A Passive house, what’s in a name ? - 03

The term Passive house comes from Swedish and German research into creating low energy houses, in terms of running costs. The concept was taken further in Germany with actual working models constructed that became known as Passivhaus or Passive House. The Passivhaus Standard is a holistic view of how a house may be constructed with the primary aim being energy efficient living. The Passivhaus Institut in Germany has set criteria for achieving the Passivhaus standard which, in effect, calculates the dwellings energy consumption.     The key to linking the name ‘Passive’ with the function of the building is to remember that the house is Passive in terms of its impact on the environment and its active energy requirement. To achieve this, these houses must incorporate features that other houses do not have at present.

This Passive House technology is now readily available, but is often poorly understood. It is also perceived as much more expensive to implement. The additional cost figures vary at between 10-15% more expensive that conventional building to the current building regulations. When taking a view as to the extra cost of achieving Passive standards it must be remembered that the energy saving year on year will pay-back the difference in of 5-10 years, after which time the dwelling is saving the occupants money. This future proofing of the house, in terms of energy use means that the houses inherent value would be maintained over the future years.

How do we achieve a Passive house? It is an area that needs professional advice and a ‘whole house’ approach as there are a number of permutations in achieving the standard.  The technology is also advancing making the choice of materials and technology reliant on the latest market innovations.

In order to put a marker down we have itemised the main principles and elements to make a passive house function. These ‘Passive house’ standards  are intended for the  European climate and would not be suitable for more extreme climates.

Characteristics of a Passive house

A ‘whole house’ approach is needed to the design a passive house.  The technology available to achieve the considerable energy efficiencies is advancing, making the choice of materials and technology reliant on the latest innovations in the market. The skill of the designer is to best employ the available materials and systems to achieve this low energy standard.  

The ‘Passive house’ must have all of the following features to function in the typical European climate.

Super Insulation and elimination of cold bridges

The most important aspect of low energy housing is the most simple to comprehend. It is a super-insulated envelope ( walls, floors, roof, windows and doors ) to the dwelling. It requires very high levels of insulation and special detailing of openings and junctions to ensure there are no ‘cold bridges’ to allow for the transmittance of heat through the building fabric. The windows for a passive house also have to have exceptional insulation standards in keeping with the other elements. It is often a misconception that the employment of renewable energies will produce a passive or low energy house but without the fundamental insulation element the use of renewable energy technology can be wasted.

Air-tightness of building fabric

In conventional houses up to 20% of energy may be lost through air infiltration or drafts that occur commonly at junctions of floors, doors and windows. For this reason passive houses are extremely well sealed in terms of air-tightness and this aids retention of heat. In current building an air pressure test is carried out on new dwellings. The building regulations are relatively easy to obtain, in terms of workmanship. The passive house standard is more onerous and relies on more careful consideration and detailing of the building fabric. Good on site construction techniques and supervision is recommended.

Circulation of fresh air

The need for active ventilation to passive houses is recognised due to the air tightness and this requirement has been developed as an advantage. A mechanical ventilation system is employed to maintain air quality. The rate of air change can be optimised and carefully controlled at about 0.4 air changes per hour. Passive houses have active ventilation to all rooms in the dwelling. The air in rooms that produce active heat, and odours, such as kitchen, bathrooms and utility room is extracted. Rooms that require heating such as bedrooms and living spaces have fresh air vented to them.  Overall the balance of air is maintained within the house by air movement below internal doors and an open plan house. The reason for this ventilation is to provide fresh air to the house and to avoid the build up of water vapour or condensation in the house due to the air-tightness. It should also be remembered that at any time a window can be opened for additional ventilation although in winter opening a window for ventilation would cause heat loss.

Heat recovery from circulated air.

The circulation of air is handled with mechanical ventilation which has the crucial element of heat recovery, commonly known as MVHR ( mechanical ventilation with heat recovery). The clever aspect of the system is that it literally ‘transfers’ heat from the extracted stale air to the incoming fresh air (which is ducted from the outside). The efficiency of heat transfer is over 85% and the majority of the heat from extracted air is therefore not lost to the building envelope, as with conventional extraction.  The system of heat transference to the cold incoming air means that the houses environment may be regulated and maintained at a higher temperature, for most of the time, without the need for an additional energy source except the low running cost of the MVHR. This system is often the least understood in terms of convincing the public of the benefits of passive houses and perhaps most off-putting to the public. Negative images of ‘air conditioning’ units humming on the ceiling are brought to mind. In fact this is not ‘air conditioning’ of the conventional type. The MVHR runs on less energy needed for a 100 watt light bulb and functions continuously, the unit may be located in utility rooms and have health benefits in that the air is filtered before entering the house. A significant amount of heat is produced with common domestic processes such as cooking, showering and the use of various electrical appliances.  Human bodies give off a significant amount of heat (People, on average, emit heat energy equivalent to 100 Watts) and this is significant to a passive house as the energy loss through the fabric of the building is so low. )To put this in perspective a 100 watt light bulb should be capable of maintaining the space heating of a 10 metres square room. Two 100 watt light bulbs would therefore have the capability of heating to a higher temperature the same room.)

