What is a Passive House?

‘Passive House’ is a design standard that achieves thermal comfort with minimal heating and cooling by using insulation, airtightness, appropriate window and door design, ventilation systems with heat recovery, and elimination of thermal bridges. Originally developed in Germany in the 1990s, Passive House principles are now being used throughout the world. Passive House standards are performance-based: they set performance targets to be met but do not dictate specific materials or products. The Passive House Institute administers a certification scheme that allows a building to be called a Certified Passive House once it has met certain performance standards.

What is ‘Passive House’?
‘Passive House’, or Passivhaus as it is known in German, is a design standard that delivers healthy, comfortable and efficient buildings. A Passive House takes a ‘fabric first’ approach. The focus of the design is ensuring the thermal envelope (the layer that separates inside from outside) is optimized to provide comfortable, healthy, indoor conditions.

The definition of a Certified Passive House is: “… a building, for which thermal comfort (ISO 7730) can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions, without the need for additional recirculation of air” according to Passivhaus Institut (PHI)

Since air is a poor carrier of heat energy, the heating and cooling energy requirements of the building must be very low because there is not sufficient air flow to deliver large amounts of heating or cooling energy. A Passive House is appropriately insulated, airtight, has quality windows and reliable ventilation systems with heat recovery. A certified Passive House undergoes a quality assurance process that ensures that it is built as designed and meets the comfort standards required by the Passive House standard.

The concept originated from international research into delivering comfort in homes where heating was unavailable or fuels were scarce including the Saskatchewan Conservation House in Canada in 1977 and a ‘zero energy house’ near Hanover, Germany in 1989. The first home constructed to test the Passive House concept was completed in Darmstadt, Germany in 1991 by Dr Wolfgang Feist and Dr Bo Adamson. It has been continuously occupied by Dr Feist and his family ever since and data monitoring and testing show that the building still performs as predicted after nearly 30 years. Dr Feist established the Passive House Institute (PHI) in 1996 as an independent research institute.

The Passive House Standard and Certification
• Space Heating Demand: not to exceed 15kWh annually OR 10W (peak demand) per square meter of usable living space
• Space Cooling Demand: roughly matches the heat demand with an additional, climate-dependent allowance for dehumidification
• Primary Energy Demand: not to exceed 120kWh annually for all domestic applications (heating, cooling, hot water and domestic electricity) per square meter of usable living space
• Airtightness: maximum of 0.6 air changes per hour at 50 Pascals pressure (as verified with an onsite pressure test in both pressurized and depressurized states)
• Thermal Comfort:Thermal comfort must be met for all living areas year-round with not more than 10% of the hours in any given year over 25°C

Certification is carried out by a PHI-approved Passive House Certifier, and there are certifiers working throughout the world. The certification process is usually undertaken in 3 stages: a pre-construction check of documentation and modeling, evidence of construction during the build, and a post-construction blower door test.

There are 3 levels of Passive House certification available: classic, plus, and premium. Classic does not require the installation of renewable energy generation. Passive House Plus buildings are more efficient and will produce about as much energy as they consume over a year. Passive House Premium certified buildings are more efficient again and will produce a significant excess of energy over the year. Although Passive House principles are often used in new homes, buildings can be retrofitted to a Passive House standard. EnerPHit is the Passive House certification system for retrofits and renovations. It uses the same principles and processes as Passive House with slightly less stringent metrics because of the practical challenges of achieving high performance in existing buildings.

The Passive House standard uses its own calculation methods, and therefore can be difficult to directly compare with other rating tools. For example, a Passive House assessment does not directly correlate to the Nationwide House Energy Rating Scheme (NatHERS) star ratings. Research undertaken by the PHI has shown that a Certified Passive House generally achieves a 50 to 90% reduction in energy demand compared with local minimum building-code-compliant buildings in Europe, North America and China. A similar analysis has not yet been undertaken in Australia.

PHI has developed thermal modeling software which uses climate data to calculate monthly heating and cooling demands and the overall energy consumption (PER). The Passive House Planning Package (PHPP) is open-source software that has a high correlation between its prediction and the real-world performance of buildings. The software can be used to compare performance results between alternative scenarios and, if cost data are available, to calculate financial metrics to estimate cost benefit. The SketchUp extension, designPH, also allows 3D modeling and a PHPP interface.

The PHPP has been developed in conjunction with dynamic simulation and performance testing and verified against the standard ASHRAE 140. This globally recognized standard (Standard Method of Test for Building Energy Simulation Computer Programs) ensures that all validated software delivers comparable results so as to give confidence to industry that the software is appropriate for use. The PHPP uses climate data collated by the Passive House Institute from national observation organizations. It is reviewed periodically and updated as needed. There are also climate data files available with future climate predictions enabling a design to be tested for future-proofing.

