Energy Efficiency and the Passive House

The past year has been one of weather extremes. The winter of 2009-2010 in Virginia saw more snow than had fallen in over a quarter of a century. The summer saw almost seven weeks of days near 100 degrees. Nonetheless, Virginia has a moderate climate and I benefit in the winter by having my roof absorb heat and the southern open orientation of my house. Throughout the summer the same southern orientation results in extra load on the air conditioning/heat exchanger, but provides the optimal conditions for my solar photovoltaic panels. (My husband enjoys the cooler months when he is allowed to open the southern drapes to experience the full beauty of our setting.) My most successful sustainable living project to date has been the insulation of my attic which effectively thermally isolated the attic from the rest of the house. According to the US Department of Energy heating and cooling account for about 56% of the energy use in a typical U.S. home, making it the largest energy expense for most homes.

To further improve the energy efficiency of my home I have been saving up to install a ground source heat exchanger (commonly called a geothermal heat exchanger) when my existing system reaches the end of its life or just before the 30% federal tax credit expires whichever comes first. This past spring, the O’Neill home in Sonoma, CA became California’s first certified Passive House, and the first certified to the new retrofit standards. A Passivhaus or Passive House in English is simply a very well-insulated, virtually air-tight building that is primarily heated by passive solar gain and by internal gains from people, electrical equipment, etc. Energy losses are minimized. Any remaining heat demand is provided by a small heat exchanger. Though the standard was developed in cooler climates and at this time tends to work best at passive heating avoidance of heat gain through shading and window orientation is also part of the standard and helps to limit the cooling load. An energy recovery ventilator provides a constant, balanced fresh air supply. Overall, a Passivhaus is reported to have an R-value of 60.

Though the concept was first pursued in New England in the 1970’s, the first Passivhaus were built in Darmstadt, Germany in 1990 and the standards were developed out of those projects. The Passivhaus standard includes an airtight building shell measured by blower-door test; an annual heat requirement ≤ 1.39 kWh/sqft/year; and primary energy ≤ 11.16 kWh/sqft/year. In addition, there are some recommendations that vary with climate. Worldwide the estimated number of passive houses ranges from 15,000 to 25,000. Passive houses incorporate high performance triple-glazed windows with solar films and argon gas, super-insulation, an airtight building shell, limitation of thermal bridging and balanced energy recovery ventilation make possible extraordinary reductions in energy use and carbon emission. The result is home that reportedly saves up to 90% of space heating costs, and reportedly provides excellent indoor air quality. If every building in America was a Passivhaus we could reduce our energy use by almost half.

The O’Neill home in Sonoma cost $500/sqft to retrofit, my house cost less than one fifth of that to purchase and came with acres of land. It is fair to say that I will never spend $500/ft sq to retrofit my home as a certified Passivhaus nor for that matter a LEEDs certified home. (About 39-percent of the points for LEED certification are energy related.) However, I think that despite the currently fashionable push into renewable energy, the real progress in reduction of energy use will be in insulation, strategies to reduce thermal bridging, and passive house techniques. Modifying our transportation behavior and reducing the energy used in our homes and buildings could change our national energy use significantly and it is within our control. Now that I have taken care of the attic insulation retrofit, I suspect that thermal bridging on the exterior walls combined with air leakage are the primary locations of heat loss and gain for my home and many others. To maximize the effectiveness of any future planned energy use improvement projects, I plan to investigate and incorporate (if possible) some of the design principals gleaned from the new Passive House retrofit standard and US Department of Energy recommendations in my next energy project.