Equipment Versus Envelope

Home Performance Improvements: What’s the Best Investment?

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 pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures.?

Saving Energy Makes More Sense Than Generating Energy
Needless to say, it’s always possible to make any home into a zero-energy home by simply installing a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) 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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment.

Diminishing Returns
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.

Finding the Least-Cost Path
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 Christensen, and Scott Horowitz determined that for a new 2,600 square-foot house in Sacramento, California, it doesn’t make any sense to add PV until the home’s energy use has been reduced by 44%. The cost of the measures required to achieve these savings is only $9,729.

Although modeling software like BEopt goes a long way towards helping designers make these decisions, human wisdom is still useful. While software can determine that investing X dollars in a ground-source heat pump saves Y kilowatt-hours per year, it rarely considers such questions as, How long will the equipment last? What are the likely maintenance costs?

Most designers know that envelope improvements should always be favored over equipment improvements. While attic insulation should last for the life of the house, a solar hot water system needs regular maintenance and eventually develops leaks.

Zero-Energy or PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates.?
While cutting-edge green home designers in the U.S. have been pursing the goal of the zero-energy homes, German designers have taken a different approach. The German Passivhaus standard includes insulation and window specifications that are considered extreme in the U.S. It’s common, for instance, for German Passivhaus buildings to include triple-glazed windows and R-40 subslab insulation. In the U.S., on the other hand, triple-glazed windows are still rare — even on zero-energy homes — and very few homes have subslab insulation better than R-20.

Passivhaus designers are far more likely to question the wisdom of installing expensive gadgets or equipment — including PV arrays, solar hot water systems, and ground-source heat pumps — than their U.S. counterparts.

While the economics of these decisions can and should be argued, there are several reasons to admire the German approach:

• Envelope improvements make a house more comfortable.
• Envelope improvements usually require no maintenance and are very durable.
• Envelope improvements increase a home’s passive survivability during power outages.

As Dr. Wolfgang Feist, the director of the Passivhaus Institut in Darmstadt, Germany, noted in a 2007 interview, “Most builders I have talked with in North America still think that increasing insulation is an expensive thing. ... I’m surprised, because insulation is the cheapest thing you can do. … To builders I would say, ‘Don’t be afraid of insulation.’ ”