What Is Passive House?

Passive House Diagram

Passive House is both a building energy performance standard and a set of design and construction principles used to achieve that standard. The passive house standard is the most stringent building energy standard in the world: buildings that meet the standard use 80 percent less energy than conventional equivalent buildings, and provide superior air quality and comfort.

A systems-based design strategy carefully models and balances a comprehensive set of factors including heat emissions from appliances and occupants--to keep the building at comfortable and consistent indoor temperatures throughout the heating and cooling seasons. Continuous mechanical ventilation of fresh filtered air, assures superb air quality.

Concept

“Maximize your gains, minimize your losses”. These are the basic tenants of the passive house approach. A Passive House project maximizes the energy efficiency of the basic building components inherent in all buildings; roof, walls, windows, floors and the utility systems: electrical, plumbing & mechanical. By minimizing a building's energy losses, the mechanical system is not called to replenish the losses nearly as frequently, saving resources, operational costs and global warming related pollution. Unlike any other structures, passive house buildings maintain occupant comfort for more hours of the year without the need for mechanical temperature conditioning of the indoor air. The opposite has been the norm in this country with a history of inexpensive fuel and construction techniques with little consideration for energy losses through thermal bridging, air-infiltration, let alone being conscious of using some or even any insulation.

Thus, to minimize losses, passive house focuses on insulation. The research shows that energy conservation is the most cost effective and immediate solution to global energy production and resulting global pollution concerns.  The cost curve analysis chart below, from the McKinzie Report (McKinsey & Company, 2007 (page 38)) looking at the global potential for carbon abatement illustrates how insulation is the most cost effective approach to this issue:

The Passive House approach focuses on the following:

Strategic Design and Planning:

Passive house building projects are carefully modeled and evaluated for efficiency at the design stage. A Certified Passive House Consultant (CPHC) is trained to use the WUFI Passive software,  House a tool that allows designers to test “what-if” scenarios before construction begins. They are also trained to use the software to identify and address potential thermal bridges and moisture issues at the design stage.

Specific Climate, Siting and Sizing:

Passive house design uses detailed, specific annual weather data in modeling a structure’s performance. Orientation of the windows can maximize or minimize solar gain and shading. Passive House theory leans towards minimizing the surface area to interior volume ratio, favoring an efficient shape to minimize energy losses.

Super-Insulated Envelope:

To keep the heating/cooling in, wall assemblies require greater insulation values to “stop the conditioned air” from leaving. Walls are typically twice as thick as today’s standard construction, for good reason. Wall assemblies are analyzed to allow for proper water and moisture management to make a long lasting and an exceptionally healthy building.

Thermal Bridge-Free Detailing:

Breaks in the insulation layer usually caused by structural elements and utility penetrations in the building envelope create a “thermal bridge,” allowing undesirable exterior temperatures to migrate to and “un-do” expensive interior conditioned air. Passive House design attempts to eliminate thermal bridges via progressive mindful architectural detailing.

Air-Tight Envelope (But Diffusion Open):

Any one who has been in an older drafty home understands how stopping unconditioned air from squeezing to the inside, effects comfort and the efficiency of the mechanical system. Passive House takes great care in designing, constructing and testing the envelope for an industry-leading control of air leakage to the interior. Blower door testing is a mandatory technique in assuring high performance. Walls are designed to be virtually air tight, while allowing water vapors to dry out providing an excellent strategy to maintain a healthy environment.

Advanced Windows and Doors:

Historically these items are the weak link of a building’s envelope and thermal defense system. Passive House places significant emphasis on specifying high performance windows and doors to address concern. To meet the high performance needs of various climate zones, windows must meet strict performance standards regarding: component insulation, air tightness, installation and solar heat gain values.

Energy Recovery Ventilation:

The “lungs” of a Passive House come from a box called a heat (or energy) recovery ventilator (HRV/ERV). It provides a constant supply of tempered, filtered fresh air 24/7 and saves money by recycling the indoor energy that is typically found in exhaust air. The heat from outgoing stale air is transferred to the unconditioned incoming fresh air, while it is being filtered. It provides a huge upgrade in indoor air quality and consistent comfort, especially for people sensitive to material off-gasing, allergies and other air-borne irritants.

Heating:

One of the best benefits to implementing Passive House design is the high performance shell and extremely low annual energy demand. This allows owners to save on operational costs as they can now significantly downsize a building’s mechanical system. Passive solar gains, plus heat from occupants and appliances supply most of the needed heat. Thus, Passive House design puts a project within reach for achieving true “Net Zero” performance (the building generates as much energy as it consumes over the course of a year), making use of alternative energy systems smaller thus more affordable and attainable.

Efficient Systems:

Through fitting buildings designed to the Passive House Standard with efficient appliances, hot water distribution, and energy efficient lighting, electricity consumption is also slashed by 50% compared to the average housing stock, without any loss of convenience. Most residential ventilation systems, for instance, are typically driven by a highly efficientient motor only consuming 40w.

Alternative Energy:

Considering alternative energy systems on your project? Building to meet the Passive House Standard is the smartest starting point. The significant reduction in energy use, allows alternative energy to power a greater percentage of a buildings demands. Likewise smaller demand equates to smaller and more affordable alternative energy systems providing higher cost-benefit value. Passive House design puts a project within reach for achieving true “Net Zero” performance (the building generates as much energy

Performance Criteria & Certification

Performance Criteria

  • Maximum Heating or Cooling Energy: 1.4 kWh/ft2 or 4750 Btu/ft2 (15 kWh/m2) per year
  • Maximum Total Source Energy: 11 kWh/ft2 or 38,100 Btu/ft2 (120 kWh/m2 ) per year (“Source Energy” by definition includes the energy required to produce and deliver the energy to the site, and can be offset with solar thermal and other measures. Photovoltaics cannot be used to offset this energy, but are recognized, at this time.)
  • 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)

Verification/Certification

PHIUS+ adds third-party Quality Assurance/Quality Control to the passive house evaluation process, providing an added level of quality assurance and helping qualified projects earn financial incentives through HERS ratings. Since 2013, PHIUS+ Projects also earn U.S. Department of Energy certification as Zero Energy Ready (previously known as Challenge Home status). PHIUS+ Certification utilizes RESNET energy raters to make on site inspections, as well as to perform final blower door tests.

 

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