As you start to research the Passivhaus philosophy, you might come across some terms and concepts that you are unfamiliar with. We hope to make some of them clearer here.
The Passivhaus standard
The Passivhaus Standard is a clearly defined, transparent and proven pathway to better, healthier buildings. The international performance-based standard takes an efficiency first approach to building design, achieving durable, resilient buildings that reduce heating and cooling demand by as much as 90% in some countries. This dramatically reduces building-related carbon emissions and running costs. Passivhaus buildings combine high levels of thermal comfort and indoor air quality with low energy consumption, creating a healthy and comfortable indoor climate.
Thermal comfort
This is a subjective perception of the body and is based on whether a person feels comfortable in their surroundings. Among other things, the indoor air temperature, humidity and the surface temperature of the building components and the air velocity affect the perception of comfort (or absence of discomfort).
Main thermal comfort requirements
Internal temperature between 20°C and 25°C.
Frequency of overheating
- Without active cooling: below 10% without active cooling
- With active cooling: cooling system is adequately sized to prevent overheating
Frequency of excessive air humidity:
- Without active cooling: below or equal to 20%
- With active cooling: below or equal to 10%
Frequency of overheating
This describes the percentage of hours in a year on which the average indoor temperature exceeds 25°C in buildings that are not actively cooled. For the building energy standards of the Passive House Institute, this may not be higher than 10%. Values below 3% are recommended in Australia.
Frequency of excessively high humidity
This describes the percentage of hours in a year on which the absolute humidity of the indoor air is higher than 12g/kg.
Energy efficiency requirements
Heating demand [kWh/(m²a)]
This is the total energy necessary to maintain the desired indoor air conditions within the thermal comfort limits in the cold season. In the Passivhaus standard, the total energy is divided by the Treated Floor Area (see below) of the building and the result can’t be higher than 15KWh/m² (kilowatt-hours per square meter) per annum.
Heating load [W/m²]
This indicates the required capacity of your heating system to keep the building above 10°C in the coldest days of winter. In the Passivhaus standard, the total energy load is divided by the Treated Floor Area (see below) of the building and the result can’t be higher than 10W/m².
Cooling and dehumidification demand [kWh/(m²a)]
This is the total energy necessary to maintain the desired indoor air conditions within the thermal comfort limits in the warm season. In the Passivhaus standard, the total energy is divided by the Treated Floor Area (see below) of the building and the result can’t be higher than 15KWh/m² (kilowatt-hours per square meter) per annum.
Cooling load [W/(m²a)]
This indicates the required capacity of your cooling system to keep the building below 25°C in the hottest days of summer. There is no specific target for cooling load in the Passive House Standard, but it is highly recommended to keep the number as low as possible to avoid overheating and keep your electricity expenses low.
PER – Renewable Primary Energy
The PER (Renewable Primary Energy) demand indicates how much total renewable energy needs to be produced to meet the energy demand from your house all year round. The Passive House Standard considers that for every kWh of electricity consumed by a household, around 2kWh is lost in production, storage and transmission processes. To meet the Passivhaus Standard (Classic), the PER demand must not be higher than 60kWh/m² per year.
Airtightness
This indicates how well sealed a building is against unwanted air exchanges with the exterior environment. An excellent level of airtightness of the building envelope is a prerequisite for the ventilation system to function efficiently and it is a necessary measure for achieving the advantages of a Passivhaus: low energy demand, thermal comfort and damage-free construction. In the Passivhaus standard, the airtightness test result can’t be higher than 0.6 air exchanges per hour at a pressure of 50 Pascals.
Passive House Classes
Passive House Classic, Plus and Premium
The Passive House Classic class is the typical certification class of the Passive House Standard that can be achieved by fulfilling the energy efficiency and thermal comfort requirements mentioned above. The Passive House Plus and Premium classes have the same requirements of energy efficiency and thermal comfort as the Passive House Classic class, however they require higher level of renewable energy generation on site using, for example, photovoltaic panels. To achieve the Passive House Plus standard, a building is required to have PER demand equal or inferior to 45 kWh/(m²a) and a Renewable energy generation of at least 60kWh/(m²a) in relation to the Projected Building Footprint (see below).To achieve the Passive House Premium standard, a building is required to have PER demand equal or inferior to 30 kWh/(m²a) and a Renewable energy generation of at least 120 kWh/(m²a) in relation to the Projected Building Footprint.