A Passive House is designed to maintain a comfortable indoor air temperature, which is defined as being between 20°C and 25°C. There is also a requirement for all internal surfaces to remain warm enough to prevent radiant asymmetry (differences between air temperatures and surface temperatures) and avoid risk of mould and condensation.

Many of the principles of Passive House are common to passive solar design, though with some key exceptions. A Certified Passive House must have:
• appropriate levels of thermal insulation
• a design that reduces thermal bridges
• airtightness
• high-performance windows and doors
• mechanical ventilation with heat recovery.

Appropriate levels of insulation are required to control heat loss and gain. Depending on the location, the amount of insulation needed to meet the Passive House standard may be similar to the California Building Code, or only slightly greater, for a similar building.

Thermal bridges are points where heat and cold can cross from the inside to outside (or vice versa) through floor, walls and roof components. Thermal bridges cause cold spots inside buildings in winter. Warm, moist, internal air meeting a sufficiently cold surface will also form condensation. Cold and damp conditions substantially increase the risk of mold. In a Passive House this conduction needs to be eliminated or, as a minimum, calculated and ameliorated so it does not cause condensation, mould or excessive heat loss and gain.

Common construction materials with high conductivity that are most likely to create large thermal bridges include aluminum, steel and concrete. Common locations for thermal bridges include aluminum (non-thermally-broken) framed windows, steel beams projecting from inside to outside and uninsulated concrete slab edges. A Passive House must be free of thermal bridges. The California Building Code does not currently require thermal bridges to be reduced or quantified.

A Passive House must be airtight to control water vapor transfer and the internal air quality. This prevents uncontrolled infiltration through the walls, roof and floor, which can draw pollutants into the building and cause moisture damage. There are various strategies and materials available to achieve airtightness, including orientated strand board, plywood and specialized membranes.

Airtightness is measured using a blower door test. A fan is placed inside an adjustable door in an external opening in the building, the fan pressurizes and depressurizes the building to at least 50Pa of pressure difference. This equates to a 20mph wind blowing on the building. The air volume passing through the fan is used to calculate the leakage of the building; the result is given as air changes per hour at 50Pascals. A blower door is installed in a door way which consists of a frame and a flexible panel fitted in the doorway with a fan and an air pressure gauge.

Windows and Doors
As part of the airtightness standard, a Passive House must have high-performance windows and doors. These must also be sized and placed carefully, with appropriate shading, to control the amount of solar heat gain in winter while reducing unwanted heat gain in summer. The windows must be very well sealed and double or triple glazed, depending on the climate. The use of double or triple glazing reduces the heat flow outwards in winter and inwards in summer by a factor of up to 6, compared with single glazing. It also reduces the risk of condensation forming on the inside of the glass. Window frames must reduce heat transfer from inside to outside.

Because they are often sourced from overseas or use European technology, appropriate windows are often ‘tilt and turn’ and large doors use a ‘lift and slide’ mechanism to ensure they are airtight when closed. While the building is required to be airtight, openable windows are still required in a certified Passive House.

Adequate Ventilation
Because a Passive House is airtight, ventilation strategies must be included to deliver fresh air for occupants. Passive House buildings may be naturally ventilated; however, Passive House buildings use a simple, mechanical ventilation system with heat recovery.

Natural ventilation requires regular occupant intervention, including overnight, if appropriate internal carbon dioxide levels are to be maintained. In most climates, opening windows to bring in outdoor air will result in less comfortable temperatures for a substantial portion of the year (for example, winter night times, summer daytimes). Natural ventilation can also be unreliable because of naturally occurring factors: the wind may not blow when needed, or the outdoor temperature may not decrease sufficiently overnight in summer to provide night-time cooling.

A mechanical ventilation system is a very simple and low-energy setup, with fans that drive air across a heat exchanger. The heat exchanger transfers the heat energy of outgoing air to the incoming air, ensuring that the outdoor air enters the building at approximately the same temperature as the outgoing air. In winter, this pre-heats the incoming air; in summer, heat is rejected back to the outdoors. Centralized, ducted systems are most appropriate and efficient in most cases, but a large range of solutions exist, including for retrofits.

In humid climates, an energy recovery ventilation unit may be used. Energy recovery ventilation (ERV) recovers both heat and humidity from the outgoing air. In a humid climate where a dehumidifier is used, this enables the home to maintain lower humidity levels than outdoors. In cold, low humidity climates, an ERV retains relatively higher humidity inside to prevent the internal air becoming too dry.

For Further Reading:
• Wikipedia has an excellent overview of the Passivehouse concept at: https://en.wikipedia.org/wiki/Passive_house.
• “What is a passive house?” by HowStuffWorks.com is presented at the page: https://home.howstuffworks.com/home-improvement/construction/green/what-is-passive-house.htm.
• Passipedia.org details this subject in depth. The page is: https://passipedia.org/basics/what_is_a_passive_house.

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