According to the US Department of Energy heating and cooling account for about 50-70% of the energy use in a typical U.S. home, making it the largest energy expense for most homes. In addition, as was once demonstrated in a mathematical proof in an economics lecture, Americans consume too much house. (As an engineer, mathematical proofs always made great impressions on me.) My home is bigger than I want, but my husband has what I think of as “very special real estate needs.” Those needs are a book collection of that exceeds 20,000 volumes and other printed media including storage of his deceased mother’s collections. The collections sized our home and I spend time, money and personal resources trying to minimize our energy consumption.
I have looked at some of the strategies used by the Department of Energy in their Colorado demonstration project, the National Renewable Energy Laboratory, NREL, reported to be the greenest office building in the nation, and at Passivhaus standards, but it is not yet clear how these could be incorporated into a house in Virginia. It has been pointed out that some of the technology used at NREL is best suited for high-sunlight, low-humidity climates like Colorado and wouldn't work well elsewhere. The building also demands that the occupants adjust to fluctuating temperatures throughout the day by opening windows, adjusting shades and other actions which require being in the space as much as possible (ideally suited for my stay at home indoor life). During warm weather, the NERL building relies heavily on the fact that temperatures Colorado cool off quickly when the sun goes down. Windows are opened at night to cool the building using the night air to store the cool in the thick concrete walls inside, keeping offices comfortable long into the next day. Such strategies would not work in the hot humid summers of Virginia and the Washington DC metropolitan area. The cost of the NREL building was $259 per square foot to build, the land and the design costs are not included in that number.
Passive solar design is not new, it was used by ancient civilizations to make their living more comfortable in whatever environment they lived in. Passive solar heating techniques are based on three methods of heat transport: direct gain, indirect gain, and isolated gain. Direct gain is solar radiation that directly penetrates and warms the living space (opening the southern facing drapes on a sunny winter’s day to warm a room). Indirect gain collects, stores, and distributes solar radiation using some thermal storage material (maybe the books could be a thermal storage mass, but I have to be careful to keep direct sunlight away from the collection). Conduction, radiation, or convection transfers the energy indoors. Isolated gain systems (like the ubiquitous Virginia sunroom or conservatory) collect solar radiation in an area that can be selectively closed off or opened to the rest of the house, though the cat literally begs to have the sunroom door opened in the dead of summer when it is an oven so she can curl up and sleep out there.
A popular German design concept is the 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. 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 and tilted towards the colder climates.The first certified passive energy home in California 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 passive house (Passivhaus) nor for that matter a LEEDs certified home. (About 39-percent of the points for LEED certification are energy related.) However, I might incorporate more passive solar heating techniques and additional insulation into my home.
Despite the current 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. My first energy project was to take 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.
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