Ventilation system used for active space heating.

The MVHR system is required to maintain air quality. The MVHR also acts as ‘air source’ space heating for the house in that the heat recovered from extracted air is transferred to incoming air and circulated throughout the house. This benefit along with a super-insulated envelope means that passive houses are able to dispense with conventional heating system as they would quickly overheat the building. Some additional source of heating is recommended as this would be required when the house is not occupied (no passive heat is generated ) or in severe weather conditions. Most Passive buildings do include a system to provide supplemental space heating. This may be distributed through the low-volume ( MVHR ) mechanical ventilation system. One recommendation is to have a dual purpose electrically operated 800 to 1,500 Watt heating and/or cooling element integrated with the fresh air supply duct of the heat exchanger ventilation system. It is important that the (MVHR) ducting is adequately sized to allow for this element of active space heating and the volume of air required for space heating ventilation.

Energy source for active space heating to the passive house.

The air-heating element associated with the fresh air intake air to the MVHR units can be heated by a small heat pump, by direct solar thermal energy, or simply by a natural gas or oil burner. In some cases a micro-heat pump is used to extract additional heat from the exhaust ventilation air, using it to heat either the incoming air or the hot water storage tank.

In some instances small wood-burning stoves can also be used in space heating and the water tank. Care is required to ensure that the room in which the stove is located does not overheat. The ventilation system would also need to be configured to circulate this hot air around a house that may not be open-plan.

Triple glazed high specification windows

A requirement of the super-insulation of passive houses is that the windows frame and glazing must be of a suitable insulation quality to stop excessive heat loss. Conventional double glazed windows are unable to meet the thermal values required and for this reason triple glazing is required along with specially designed window frames, with thermal breaks, that have a low thermal conductivity. It should be noted that the window glass may be coated and an insulating gas used in the sealed system to achieve the desired values.

Passive solar gain / orientation

One last feature of passive houses is the orientation of living spaces in terms of making use of solar gain. The capture of passive solar energy by glazing within the fabric of the building may be used to heat the Thermal mass of the building during the day and this heat is then released or radiated during the night stabilising the temperature of the house. The use of concrete floors to act as a thermal store for the suns heat during the day is a simple way of retaining heat and for this reason the glazing to the south may be sized to take advantage of this. The passive houses we are designing are not ‘Passive Solar Houses’ and overheating due to thermal gain can be a problem in sunnier climates and potentially lead to overheating of Passive houses.

Some characteristics of Passive Houses

The air is fresh, and very clean. Note that for the parameters tested, and provided the filters are maintained, quality air is provided. 0.3 air changes per hour are recommended, otherwise the air can become "stale" (excess CO2, flushing of indoor air pollutants) and any greater, excessively dry (less than 40% humidity). The use of a mechanical venting system also implies higher positive ion values. This can be counteracted somewhat via opening the window for a very brief time, plants and indoor fountains. However, it should be noted that failure to exchange air with the outside during occupied periods is not advisable.

Inside temperature is homogeneous; it is impossible to have single rooms (e.g. the sleeping rooms) at a different temperature from the rest of the house. Bedroom windows can be opened slightly to alleviate this when necessary.

The temperature changes only very slowly - with ventilation and heating systems switched off, a passive house typically loses less than 0.5 °C (1 °F) per day (in winter), stabilising at around 15 °C (59 °F) in the central European climate.

Opening windows or doors for a short time has only a very limited effect; after the windows are closed, the air very quickly returns to the "normal" temperature.

The air inside Passive Houses, due to the lack of ventilating cold air, can be drier than in 'Standard' Houses. This may be counteracted by slowing the rate of ventilation to allow water vapour to build up within the house as required to comfort levels.

Appendix 1.   Technical energy targets for passive house in Western Europe Climate.

The Passive House standard for central Europe requires that the building fulfils the following requirement,

The building must not use more than 15 kWh/m² per year heating and cooling energy.

 Total energy consumption (energy for heating, hot water and electricity) must not be more than 42 kWh/m² per year

Total primary energy (source energy for electricity and etc.) consumption (primary energy for heating, hot water and electricity) must not be more than 120 kWh/m² per year

Recommended:

 With the building de-pressurised to 50 Pa (N/m²) below atmospheric pressure by a blower door, the building must not leak more air than 0.6 times the house volume per hour (n50 = 0.6 / hour).

 Further, the specific heat load for the heating source at design temperature is recommended, but not required, to be less than 10 W/m².