Eco Friendly Building - PassivHaus Design
The term PassivHaus refers to a voluntary, low energy construction standard, first developed over a decade ago by Dr Wolfgang Feist of the PassivHaus Institut in Darmstadt, Germany. Buildings of this construction standard have been built mainly in Germany but also in Austria, Switzerland, USA and more recently in the UK
PassivHaus Design Standard
The PassivHaus Institut lays down the standard for PassivHaus construction and, on completion, provides certification for buildings which meet its standard. The technical definition of this standard is very clear and straightforward for climate conditions, between northern hemisphere latitudes 40 degrees (Madrid and Ankara) and 60 degrees (Oslo and Helsinki).
A comfortable internal climate can be maintained without the use of a conventional central heating system or cooling system. The building heats and cools itself and is therefore passive. The cost savings from dispensing with the conventional heating system can be used to fund the upgrading of the building envelope and the heat recovery ventilation system. After the main criteria have been followed, additional energy requirements can be catered for using renewable resources.
PassivHaus - Passive House Design
The PassivHaus design standard is integrated with the architectural design and although it has generally been used for new buildings, it has also been used for refurbishment projects. A building can be certified as a PassivHaus when the following criteria are met:
- The building must not use more than 15 kwh/m²/year in heating energy
- The standard requires very precise levels of insulation for every construction element. Every external surface must have a U-value lower than 0.15 W/m²K and there are tight restrictions on the relative window surface.
- The total use of primary energy (e.g. fuels) for all uses combined (heating, hot water and specifically electricity) may not exceed 120 kwh/m²/year.
- With the building depressurised to 50 Pa (N/m²) below atmospheric pressure using an approved air leakage testing method, the building must not leak more air than 0.6 times the house volume per hour.
The following are the basic features that distinguish PassivHaus construction:
High Level of Thermal Insulation in the PassivHaus
In order to reduce the area of the external envelope, PassivHaus buildings are usually compact in shape. They must employ a high standard of insulation to reduce the heat transfer through the walls, roof and ground floor. A wide range of thermal insulation materials can be used to provide the required low U-values typically in the 0.15 to 0.10 W/m²K range. Special attention must be given to the external walls in order to eliminate cold bridges which can cause heat loss and result in damaging condensation.
PassivHaus and Passive Solar Design
The main principle of passive solar design is to allow the energy of the sun to enter the building and not let it out again. In order to achieve this, the main windows will be orientated towards the equator to maximise passive solar gain.
In these low energy houses, south facing windows act as solar energy absorbers. The entire building will be designed so that solar energy can be distributed from the south façade.
Providing large south facing spaces is usually a straightforward process when dealing with a new building. However, it is more problematic in retrofit projects because the orientation of the building is not always ideal. In such cases, increasing the size of windows on the southern elevation will be an advantage.
Carefully Chosen Construction Materials for the PassivHaus
PassivHaus buildings can be constructed from dense or lightweight materials. Some internal thermal mass is usually incorporated to reduce summer peak temperatures and to stabilise winter temperatures. Dense materials can be integrated to provide thermal mass which will absorb and store heat and then release it when the outside temperature drops. The positioning and dimensioning of these architectural elements must be handled carefully to maintain a comfortable temperature balance for the occupants.
Energy Efficient Windows
In order to meet the requirements of the PassivHaus standard, windows must be manufactured with exceptionally high U-values. These will typically have U-values of 0.8 W/m²K for the entire window, including the frame.
Typically, these will be triple glazed windows, with a good solar heat gain coefficient, low emissivity glass, argon or krypton gas fill, ‘warm edge’ insulating glass spacers with air seals and specially developed thermally broken window frames. Expertly designed, assembled and installed triple glazed windows will significantly reduce energy usage.
PassivHaus - Airtightness of the Building Envelope
The PassivHaus standard requires the building envelope to be extremely airtight. Careful attention to airtightness minimises the amount of warm (or cool) air that can pass through the structure; enabling the mechanical ventilation system to recover the heat before expelling the air.
Warm air leaking out through gaps in the building’s envelope is a major cause of heat loss and, consequently, wasted energy. The building design and the quality of its construction will have a major effect on the amount of air leakage. The issue of airtightness must be addressed at an early stage when the designer should identify where in the building envelope the barrier to air leakage will be. This will be a continuous line around the building, passing through all the elements in the building that separate heated from unheated spaces.
Mechanical Heat Recovery Ventilation in a PassivHaus
Mechanical heat recovery ventilation is the process of exchanging the heat energy contained in air that is extracted from a dwelling and transferring it to the incoming replacement air. This type of installation will include either a central extract system or individual room fans (or a combination of both). Mechanical heat recovery ventilation systems, with a heat recovery rate of over 80 per cent, will be installed to maintain air quality and recover sufficient heat to dispense with a conventional central heating system. Since the building is essentially airtight, the rate of air change can be optimised and carefully controlled at about 0.4 air changes per hour.
PassivHaus and Hot Water Supply - Using Renewable Energy Sources
The most straightforward method of supplying energy for domestic hot water is solar heating panels on the roof of the building or a ground source heat pump in the land outside.
PassivHaus - Solar Water Heating
A well designed solar heating system can provide half of the heat required to
supply a household's domestic hot water throughout the year. Ideally, a home needs to have a south facing roof that has uninterrupted exposure to the sun. Solar heating panels can face anywhere in the south-east to south-west quadrant without losing
much efficiency and should be angled about 35 degrees from the horizontal.
PassivHaus - Ground Source Heat Pumps
Radiation from the sun heats the earth all year round. In the UK winter, the temperature of the earth, close to the surface, is between 8° and 12°C. The technology used in the ground source heat pump is the same as that used in a domestic refrigerator. A refrigerator extracts heat from its contents and pumps that heat to a radiator on the outside of the cabinet, where it is dispersed. Similarly, a ground source heat pump extracts heat from the earth and pumps it into a building, where it can be used for heating.
Space Heating Requirements in the PassivHaus
By achieving the PassivHaus standard, qualified buildings are able to dispense with conventional heating systems. While this is an underlying objective it is usual to provide some supplemental space heating. This is normally distributed through a, low volume, heat recovery ventilation system that is required to maintain air quality. PassivHaus buildings make extensive use of intrinsic heat from internal sources such as waste heat from: lighting, white goods, other electrical non-heating appliances, as well as body heat generated by the occupants of the building.
Beyond the recovery of heat by the mechanical heat recovery ventilation unit, a well designed PassivHaus building, in the European climate, should not need a supplemental heat source when the heating load is kept below 10 W/m². If desired, PassivHaus buildings can have heaters fitted to the supply air duct of the ventilation system to provide heat on the coldest days.
The PassivHaus Institut report that a passive house is cost effective when the combined capitalised costs (construction, including design and installed equipment, plus operating costs for 30 years) do not exceed those of an average new home
What is PassivHaus? - PassivHaus Standard - Passive Design