Designers of high-performance homes know that there is always more than one way to reduce energy use. It can be daunting to optimize investments in energy-saving measures: even with the help of computer modeling software, designers need to exercise judgment.
Designers face such questions as: Does it make more sense to upgrade the attic insulation from R-40 to R-60, or to upgrade the water heater to a more efficient unit? Does it make more sense to upgrade from double-glazed to triple-glazed windows or to upgrade from a gas furnace to a ground-source heat pump?
Needless to say, it’s always possible to make any home into a zero-energy home by simply installing a photovoltaic (PV) array large enough to meet its needs. However, it make no sense to buy a $300,000 PV array for a wasteful house if less expensive measures can be used first to lower the home’s energy requirements.
Zero-energy home designers start with the smallest possible home. They then optimize the home’s orientation and improve the home’s air tightness, insulation thickness, and window performance. They also specify very efficient HVAC equipment.
Each new incremental improvement — for example, more attic insulation — saves less energy. At some point, it’s cheaper to install more PV than to further upgrade the envelope.
Energy consultants who need to determine when to stop improving the envelope and equipment, and when to add PV, can choose from more than a dozen available residential energy modeling programs. All existing modeling programs have limitations.
One of the most interesting modeling programs is Building Energy Optimization (BEopt), a software program developed in 2004 at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. The program determines the least-cost path to develop a zero-energy home. For example, an NREL study conducted by Ren Anderson, Craig…