof surface, rainwater is captured in a 3000-gal. underground cistern. This water will be used to irrigate the vegetable gardens, meadows, and site plantings. The sump pump is also directed to the cistern to collect storm-event groundwater. The house has municipal potable water provided by the town, but because of its coastal location, there are often conservation bans in place to limit irrigation or other nonessential uses. By collecting the rainwater from the roof, the owners are free to irrigate their gardens without impacting the municipal water supply. Overflow from the cisterns during large rain events is directed to the rain garden to further reduce any chance of runoff.
The landscaping features a native-pollinator-species meadow that replaced the conventional mowed-grass areas. The meadows provide a habitat-rich, low-maintenance, and beautiful landscape that will flower throughout the spring and summer, and will help restore the soil quality after years of turf grass and associated fertilizers. The meadows and other native plantings also require less irrigation, and will thrive in the local climate. There is no turf grass anywhere on the site that needs to be mowed—the only grasslike surface is the driveway, which is actually a mix of clovers and other ground cover.
Also check out Garth Schwellenbach’s 6 Tips for Net-Zero House Design.
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