For more than 10 years the Germans and Austrians have been building homes with very low energy requirements for heating and hot water. In true orderly fashion they have developed the systems, design details and product approvals necessary and, with more than 6,000 houses built to this standard, there are many lessons to learn from their experience.

The PassivHaus principles have been driven by the simple desire to reduce drastically the demand for energy in buildings. They cover both new and upgraded buildings and have been broadened beyond dwellings to include educational and commercial buildings.

The PassivHaus standard is considered by the Department of Communities and Local Government (CLG), which launched the Code for Sustainable Homes (CSH) in April, to be broadly equivalent to Level 4 in the six-tier Code. Energy efficient houses built to the PassivHaus standard usually cost more than those achieving minimum German building standards, typically by about £5,000 per unit. Who pays for this increase?

Housebuilders here will be envious to know that, although PassivHaus is an entirely voluntary scheme in Germany, funding mechanisms have been set up to make it commercially attractive to the developer.

One source of funding is proKlima, a not-for-profit organisation based in Hannover, which uses funds to encourage the reduction of energy use in buildings in the local region.

proKlima receives its funding from a levy placed on energy producers around Hannover and these funds have been used to pay for developing the technical infrastructure for PassivHaus and to provide grants for design professionals and developers. Developers have been able to qualify for grants, providing they meet all the design checks, audit and testing requirements. The grant is designed to offset fully the increased cost of construction, allowing the developer to market the properties as higher value (because of reduced running costs), but at no extra cost or risk to themselves.

By comparison, there is little practical expertise in the UK and there is no financial incentive for developers, with the exception of no stamp duty on zero carbon houses (which at Level 6 of CSH is a much tougher standard), up to a maximum sale value of £500,000, on the first sale of the house only.

The PassivHaus system is based on four clear principles:

• the building must not use more than 15kWh/m2/year in heating energy;

• the specific heat load for the heating source at the desired temperature must be less than 10W/m2;

• air leakage levels at 50Pa must not exceed 0.6 times the house volume / hour (approximately 1m3/hr/m2 @ 50Pa);

• total primary energy consumption (heating, hot water and appliance electricity) must not exceed 120kWh/m2/year.

To achieve them requires excellent levels of thermal insulation, including minimising thermal bridging, to the extent that very little heating/cooling is needed, rendering a traditional heating system unnecessary.

It will take a long-awaited cultural revolution in UK site practice to embrace the German model

PassivHaus design means that the house and glazing system make good use of solar energy. With high thermal insulation levels, it becomes economically and practically realistic to use solar energy to heat the dwelling during the colder months. To achieve this, significant glazing areas are required on the southerly aspects of the building – glazed areas on south facing walls can often be almost wall to wall and ceiling to floor – but solar shading is needed to counter solar gain in the summer.

Having made the building airtight, however, it is necessary to install mechanical ventilation throughout the property. When combined with heat recovery systems, it is possible to retain most of the heat energy and minimise energy use, but this is not without problems. Frequent air changes are necessary if damp/mould and poor air quality are to be avoided. Passive air stacks (not to be confused with passive solar heating) cannot be relied on, because the system is dependent on sufficient external wind conditions. Controlled ventilation is not just desirable, but absolutely necessary.

The PassivHaus system will not translate all that easily into the UK, as things stand. There are several areas where PassivHaus and UK practice are out of step, not least in terms of insulation. With PassivHaus, components of the exterior shell (excluding windows) are insulated to achieve a U-value that does not exceed 0.15W/m2/K. Special attention is required to eliminate thermal bridging. In practice this can be achieved in the UK, too, with a variety of materials, including timber frame, but components in a typical PassivHaus are usually much thicker in dimension than current UK practice to achieve the required level of insulation.

At present, England and Wales Building Regulations set limiting U-values of approximately 0.25-0.35W/m2/K. Thermal bridging is only loosely addressed through Part L’s ‘Approved Construction Details’, which are not compulsory.

While the UK aims simply to reduce heat loss through windows to an ‘acceptable’ level, typically 1.8-2.0W/m2/K, PassivHaus designers want windows (glazing and frames combined) with U-values not exceeding 0.80W/m2/K, with solar heat-gain co-efficients around 50%.

The windows typically combine triple-pane insulated glazing (with good solar heat-gain co-efficient, low-emissivity coatings, argon or krypton gas fill, and ‘warm edge’ insulating glass spacers) with air-seals and specially developed thermally broken window frames. Triple-glazing is more effective at reducing down-drafts than double-glazing with the same U-value. Window frames need airtight details around the perimeter, typically achieved using proprietary foils.

The heating provision from solar gain is not sufficient for all the heating needs throughout the year, of course, so some supplementary heating is required. This is normally distributed through the low-volume heat recovery ventilation system, rather than by a conventional hydronic or high-volume forced-air heating system. Waste heat from appliances and lighting, as well as body heat, all contribute significantly. Part L and the SAP 2005 software have led to the majority of house designs including efficient condensing gas boilers and there is now an encouragement within Part L to adopt low or zero carbon technologies to provide secondary heating and/or hot water.

Although PassivHaus is entirely voluntary in Germany, its popularity has grown particularly because funding has been set up to make it commercially attractive to developers

As mentioned above, the air leakage value in a PassivHaus is less than 1m3/hr/m2. In England and Wales, design air permeability of 7-10m3/hr/m2 @ 50Pa is required as a minimum in most cases – 10 times poorer than the PassivHaus standard.

And, in true German fashion, the scheme is carefully monitored at every stage. An architect trained in PassivHaus principles designs the dwelling in accordance with all PassivHaus requirements. The design must be successfully appraised using the PHPP (Passive House Development Package), a software program similar in concept to SAP2005, designed to consider many aspects of the dwelling’s energy performance at the same time. An airtightness test is carried out at an optimum time in the build process. It is the architect who takes responsibility for quality control on site throughout.

PassivHaus may well be held up as a benchmark for the Code for Sustainable Homes, but it will take a cultural revolution in design and site practice in the UK to embrace the German model fully – not to mention funding incentives. If CSH Level 4 sets these high standards, zero carbon poses quite a challenge.

Keywords: PassivHaus