What is a Passive House?
Developed in Germany and Sweden during the 1990s, the Passive House Standard (PassivHaus in German) is a design approach aimed at delivering very large reductions - 60-80% compared to current building code - in the energy used for heating and cooling buildings. The idea is to maximize a building’s thermal shell or envelope in order to minimize the mechanical system required for heating and cooling. Efficient buildings with small mechanical systems use much less energy to heat and cool.
Passive Houses- not to be confused with passive solar houses although they share some of the same design attributes- achieve high levels of energy savings by following a prescribed set of design elements. These key elements include good solar orientation, a compact building shape, avoiding design details that can accelerate heat loss (known as thermal bridges), high levels of insulation in exterior walls, roofs, and under slabs/foundations, and high performance windows. Control of air leakage is also critical so Passive Houses need to be built very airtight, with precise levels of fresh air supplied by a ventilator that incorporates high-efficiency heat recovery.
Passive Houses- not to be confused with passive solar houses although they share some of the same design attributes- achieve high levels of energy savings by following a prescribed set of design elements. These key elements include good solar orientation, a compact building shape, avoiding design details that can accelerate heat loss (known as thermal bridges), high levels of insulation in exterior walls, roofs, and under slabs/foundations, and high performance windows. Control of air leakage is also critical so Passive Houses need to be built very airtight, with precise levels of fresh air supplied by a ventilator that incorporates high-efficiency heat recovery.
Building Envelope Efficiency
Efficiency is a term that refers to how easily or effortlessly a system does what it was designed to do. It differs from effectiveness which refers to how well a system does what it was designed to do. A tree house uses very little energy, that is it's very energy efficient, but it's not very effective at keeping its occupants comfortable throughout most of the year. In the world of buildings, efficiency can refer to different things. Mechanical efficiency is a measure of how a system, for example a heat pump, converts an input amount of energy, in this case electricity, into an output amount of energy like heating or cooling. Building efficiency, on the other hand, refers to how easily a building retains heat (or coolness in the summertime).
The part of a building responsible for retaining heat is referred to as the thermal envelope. It's the aggregate of parts - walls, roofs, windows, doors, foundations, footings, slabs - that are in contact with the outside air and the ground. These individual parts need to work together and be assembled in such a way that minimizes thermal bridging, and results in a high level of air tightness, as uncontrolled air leakage compromises efficiency by wasting energy.
In Passive House design, the unit of measure used to evaluate a building's efficiency is called the Annual Heat Demand and is defined as annual energy use per treated floor area and is expressed in kilowatt-hours per square meter per year (kWh/m2/yr).
The part of a building responsible for retaining heat is referred to as the thermal envelope. It's the aggregate of parts - walls, roofs, windows, doors, foundations, footings, slabs - that are in contact with the outside air and the ground. These individual parts need to work together and be assembled in such a way that minimizes thermal bridging, and results in a high level of air tightness, as uncontrolled air leakage compromises efficiency by wasting energy.
In Passive House design, the unit of measure used to evaluate a building's efficiency is called the Annual Heat Demand and is defined as annual energy use per treated floor area and is expressed in kilowatt-hours per square meter per year (kWh/m2/yr).
PHPP- Powerful Energy Modeling Software
Passive House designers calculate a building's Annual Heat Demand by using a comprehensive energy modeling tool known as PHPP- Passive House Planning Package. PHPP is an Excel-based program that analyses the energy balance between heat losses through all of the various thermal envelope components and air leakage, and heat gains from solar radiation (via sunlight through windows), as well as heat generated by appliances, electronics, lights and human occupants. The PHPP also considers the impact of various other factors like climate zone, solar orientation, shading, air tightness, and efficiency of the ventilation system. The difference between gains and losses represents the Annual Heat Demand. The AHD is updated each time an input component is changed so designers can immediately see the impact on the energy balance of resizing a window or adding more insulation to the walls.
The Passive House Standard
Passive House, apart from being a design approach, is also a performance-based standard. In order to achieve Certified Passive House status, a building needs to fulfill the following performance criteria:
Specific Space Heat Demand < 15 kWh/m2/yr
Primary Energy Demand < 120 kWh/m2/yr
Airtight Building Envelope < 0.6 ACH @ 50 Pascals (as measured by a blower door test)
A building not quite meeting the 15 kWh/m2/yr but having a Heat Load of no more than 10 W/m2 would also qualify if the other criteria were met.
