8 Design Strategies for Building a Net-Zero Energy House
A net-zero energy house is a house that has no net energy consumption and no net carbon emissions. It does not mean that the house does not consume electricity. It means that the electricity it generates is greater than the electricity it consumes.
It produces all its required power on site from renewable energy sources and is completely independent from a utility electricity grid. Net-zero energy homes not only generate their own electricity, but are also designed to shield themselves from outdoor thermal conditions using energy efficient appliances and mechanical equipment to reduce consumption. The following strategies have been used in the design of real net-zero energy houses.
1. Diminish heat loss with heavy insulation. Homes that are covered heavily with insulation require minimal heating. Use 6-inch thick or better structural insulated panels under roofs and on walls. Insulated panels can also be placed underneath the ground floor to prevent heat loss through the basement. Wood-framed roof and wall structures are better than steel structures for insulating the house.
2. Install high performance windows. The most important energy conservation factors in a window are the solar heat gain coefficient and the U-factor. The lower these values, the better the window is at rejecting solar radiation during the summer and insulating the home during the winter. Windows with values less than 0.3 for both of these categories are outstanding.
An excellent example of a high performance window would be a double-pane or triple pane fiberglass, low-e, argon-filed window with a thermal break. Place overhangs over all west-facing and south-facing windows in the house. The overhang provides a shade that blocks solar heat. Sometimes the roof itself or planting a tree serves as a natural shade for windows.
3. Use a ground source heat pump to heat and air condition the house. A ground source heat pump takes advantage of free cooling and free heating that exists in the ground and can have an EER of 18 or better. Use a water-to-water ground source heat pump to provide hot water for a radiant floor heating system. Radiant floor heating systems use lower temperature water (95°F) circulating underfloor than is required for conventional hydronic heating systems (130°F), which translates into additional energy savings.
4. Install a solar photovoltaic (PV) system to produce electricity. An energy efficient home combined with a complete solar PV system can make a house 100% independent of the grid. The extra electricity generated can be sold back to the utility company for credits that can be used at later periods when there is no sunlight. Solar panels will also absorb solar radiation that would otherwise increase a home’s cooling load. Any place on the roof that is not covered with a solar panel should be shielded with a highly reflective metal to reject solar heat.
5. Furnish your home with Energy Star appliances. Your washer, dryer, refrigerator, freezer, televisions, home theater systems, DVD players, Blu-ray players, computers and dishwashers should all be Energy Star rated or better. Check the Energy Star website for information on brands that qualify for these products.
6. Seal air leaks in your home to stop air infiltration. Spraying polyurethane foam, sealing with silicone caulk and weather stripping spots where air infiltration occurs will have a substantial impact on cooling and heating loads. Use silicone caulk to seal small cracks and gaps around windows and doors. Spray polyurethane foam insulation for medium sized gaps in attics, between insulation panels, and around plumbing pipes and vents. Seal in between walls and floors and everywhere you feel a draft. Seal your home airtight.
7. Use a solar thermal system to heat domestic water. Place solar collectors on the roof to provide heat and hot water for bathrooms, kitchens and laundry room equipment. Solar thermal systems have insulated storage tanks that store hot water that lasts up to three days without requiring heat from the sun. Back-up heat for the system can be provided with an electric resistance heat exchanger.
8. Use CFL bulbs and LED fixtures for lighting. Lighting energy can account for 20% of a home’s electricity use. That is why it is important to use lighting fixtures with low energy consumption. Use LED fixtures for all down-light applications and use CFL’s for lamps in nightstands, bathrooms, desks, and every lamp that uses a screw-in type of base. The economic benefits of using CFL bulbs over incandescent bulbs can be seen with our lighting energy calculator.
Although following these guidelines will make it likely that a home becomes a net-zero energy house, good judgment must be used. If the house has a hot tub running all day or a data center in its garage, the house will not achieve net-zero energy use.
Besides the obvious utility savings, a net zero energy home aims for the goal of being 100% independent of the grid. However, even when the energy required from the utility grid is zero, we cannot get rid of basic utility service charges. Utility companies need money to provide power lines and service at nights when solar energy systems are not generating electricity.