PHI Low Energy Building
This class is suitable for buildings were designed under Passive House Principles but do not achieve the Passive House Standard for various reasons. There requirements for this class are less stringent than for Passive House buildings. To achieve this standard, the heating demand of a building must be equal to or below 30 kWh/(m²a), the cooling and dehumidification demand must be equal to or below 15 kWh/(m²a), the airtightness test result can’t be higher than 1.0 air exchanges per hour at a pressure of 50 Pascals, and the PER demand must be below or equal to 75 kWh/(m²a).
Enerphit
The Enerphit class was developed to cater for retrofitting Passive House projects (renovations). Considering it can be harder to achieve the Passive House requirements when a building has already existing structures that were not designed taking in consideration the Passive House principles, in the Enerphit class, some of the Passivhaus benchmarks were relaxed, at the same time, and an alternative method of compliance (component method) was introduced for buildings that cannot achieve the energy demand requirements.
Enerphit - Energy demand method
If a building is to be certified under this method, the total heating demand must be below the limits specified for the climate zone where the building is located. In Australia, those limits vary from 15 to 30 kWh/(m²a). The cooling and dehumidification demand must comply with the Passive house Classic standard and the airtightness test result can’t be higher than 1.0 air exchanges per hour at a pressure of 50 Pascals (instead of the 0.6 air exchanges per hour specified by the Passive House Classic Standard.
Enerphit - Component method
If a building is to be certified under this method, the different components in the building (walls, floors, ceilings, doors, windows and ventilation system) must comply with the limits specified for the climate zone where the building is located.
Ventilation System
Air change rate (via ventilation system)
This measures how often the volume of air inside the building is replaced with fresh air from outside the building in one hour by the ventilation system. In residential Passivhaus buildings, this value is usually between 0.3 and 0.5 1/h, which means that the air inside the house will be recycled 7.2-12 times every day.
Heat recovery efficiency [%]
This is the percentage of the heat energy contained in the air extracted from the building that is transferred to the supply air by the heat exchanger and therefore is not lost. The effective heat recovery efficiency of the ventilation system considers the heat recovery efficiency of the ventilation unit and the heat losses through the ventilation ducts between the unit and the thermal envelope of the building.
Other key concepts
Treated Floor Area [TFA]
This is the net floor area of a building which is to be heated and/or air-conditioned. The TFA is approximately equivalent to the gross internal floor area, the main difference being that the TFA excludes the areas occupied by internal walls. It is therefore a measure for use of the building. The areas are weighted differently depending on the use of the rooms.
Thermal Envelope Area
This is the sum of all the surfaces that separate the conditioned areas of a building to the exterior. In other words, it is a sum of the areas of walls, floors, roofs/ceilings, doors and windows that separate the internal and external environments.
Heat Loss Form Factor
This is the ratio of thermal envelope area to the treated floor area (TFA). It indicates how susceptible a building will be to lose heat according to its shape. The more compact a building is, the easier it is to insulate and achieve high levels of energy efficiency. Lower form-factor values indicate that a building is compact and therefore can more easily achieve higher levels of energy efficiency. The Passivhaus standard has no specific targets for form-factor, but it is recommended that residential single dwelling projects try to achieve a form-factor of 3.
Window to TFA Ratio
This is the total area of all the windows in a building divided by the treated floor area. It indicates how susceptible a building is to lose and gain heat depending on the presence of solar radiation. In Passivhaus building it is usually recommended to keep this number below 30%.
Projected Building Footprint
This is the orthogonal projection of the heated or air-conditioned building envelope on a horizontal plane. It is used to describe the ground surface occupied by the building. The projected building footprint serves as a reference area for assessing renewable energy generation, as it basically corresponds to the area that can be used to produce solar energy.