Passive House

The term ‘Passive House’ is a general description for an extremely well insulated house designed to be substantially heated from passive gains such as those from the sun and the occupants of the house. Passive houses generally require little or no energy for heating.

The term 'PassivHaus' refers to a specific construction standard which has excellent comfort conditions in both winter and summer. The PassivHaus standard was developed by the PassivHaus Institute in Germany. Homes built to the PassivHaus standard typically require 90% less energy than conventional homes.

Key features of a Passive house include;

  • INSULATION - very low U-values and almost no thermal bridging
  • AIR-TIGHTNESS - very high level of air-tightness
  • PASSIVE GAINS - high utilization of solar & internal gains
  • MHRV - excellent air quality through efficient MHRV system

Insulation & thermal bridging

The specification and construction of the thermal envelope is critical to achieving a passive house. In Ireland, walls, roofs and floors should generally have a U-value of 0.15 W/m2K or better. Windows and doors should have an average U-value of 0.8 W/m2K or better.

Thermal bridging is also very important in a passive house. A thermal bridge refers to part of a building where there is more heat loss than the surrounding elements. These usually occur at junctions, for example where a window connects with a wall or where a wall connects with a floor. To reach the PassivHaus standard, thermal bridging needs to be eliminated as far as possible. Click here for more information on Thermal Bridging.

Air-tightness

Extreme levels of air-tightness are required to achieve the PassivHaus standard. Air-tightness is measured in AC/hr (air changes per hour) at 50Pa pressure difference between inside and outside. This is different to the air-tightness result used for a BER which is measured in cubic meters of air leakage per hour per square meter of building envelope. The main difference is that the PassivHaus method is based on the volume of the house and the BER method is based on the envelope surface area. A maximum air permeability of 0.6 AC/hr is required to reach the PassivHaus standard. To achieve this level of air-tightness will require a great deal of attention to all details and has to be properly planned out in advance of commencing construction. We design for and test all our homes for air-tightness. Click here for more information on Air-tightness.

Passive heat gains

With such high levels of insulation and air-tightness, passive heat gains can make up a substantial proportion of the space heat required in a passive house. Ideally the house should be orientated east-west with the largest elevation facing south with large glazing to benefit from solar gain. A large south facing window in a passive house will have more solar heat gain than heat loss per annum.

Passive internal heat gains also contribute to the space heating requirement of a passive house. These include the occupants of the house, household appliances, kitchen and utility equipment, electronic equipment and artificial lighting.

MHRV

With such levels of air-tightness a MHRV (Mechanical Heat Recovery Ventilation) system is essential to supply fresh air to the building. A highly efficient system will reduce the amount of heat loss associated with ventilating the house.

The space heat requirement in a passive house is so low that a traditional heating system is not required. A small supplementary heating system can be combined with the MHRV system by post-heating the fresh air after it has already been warmed by the MHRV heat exchanger.

The PassivHaus standard or similar is being widely taken up across Europe and is seen as a key step towards building low or zero carbon homes in the very near future. The UK has set 2016 as a target for all new homes to be built zero carbon.

Building a passive house does not have to cost much more than a conventional house, but a lot will depend on the design.

If you have already designed your new home and secured planning permission, your design may not be suitable to reach the PassivHaus standard. However, you can make use of passive house technologies to build a 'low-energy' or ‘near-passive’ home.

ECO Timber Frame has experience in designing and manufacturing timber frame houses to the PassivHaus standard. Our I-beam stud walls are ideal for achieving the extremely low U-values required for a passive house. Click here to view our passive wall.

Please contact our technical department to discuss your passive house project.