Primary Energy is a measure of the total impact on the environment from energy that needs to be generated from non-renewables to supply all of the requirements of a building- space and water heating, cooling, and all the energy (electricity, natural gas, oil etc.) needed to run the building and its systems. If a home uses electricity that is generated from a 30% efficient coal-fired power plant, then it would take more than 3 kWs of energy generated at the plant to deliver a single kilowatt to the end user. Another way to look at this is that every kWh saved at the site, can save multiple kWs at the point of origin. Because of the high number of inefficient power generating sources, and all of the variables regarding the systems used to heat and cool (mechanical efficiency), the Primary Energy target of 120 kWh/m2/yr can be difficult to achieve.
In addition to the above-noted performance criteria, PH designers also incorporate the following attributes into their low-energy buildings:
High performance windows (U-value < 0.8 W/m2/K ) contribute significantly to the energy balance and to the thermal comfort of the occupants. Windows will be further-discussed in the Projects section of this site.
Thermal Bridge Free Construction also contributes to the comfort of the indoor space and prevents cold spots that can lead to condensation. Excess condensation can cause building envelope damage/failure and promote the growth of mould.
Ventilation system with high efficiency heat recovery > 75%. One of the greatest benefits of Passive House design is the extremely high quality of the indoor air enjoyed by the occupants. Passive Houses achieve this by supplying a precise amount of fresh, filtered air into the home while extracting stale air and polutants. Bringing fresh, ice-cold air into the home in the middle of winter can be a problem so a special ventilator (HRV or ERV) recovers the heat energy of the indoor space from the exhaust stream and transfers it to the incoming fresh air stream thus avoiding the huge energy penalty that would be present without this system. In addition to the ventilation system, windows can be opened any time the weather is amenable, to take advantage of "free" passive ventilation.
Specific Space Heat Demand < 15 kWh/m2/yr
Primary Energy Demand < 120 kWh/m2/yr
Airtight Building Envelope < 0.6 ACH @ 50 Pascals (as measured by a blower door test)
A building not quite meeting the 15 kWh/m2/yr but having a Heat Load of no more than 10 W/m2 would also qualify if the other criteria were met.
Primary Energy is a measure of the total impact on the environment from energy that needs to be generated from non-renewables to supply all of the requirements of a building- space and water heating, cooling, and all the energy (electricity, natural gas, oil etc.) needed to run the building and its systems. If a home uses electricity that is generated from a 30% efficient coal-fired power plant, then it would take more than 3 kWs of energy generated at the plant to deliver a single kilowatt to the end user. Another way to look at this is that every kWh saved at the site, can save multiple kWs at the point of origin. Because of the high number of inefficient power generating sources, and all of the variables regarding the systems used to heat and cool (mechanical efficiency), the Primary Energy target of 120 kWh/m2/yr can be difficult to achieve.
In addition to the above-noted performance criteria, PH designers also incorporate the following attributes into their low-energy buildings:
High performance windows (U-value < 0.8 W/m2/K ) contribute significantly to the energy balance and to the thermal comfort of the occupants. Windows will be further-discussed in the Projects section of this site.
Thermal Bridge Free Construction also contributes to the comfort of the indoor space and prevents cold spots that can lead to condensation. Excess condensation can cause building envelope damage/failure and promote the growth of mould.
Ventilation system with high efficiency heat recovery > 75%. One of the greatest benefits of Passive House design is the extremely high quality of the indoor air enjoyed by the occupants. Passive Houses achieve this by supplying a precise amount of fresh, filtered air into the home while extracting stale air and polutants. Bringing fresh, ice-cold air into the home in the middle of winter can be a problem so a special ventilator (HRV or ERV) recovers the heat energy of the indoor space from the exhaust stream and transfers it to the incoming fresh air stream thus avoiding the huge energy penalty that would be present without this system. In addition to the ventilation system, windows can be opened any time the weather is amenable, to take advantage of "free" passive ventilation.
Passive House websites
The information contained on this page is only meant to be a brief summary of Passive House design. Those seeking a more detailed analysis are encouraged to check-out the following websites:
Passive House Institute (PHI), Darmstadt, Germany http://www.passiv.de/en/index.html
Passive House Institute US (PHIUS) http://www.passivehouse.us/passiveHouse/PHIUSHome.html
Passive Buildings Canada http://www.passivebuildings.ca/
Passipedia http://www.passipedia.org/
Passive House Institute (PHI), Darmstadt, Germany http://www.passiv.de/en/index.html
Passive House Institute US (PHIUS) http://www.passivehouse.us/passiveHouse/PHIUSHome.html
Passive Buildings Canada http://www.passivebuildings.ca/
Passipedia http://www.passipedia.org/