Airtight Home Pays Its Own Way in Energy Use

“Why aren’t zero-energy healthy homes the norm for new homes in our market?” The answer from builders: “Because people aren’t asking for them.” The answer from buyers: “Because builders aren’t building them, or I can’t afford the upgrades.”

Sustainable living advocate green|spaces sought to solve this problem by designing, developing and ultimately selling a series of spec-built net-zero-energy homes. In the process, the nonprofit educated the entire market, including designers, builders, real estate agents, appraisers, buyers, lenders, students and teachers. With simple, off-the-shelf features, the design team delivered a home that can produce as much energy as it consumes, protects and promotes the health of its occupants, and uses water and materials responsibly—all at a conventional price point for the neighborhood in which it is located.

Multi-purpose planning in Tennessee. The green|spaces net-zero-energy home includes Greenguard-certified open-cell spray foam for insulation, and a standing steel metal roof for durability and easy solar panel install. Credit: Tennessee Office of Sustainable Practices

green|spaces began its net-zero-energy homes project in 2014, with a list of what it wanted to achieve with the homes. They needed to be visible from afar, located on a steep site, and able to be built and sold at a reasonable rate. To ensure these components were possible, the group looked at other net-zero energy case studies. These helped it decide which products and techniques were best to achieve net-zero energy, as well as to build an affordable house.

The nonprofit held a design competition featuring 11 teams, each with a builder, designer and engineer, to design the home with components that green|spaces wanted to include. Each team had to demonstrate that its design could realistically be built net-zero energy. The final project team included green|spaces, Workshop Architecture, WM Whitaker Landscape Architects and Collier Construction. Their consultants were Conditionaire, SK Collaborative and Vandemusser Design.

A Passive Design

Energy costs can be reduced by simply taking advantage of the movement of the sun. For the Chattanooga area, it is best to shade any south-facing windows to eliminate direct sunlight in the summer, but allow for heat in the winter. Providing overhangs and vertical shading elements helps to improve energy performance. On the homes, the windows at the top of the house have an overhang, which allows the solar panels to pick up the sunlight, conserve energy, and reduce excess heat, glare and ultraviolent radiation.

A building envelope: walls, roof and foundation

The design of the walls, roof and foundation can help achieve a high-performance home.

VaproShield, a vapor permeable air barrier, was used on the walls to keep the wood dry. This secures a long life for the wood in the home.
Greenguard-certified open-cell spray foam was used to insulate the walls. The crawlspace was sealed, insulated and conditioned with a 10-mil vapor barrier and closed-cell spray foam.

Casement windows, thermally broken frames and multi-point lock doors, which provide better air tightness, were installed to improve energy efficiency.

The roofing material required something that would work well with the solar panels, so green|spaces used a standing steel metal roof. The solar racking can be clamped to the seams, with open-cell spray foam between the rafters, instead of having to penetrate like a regular roof. Also, it is one of the longest-lasting roof materials—one that will outlast the solar panels—so it prevents the owner from having to remove the solar to replace a shingle roof and then reinstall the solar.

In the air: heating, air conditioning  and ventilation

The passive design also plays a role in keeping the house well ventilated. Natural ventilation with high exhaust windows pulls air up through the house during months when active heating and air conditioning is not necessary.

For heating and cooling, green|spaces installed two Mitsubishi Hyper-Heat ductless mini split systems, one upstairs and one downstairs. These eliminate the use of ductwork, increasing efficiency. The units have the ability to vary capacity, which saves energy. It can run a little, when just a little is needed—which is most of the time—in a well-sealed, well-insulated home. Another feature of this system is that it can still work effectively at temperatures as low as -13 degrees. A Schneider Electric Wiser Air thermostat was installed and works as a control panel for the home. It has intuitive scheduling and learning algorithms, which help to conserve energy in the home, and also has live weather alerts.

green|spaces wanted to seal house as tightly as possible, while also making sure the home has enough fresh air. They used an energy recovery ventilator, which takes the air from outside and brings it inside, then sucks it back out through the kitchen and bathroom. The system pre-heats and pre-cools the fresh air that is brought in from outside.

After installing the system, a blower door test showed a 1.67 air changes per hour (ACH) rating in the home. The required Energy Star rating is 4 ACH; the nonprofit is anticipating that number to be lower in the next three homes.

Better windows and doors

green|spaces found that using Energy Star-qualified windows and doors allowed for greater efficiency. They meet standards for the insulating value of glass, which is a U-factor less than 0.30, and standards for the amount of heat the window allows to enter the home from direct sunlight, which is a solar heat gain coefficient of less than 0.40.

With that in mind, casement windows, thermally broken frames and multi-point lock doors were installed to improve performance further, through better tightness of the building envelope. Unlike double-hung windows, which are pressurized at the top and bottom, casement windows are pressurized on all sides, resulting in a tighter seal and less cool and warm air entering the home. To ensure optimal tightness around the window, minimal expanding foam was applied to the window frame.

A heat pump-driven water heater

To determine the most efficient system for heating the home’s water, green|spaces referred to a study from Tennessee Valley Authority’s Campbell Creek Research Homes. The nonprofit decided to use a heat pump water heater in the home, instead of the typical electric or gas water heater.

A heat pump hot water heater uses the same compression cycle as a refrigerator to do the opposite job. It squeezes heat and humidity out of the ambient air and puts it in the water. In the process, it gives off slightly cooler, slightly dryer air.

Not only is the water heater more efficient, but in the Southeast, where there is often humid air, it assists the mechanical system to save even more energy than gas, electric or solar thermal.

For savings, think LED lighting and Energy Star appliances

 

LED was the obvious choice for lighting in the home. As LEDs have become more popular, they have also become more affordable. Using LEDs throughout the net-zero home allows for a monthly energy savings of $12, or about $152 per year, when compared to an average new home of the same size.

Energy Star-certified appliances, including the refrigerator, washer and dishwasher, were installed to save an average of $8.33 a month and $100 a year. The dishwasher also conserves water by using only 2.5 gallons to completely wash, dry and sanitize really dirty dishes. The range installed in the home is a double oven induction unit that is 75 percent more energy efficient than conventional electric.

Another important appliance feature is a heat pump dryer, which is an Energy Star appliance as well. This dryer uses the heat from the home to dry the clothes in the dryer, and since there is no hole that has to be made for a dryer vent, it keeps the house as tight as possible.

Above all, it’s solar

Solar was the final piece of the equation to get the home to net-zero energy. Based on the energy that would be produced from the homes, with the above techniques and appliances, green|spaces installed a 4.65-kw array. This will generate approximately 6410 kWh each year, and offset demand and selling excess to generate $641 per year. This value is expected to increase as electric rates rise over time. Even on a cloudy day, this array has proven to produce more energy than the home consumes.

The first of the four homes was completed in December 2016 and recently sold for $366,000. Land is currently being cleared for the other three homes, which will sit alongside the first model.