Green in Balance
If you’ve seen one net zero–energy home, you haven’t seen them all. There are many different ways to craft a house that produces as much energy as it uses. It all boils down to strategy in the face of certain “givens,” such as location, climate, site conditions, budget, and available materials. The art and science is in the tweaking.
KB Home used energy modeling software to piece together the biggest green elements and gauge how they affected each other. “You have to look at this whole thing as a series of systems contributing to an end result,” explains KB Home vice president Dan Bridleman. “You can’t ever put your finger on this one thing that did it. Net zero is almost always accomplished through a combination of techniques.” Let it be noted that this combination rated LEED for Homes Platinum and qualified for Environments for Living Certified Green, EPA WaterSense, Energy Star, and IAPMO Green Plumbing certifications. Here’s how the GreenHouse got to zero and is doing its part to help the environment:
The Envelope
Any net zero endeavor must start with conservation—the logic being that saving energy is cheaper than producing it. In Florida’s hot and often muggy climate, conservation starts with an air-tight shell. Structurally, the Greenhouse is comprised of exterior cement block walls lined with a layer of Âľ-inch-thick rigid insulation and interior framing with FSC-certified wood. The critical piece is a sealed attic, which ensures the HVAC system isn’t forced to work harder than it has to. “We sprayed the roof deck with foam insulation, and on the roof we used Monier’s cool roof tile, which reflects the thermal properties of the sun,” Bridleman explains. As a result, the attic maintains a constant temperature within five to seven degrees of the rest of the house, even though it’s not conditioned space.
Air Handling
Heat and moisture are friends to mold, but not in this house. An energy recovery ventilator in the attic recaptures as much as 80 percent of conditioned temperatures that would otherwise be lost through exhaust airflows and transfers that energy to incoming fresh air. This unit, combined with a whole-house dehumidification system, reduces the home’s cooling loads by not allowing them to escalate in the first place.
Then there’s the HVAC itself. “HVAC sizing is the biggest deal in the energy modeling equation because there are all kinds of things that can affect your energy usage, from heat-emitting light fixtures to appliances to vent fans in bathrooms, to people opening and closing exterior doors,” notes Chad Burlingame, director of purchasing and design for KB Home’s Orlando, Fla., division. All of those little details play into right-sizing an HVAC system that works efficiently and doesn’t provide more capacity than necessary. A conventionally built house of this size in Florida normally requires a five-ton HVAC system. This home lives comfortably with a two-ton, 19.5 SEER heat pump.
Building Components
Want to reduce construction waste? Don’t generate it in the first place. “Waste reduction starts with an accurate take-off so you are sending only what’s needed to the jobsite,” says George Glance, KB Home’s Central Florida division president. Building blocks in this home include pre-engineered trusses, floors, and panelized wall systems that reduce scrap lumber on site. Drywall and trim pieces left over during construction were recycled. With this approach, Glance estimates that the GreenHouse diverted 88 percent of the jobsite waste that a conventional home would normally send to the landfill.
Water Usage
The average American household wastes more than 3,650 gallons of water each year while waiting for hot water to reach the tap, according to EPA estimates, and 10 percent to 15 percent of energy use in hot water systems is wasted in distribution losses. Not so in the GreenHouse, which runs on an on-demand hot water recirculation system in lieu of a traditional boiler. Water heated by rooftop solar panels is stored in an 80-gallon thermal tank and then circulated through a loop under the slab. Each plumbing run off of the main loop is less than 10 feet long. This configuration reduces the amount of piping needed, and there’s no waiting for a hot shower. Instant hot water is activated with the push of a button in the kitchen, while motion sensors trigger it automatically in faucets and showerheads whenever someone enters the bathroom.
There are other water-wise features to boot. Wet areas inside the home are outfitted with low-flow faucets and dual-flush toilets. Outside, rainwater collection tanks store runoff from downspouts and redistribute that water into the landscaping. Sink and shower water is also filtered and redistributed for irrigation. A WeatherSmart irrigation system assesses the amount of moisture in the soil and turns on the sprinkler systems only as needed.
Electricity
Roof-mounted photovoltaics (PV) were introduced only after all of the other building and product specs were in place to ensure maximum performance on the conservation side. “Raised panels would have been more efficient, but they are also more conspicuous, and sometimes you have to make aesthetic choices,” Bridleman says. “We ended up choosing a flat panel PV system which isn’t quite as efficient, but it looks better on the house.” The 8.57 kilowatt roof system is expected to generate about 10,000 kW of electricity annually—enough to match the home’s anticipated energy consumption, which includes an electric car charging station in the garage. Although the PV system added an extra $60,000 in hard costs, the homeowner is eligible for a 30 percent federal tax credit as a result.
Homeowner Education
The final variable in any net-zero equation is consumer behavior. The GreenHouse is equipped with a simple energy monitoring system that allows homeowners to keep an eye on their electric, water, and gas consumption/production, tracking their usage history by the day, hour, week, or month, by peak usage times, and by dollar amounts. “The system even makes suggestions,” Bridleman says. “For example, during peak usage it can tell you how to adjust things like lighting controls or the condensing units in your fridge to save energy. You can access the system from home or via an application on your smart phone.”
For a virtual tour of the Builder Concept Home 2011, visit www.builderconcepthome2011.com.