Ten Misconceptions About the Passive House Standard
Arguments against the Passive House standard are based on misunderstandings
I'm a small building energy modeler, and the tools of my trade are airtightness, insulation, window placement, and heat-recovery ventilation(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. . These are also the tools of the international Passive HouseA 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. standard (known in Europe as the Passivhaus standard). And yet, almost every week, some veteran home builder patiently schools me as to why these building performance strategies — or Passive House requirements — are a waste of time or money.
I have compiled the most frequently cited arguments I hear; let's call them “Ten reasons not to build a Passive House.”
Myth #1: Too expensive
I hear this almost every time I hand out my business card. It doesn’t seem to matter whether I’m listening to the builder of a $4 million luxury home or a $400,000 townhouse; nearly everyone assumes a Passive House costs more.
The truth is counterintuitive: If you include operating costs in the equation, Passive House emerges as the most affordable way to construct any building.
Here’s the math: Passive House walls and windows cost more than code construction, while Passive House heating systems cost less. On balance, Passive House construction costs up to 10% more than code. (Costs for land, carrying and marketing remain unchanged.) Upon completion, Passive House buildings require an average of 90% less heating energy than code buildings.
Amortize the one-time construction premium over the life of a mortgage, subtract the annual savings on utility bills, and most Passive House owners save money every single year.
This inherent affordability is no accident. The Passive House standard evolved from research experiments in which European scientists sought to calculate a balance pointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit. below which the installation of more insulation provided a measurable return on investment, and above which more insulation returned no economic benefit. This economic “sweet spot” is what defines the Passive House standard, as well was what distinguishes Passive House from most green building standards.
Myth #2: Too stuffy
Passive House buildings are ten times more airtight than typical new buildings. But this does not mean they feel “stuffy.” A Passive House window opens like any other. And because the Passive House is better insulated, its residents may choose to leave windows open more days per year than the resident of a code-minimum home.
It’s when the windows are closed that the Passive House excels, however. Stale indoor air is continuously exchanged for fresh outdoor air through a high-efficiency heat recovery ventilator. The New York Times recently described the resulting air quality of a Passive House this way: “The air inside the house feels so fresh, you can almost taste its sweetness.”
Myth #3: The walls are too thick
The outer walls of a Passive House are usually more than a foot thick. Wall thickness varies by climate: thicker in Iqaliut, thinner in Vancouver.
In a conventional home, residents pull away from bay windows and sliding glass doors during the winter. In a Passive House, however, every square inch provides the same exceptional thermal comfort all year long. This unique sit-by-the-window-in-winter comfort adds more square feet of usable space than the loss of 6 inches along the perimeter due to thicker walls.
Myth #4: Too many exotic materials
The Passive House standard requires no specific products. However, it is easier to design a Passive House using assemblies that have been modeled and windows that have been certified to meet Passive House guidelines.
These products are now readily available in the U.S. and Canada. (Dozens will be on display at the upcoming Passive House North 2013 conference in Vancouver on September 27 and 28.)
Myth #5: Too complicated
Here’s the Passive House standard in a sentence: Heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. ≤ 10 W/m2, annual heating demand ≤ 15 kWh/m2, annual primary energy demand ≤ 120 kWh/m2, thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. Psi ≤ 0.0, airtightness ≤ 0.60 ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.@50Pa. That’s it.
Passive House is like hockey: It’s a simple game with a steep learning curve. It's an experience that is easier (and more fun) with an experienced team. Architect and Passive House consultant Bronwyn Barry has noted that “Passive House is a team sport.”
Myth #6: Too rigid
There’s more than one way to build a Passive House.
Creative approaches are encouraged, and learned lessons are openly shared on a unique site called Passipedia. Likewise, there’s more than one way to certify a Passive House: via the original Passive House Institute (PHI) in Germany, the Canadian Passive House Institute (CanPHI), or a growing number of certification authorities in the U.S., U.K., or Ireland.
Myth #7: Too ugly
Consider the diversity of designs evident in the Passive Houses featured on the GBAGreenBuildingAdvisor.com site, or in the buildings (of all types) that have won PHI’s annual Passive House Award. If you want, Google the offices, apartment blocks, and high-rise Passive House buildings erected in Europe during the past few years.
Passive House is a performance standard; beauty is in the eye of the deed holder.
Myth #8: Too soon
The ideas at the heart of Passive House date back to the 1970s. (For example, see the Sasketchewan Conservation House.)
The first true Passive House was built in Germany in 1991. More than 40,000 buildings have been constructed in the ensuing 22 years, as Passive House has become the most tested and most rigorously verified building standard.
Myth #9: The payback period is too long
See Myth #1: A 10% construction price premium works out to a few hundred dollars per year over the life of a mortgage. A 90% reduction in heating energy totals close to a thousand dollars per year.
The savings begin in Year One, and grow as energy prices rise. That’s why organizations such as Habitat for Humanity are following the Passive House standard.
Myth #10: Net zero is better
Absolutely: Net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. is better than 90% less heating energy. And net positive buildings, such as those built to the Living Building standard, are better still. But because generating energy on-site demands additional systems, the price of clawing back that last 10% can double the cost of construction.
The energy savings provided by Passive House is sufficient to meet or exceed even the most stringent greenhouse gas reduction targets. That’s why European cities such as Brussels will require all new buildings to meet Passive House-like requirements beginning in January 2015.
Bonus gripe: What’s with the name?
Beats me. The Germans call it Passivhaus; so do Brits and purists in North America. The PHI prefers Passive House in English, and most North American organizations follow suit.
Perhaps the linguistic chaos is a byproduct of an open-source standard. Regardless of what one calls it, the Passive House standard remains among the most affordable ways to build anything.
Monte Paulsen is principal of Red Door Energy Advisors, a building energy modeling and performance consultancy in Vancouver, B.C., Canada. He will present “A Loose History of Air Tightness” at the upcoming Passive House North 2013 conference. A similar version of this essay appeared in Canada EcoHome magazine.
- Derek Ford
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