14 October 2010
This is reposted from an article I wrote in AIBD's Design Lines Magazine.
Houses are notorious energy hogs for heating and cooling, but what if you could reduce that energy use to a meager 1500 watts, the amount needed to power your average hair dryer? The idea is far from fantasy. The Germans have been building these energy tightwads for decades using passive house design. Not to be confused with the passive solar houses first pioneered in the 1970s, which emphasized orienting buildings toward the sun, a passive house (also known as passivhaus) relies on super insulation, an airtight seal and few, if any, thermal bridges to maintain a comfortable temperature. And just like any other house, the windows can still be opened on a nice day. The result is a building that uses 70% to 90% less energy than one constructed by traditional methods.
Although the concept is new in the United States, passive houses are quietly gaining a following, and recently, I began a project to design and build them in Roanoke, Va., where my business is based. While the project details are proprietary, I can share the basic principles behind passive house design along with some specifics about the houses
I plan to build and how they can be priced competitively.
Wanted: A Half-Ton Heat Pump
The genius of passive house design is that heating and cooling systems are barely needed. They exist mainly as a backup to a heat recovery unit, which extracts heat from inbound or outbound air, to maintain a constant temperature using only fresh air. Because the houses are so airtight, hardly any heat escapes or cold seeps in, so the heat generated by appliances, people and sunshine is more than enough to warm the building, except perhaps on the coldest days when the heat is turned on. In fact, studies have found that the building envelope alone keeps the average temperature of a passive house around 60 degrees Fahrenheit in the winter. When heat from human beings, hot water or electric loads are factored in, the temperature is about 70. This has been the case even for houses built in bitterly cold climates like northern Sweden.
In Roanoke, where winters are chilly and summers are hot, a 1,500-square-foot passive house would requireonly the heat recovery unit and a half-ton heat pump (so small that few companies manufacture it) to keep the air a comfortable temperature, and the pump would run only on the hottest or coldest days. To put that in perspective, a similar-size, traditionally built home requires a heat pump that can circulate three tons of warm or cool air continually most days of the year.
In hot, humid climates, passive houses use their tightly constructed envelope differently. Here, the emphasis is on sealing in the coolness generated by air conditioning, shading or, for the truly green-minded, passive geothermal energy. Most passive houses in the South wouldn’t need
a heating system, only air conditioning, and even that would be sized smaller to account for the home’s airtight construction.
Sealing the Envelope
How that airtight construction is achieved varies because passive house design is not prescriptive. There’s no single way to build a passive house, which can be constructed from any material whether concrete, timber framing or brick. But different materials will require different airtight strategies to satisfy the three performance standards that a passive house must meet. It can’t use more than 15 kilowatt hours per square meter annually for heating and cooling, and it must pass a pressurization test of no more than 0.6 air changes per hour at 50 pascals. That’s a quantum leap considering the average U.S. home has five to seven changes per hour and a well-built green home has only as few as two.
The third performance standard measures the home’s total energy use and primary energy source, which can’t exceed 120 kilowatt hours per square meter each year. That measure essentially targets greenhouse gases so that homes getting their electricity from hydropower rather than coal-fired power plants have an advantage. In coal states like Virginia, the houses I design must use nearly three times less total energy than a similar home in the Pacific Northwest to compensate for a dirtier power source.
The key to meeting these metrics is to design a super-insulated home that minimizes thermal bridges so that heat isn’t transferred through exterior walls, particularly at building intersections. Moisture can still escape but not air. How that design is accomplished varies depending on whether the house is in icy Minnesota or steamy Louisiana. To create a home that is virtually thermal-bridge-free, I rely on two computer programs: THERM to target thermal bridges and WUFI, which simulates how heat and moisture are transferred through a wall’s multiple layers. The passive houses I design in Roanoke have minimum insulation values of r60 in the roof, r35 for the walls and r30 under the slab. Although the windows in the Roanoke homes are triple-paned casements with a double gasket, their seal isn’t entirely airtight, and we are working with the manufacturer to improve the performance.
The lack of high-performing building materials, which are common in Europe but virtually nonexistent here, may be the biggest stumbling block for designing passive houses on this side of the Atlantic. Europe’s stringent energy regulations have spurred a bonanza of new building materials and techniques, and even publishers have gotten on the bandwagon with books and catalogs that identify products and methods for creating thermal-bridge-free structures. Too bad most of them are in German. But U.S. designers can pursue passive house design by working with a certified consultant or taking courses that lead to certification through the Passive House Institute US (www.passivehouse.us).
Cost at Parity
In some ways building a passive house is a bargain compared with other net-zero-energy designs. To get the average 2,000-square-foot home to net zero requires a hefty investment of $45,000 to $65,000 in photovoltaics alone. The same-size passive house is made energy neutral with a modest $10,000 to $15,000 investment in photovoltaics. Still, passive houses cost about 5% or 10% more on average to build, and while that investment can be recouped from lower energy bills usually in 15 years, that’s too long for many clients.
To make passive houses more financially attractive, I design them so that the cost of the mortgage and energy bills combined is the same as that for a comparable traditionally built house. Instead of spending $100 each month for electricity, the owner of an energy-saving passive house spends $30, with the remaining $70 going toward a slightly higher mortgage. But the total $1,100 monthly cost is the same dollar figure the owner would need to shell out whether it’s a passive house or not.
The Roanoke project is only in the second of five phases, and we are just now beginning to attract clients for the houses, none of which have yet been built. The two-story homes will range between 1,400 and 2,000 square feet. Because clients only have our word that the monthly costs will be the same, we are offering discounts to the first few buyers in exchange for them letting us monitor the home’s energy use and costs over time. Eventually, we expect to have actual performance data that can help entice other clients to buy at full price, and maybe then, hair dryers will finally be redesigned so that they’re more energy-efficient.