Passive Houses save energy by design.
“The greenest energy to use is the energy we don't use at all,” says Katrin Klingenberg, executive director of Passive House Institute US (PHIUS). That line basically sums up the philosophy behind Passive House. Just as a thermos keeps coffee hot or a cooler keeps beer cold, a Passive House keeps its occupants warm in the winter and cool in the summer using similar principles of energy conservation: construct a super-insulated and airtight, but breathable, home that maximizes solar gain and demands little energy.
Passive Houses guarantee overall energy savings of 60%–70% and 90% of space heating. These energy misers literally make use of whatever heat is present in the home: body heat, electronics, lights. Passive Houses can be built for nearly any climate in the world, but are particularly beneficial in cold climates where so much energy goes toward space heating.
The Passive House approach to building started in Germany under the name “Passivhaus” and has since spread to the United States through the work of the Passive House Institute US (PHIUS). Passive house design is nothing new, however. Early passive houses were built in the 1970s and passive solar houses are another type of design that focus on maximizing solar gain to heat a home. These early houses of the 1970s suffered from the technological limitations of the day but provided a foundation for the development of current Passive House standards, which ensure optimal home performance.
7 Principles of Passive House
Passive House walls are typically two to three times as thick as those in typical homes. The amount of insulation varies according to climate, so whereas a home in Minnesota might need a whopping 16" of insulation to get certified, a home in California could get by on just 6".
Elimination of thermal bridges
Thermal bridges – materials that conduct heat more easily than the materials surrounding them – occur at penetrations, connections, and other points in the building, causing significant heat loss and sometimes causing condensation. Passive House builders minimize or eliminate thermal bridges by limiting penetrations and by selecting materials that resist heat transfer.
By completely sealing the building envelope, walls, windows, doors, penetrations, and joints, builders can make a home that is virtually airtight, thus maintaining internal temperature by cutting out drafts, which also prevents moist, warm air from entering the building and causing condensation problems.
Energy- or heat-recovery ventilation
With such airtight envelopes, Passive Houses need some respiratory assistance. Heat-recovery ventilation (HRV) or energy-recovery ventilation (ERV) systems are essential to breathe life into a Passive House. These systems and the associated HRV and ERV devices suck fresh air from the outside and expel stale air from the inside. They manage to transfer 75%–95% of the heat from the air without mixing it. These HRV and ERV systems mean that the home always has fresh air but is draft-free. HRV differs from ERV: ERV transfers humidity of the exhaust to the intake air. Either of these systems can be switched off in favor of opening the windows for fresh air.
High-performance windows and doors
Normally a weak point in a home, windows and doors in a Passive House are turned into a strength through the use of well-insulated, triple-pane, coated windows that effectively cut out the cold.
Passive solar/internal-heat gains
By siting a home to face south, Passive House design harnesses the power of the sun to heat a building. The sun's energy is maximized in the winter and minimized in the summer by the size of the roof eaves. Because the sun is higher in the summer, the eaves block much of the midday sun but let in all the sun in the winter. Because Passive Houses are airtight, the little heat generated inside a home by people, appliances, or lighting is maximized.
Accurate modeling of energy gains and losses
The Passive House Planning Package (PHPP) is a tool used to accurately assess energy modeling of a building that accounts for attributes such as airtightness, wall thickness, and solar orientation. Only when a building meets the following measurable requirements is the Passive House standard met: space heating requirement of less than or equal to 4.75 kBtu per sq. ft. per year, energy use of less than or equal to 38 kBtu per sq. ft. per year, and airtightness less than or equal to 0.6 air changes per hour (ACH) at 50 pascals (Pa).
As with many new technologies that people don’t fully understand, a number of beliefs have arisen about Passive Houses. Klingenberg points out some commonly believed statements, all of which are untrue:
Passive Houses have to be box-shaped,
They can only have a limited number of windows,
Homeowners cannot open the windows,
Passive Houses only work in cold climates, and
Building a Passive House requires Passive House-certified materials.
Sometimes misconceptions gain traction because they make sense. Although it’s not accurate to say that Passive Houses only work in cold climates, Passive Houses do make more sense for someone living in a cold climate than a warm climate because they save 90% of space-heating costs.
That 90% savings is guaranteed for a certified Passive House. Unlike certification from a program such as LEED, which awards points based on a rating system, Passive House certification functions solely on measurable results, such as blower-door tests, which guarantee that the home meets the 0.6 air changes per hour (ACH) or fewer mark, ensuring that the homeowner gets true value for their investment.
Passive Houses cost on average 10% more to build, a sizable margin that the average home buyer cannot overlook. Building performance specialist Corbett Lunsford believes that the extra cost is worth it, even if you take the actual figures out of the equation. “The envelope is the most important component of any residential building, so it makes perfect sense to focus single-mindedly upon it,” he says. “If you're going to spend $20,000 extra on a home, it should be on improving the envelope and decreasing the size of the HVAC – no matter what percentage improvement you get, it will always be the best investment.”
What appeals to many about Passive Houses is the ability to save energy through conservation. The choice is simple: passive or active. Do we continue to keep that coffee warm with a heater, no matter how green the energy producing the heat, or heat it once and bundle it tightly so that that heat doesn’t escape? The latter is conservation, a simple approach that’s easy to appreciate in a world with dwindling resources.