What is a Passive House?
In the simplest terms, a Passive House maximizes passive energy gains while minimizing energy losses. It is a practical approach focused on the “low hanging fruit” of energy efficiency – warm and cool the building passively and then conserve the gained energy. To be considered a Passive House a building must meet the most stringent building energy standard in the United States (and throughout the world). The Passive House standard is both a building energy performance standard and a set of design and construction principles.
Homes that meet the Passive House standard use 80-90% less heating and cooling energy and 50-70% less total energy consumption, in comparison with the usage in a typical home.
The extraordinary performance is a result of an integrated design and construction process that is informed by a common set of principles that are modeled in advanced energy-modeling software, and that are verified by third party testing during and upon construction completion.
The principles that guide a Passive House are: optimize solar and heat gains, insulate extremely well, achieve airtightness, prevent thermal bridges, install high-performing windows, and provide constant fresh air at a low volume through a highly efficient heat recovery ventilation system (HRV). A compact building form and proper solar orientation help the building reach maximum energy efficiency in the most cost effective manner. In the design process the building’s performance is an integral element which must be considered in conjunction with the usual considerations of function, form, and aesthetics. In the construction process the stringent performance tolerances require meticulous craft and execution of the construction details, or the building will fail to meet the requisite level of performance.
The success of a super energy efficient house lies in the idea of assimilation, rather than application. Passive House projects become expensive and it is harder to reach certification if the principles of energy efficiency are applied late in the design phase or after the house is been designed (when it is “shoe-horned). Keeping all the elements/ principles in balance with each other is a key to the approach. Proportionality rules – for example it doesn’t make sense to have an extremely insulated building shell and fail to pay attention to the air-tightness. The extensive energy modeling in the Passive House Planning Package (PHPP), an elaborate spreadsheet program, is an effective design tool in for optimizing the components and refining the overall design. The PHPP has been shown (it has been in use in Europe for nearly 20 years) to be an accurate estimator of predicted energy use compared with the ‘real world’ results.
The Passive House standard stipulates very low air leakage levels, commonly referred to as airtightness. Current code is 7 air changes per hour (ACH) while Passive House requires .06 or less ACH. Airtightness is the unplanned movement of air though the buildings thermal envelop (walls, floor, and roof). The reason it is important to have an airtight building, is that it saves heat energy that could be lost with escaping air, it provides better comfort, and minimizes vapor moisture entering the building’s fabric. A blower door test, which is site blower test that pressurizes the building and calculates the air permeability and air change rate, is required during construction (at least once) to verify that the criteria of .06 ACH at 50 Pascals has been met.
When a building is virtually airtight, a mechanical ventilation system is required. Heat Recovery Ventilation Systems (HRV’s) provided a constant supply of fresh air at very low rates. Does this mean you can’t open your windows for natural ventilation? Of course not! Passive Houses’ have natural ventilation and beautiful operable windows. But, they also have mechanical ventilation systems that provide a very low flow of constant air so the building and occupants stay healthy.
One of the big “leaps of faith” for most folks, when they are considering a Passive House, is that buildings don’t have traditional heating and cooling systems. No over sized forced air systems, no wood stoves! Because the building’s energy losses have been significantly minimized, mechanical systems are infrequently called upon to heat or cool the building. Therefore Passive Houses can be heated with a minisplit or electric resistant heater. Or…if the temperatures in the house drop a bit, you could bake a batch of cookies and the house will soon be warmed!
Passive House or Passivhaus?
The Passivhaus standard originated in Germany and the “haus” in Passivhaus denotes a building – therefore the broader meaning is captured in the term. Here in the “new world” it is generally being called the “Passive House” standard. Passive House should not be confused with a simple passive solar house, although passive solar design is a key one of the design principles (forget your 70’s overheating passive solar houses), nor should it be assumed that the standard is for houses alone. The Passive House standard is well applied to all building types. You will likely see it referred to as both the Passivhaus and Passive House – it is essentially* the same standard.
*In North America PHIUS has formulated a certification, known as PHUIS+, which includes the US accredited rating system Home Energy Rating System (HERS). Linking it to HERS will hopefully bring broader recognition to the Passive House standard and open up eligibility for more incentives.
A micro History of Passive House
The Passivhaus standard was developed in Germany at the Passivhaus Institut (PHI), an independent research institute, in 1996. However the roots of Passive house can be traced back to the 1970’s (those oil embargo years) in North America when concepts of super insulation and passive solar were being explored. Since the development of the Passivhaus standard there have been more than 40,000 Passive House projects built worldwide.
Passive House Criteria
Maximum Heating or Cooling Energy: 4.75 kbtu/ft2 (15 kWh/m2) per year
Maximum Total Source Energy: 38.1 kbtu/ft2
Maximum Air Leakage: equivalent to 0.6 air changes per hour at 50 Pascals (ACH50), (~0.03 ACHNAT )
In addition, the following are recommendations which vary based on specific climate region:
Window u-value ≤ 0.14 Btu/hr-ft2-°F (0.8 W/m2/K)
Ventilation system with heat recovery with ≥ 75% efficiency with low electric consumption @ 0.68 W/cfm/ft3 (0.45 Wh/m3)
Thermal bridge free construction ≤ 0.006 Btu/hr-ft-°F (0.01 W/mK)