Mon 1 Oct 2007
Passive gets active
PassivHaus may be firmly established in parts of Europe, but it is only just catching on here. However, says Roger Hunt, with a little forward planning and careful selection of construction materials it is achievable.Creating a house that provides a comfortable environment with little or no auxiliary heating, while having exceptionally low running costs and minimal environmental impact, may seem like the holy grail of building design. In fact, this 'passive house' is a reality and has been around in Germany and other parts of Europe for some years with the theory proved in numerous projects.
Broadly, the term 'passive house' refers to a construction standard that aims to reduce the heating needs in housing to a point where conventional heating systems are no longer necessary but a comfortable indoor climate is achieved in winter as well as in summer.
Criteria for the technology and standards necessary are, in the main, defined by the PassivHaus Institut (PHI) in Darmstadt, Germany, under its director Dr Wolfgang Feist. He developed the PassivHaus principles in the 1980s, constructed the first house in 1991 and founded the institute in 1996 to take forward the development and promotion of housing based on its principles; in Europe more than 6,000 homes have been built.The detail of the PassivHaus standards is readily available and a design tool, The PassivHaus Planning Package, has been produced for use by architects and designers; but, in broad terms, there are certain key considerations that have to be taken into account in order to meet the standard, which requires total energy demand for space heating and cooling to be less than 15 kilowatt hours per square metre per year.
Unsurprisingly insulation is a major factor and all components of the exterior shell of a PassivHaus are insulated to achieve a U-value that does not exceed 0.15 W/m_K. Orientation of the building and shade are also major considerations in order to achieve the passive use of solar energy and, as might be expected, energy-efficient windows are essential. Air tightness of the building envelope is another consideration as is the passive preheating of fresh air. Air-to-air heat exchangers are necessary for the recovery of heat from exhaust air. Hot water is provided through the use of renewable energy using solar collectors or heat pumps while energy-saving appliances and lighting are a prerequisite. Quality control is vital to avoid problems that may prevent the highest levels of airtightness and thermal insulation being achieved.
Among the UK housebuilders to be actively considering the passive house concept is Kingerlee Homes. Its chairman, Jonathan Kingerlee, sees passive standard housing as representing the single most important milestone as advances are made in the design, materials and workmanship in new homes.
"The principles are simple: high levels of insulation and airtightness, low levels of thermal bridging, and comprehensive heat recovery ventilation. Properly applied, these principles ensure that very little heat is able to escape the building envelope, and that any heat collected by way of solar gain or generated by the occupants and their appliances is recovered and recycled into fresh incoming air.
"Housebuilders will need to invest to create house designs that not only apply these principles on the drawing board and in energy models but are readily deliverable in the more challenging environment of the building site; simplicity will be vitally important."
At The Wintles, the Living Villages development at Bishops Castle in Shropshire, the houses utilise many of the elements that contribute to a passive house. The properties are oriented to face the sun. They use high-performance windows to draw in as much light and warmth as possible. In cold weather high levels of insulation and draft-free building techniques minimise energy loss through convection and conduction.
As the buildings are airtight and super-insulated they are ventilated to ensure that they do not become stuffy but, so that heat is not lost, a heat recovery system extracts the heat from the vented air and uses it to warm incoming air. In addition, the warmth built up through passive solar gain is saved in a sun space, which acts as an internal, thermal mass heat store. In warm weather, vents at the top of these sun spaces can be opened. The air rises as it warms up leaving the property though the open vent and the convection current established draws fresh air through the house which cools the property.
Bob Tomlinson, founding director of Living Villages, comments: "The passivity of a house depends on its usage. If the inhabitants leave windows open and don't switch on the heat recovery system many of the benefits are lost. When people use the houses at The Wintles properly, however, they could get very close to being passive houses."
David Wilson Homes is also embracing passive house principles at Upton Site C, its flagship sustainable development near Northampton. Each property has a double-height solar space and pitched roofs at variable heights to maximise access to sunlight. The company also optimised building orientation by analysing sun paths to ensure maximum window areas to the south and by avoiding overshadowing from adjacent buildings. Landscaping and its implications on the individual building's exposure to wind and solar radiation were also taken into account to ensure properties are sensitive to the local climate and wind direction. In addition, the maximisation of sunlight ensures optimum solar power gain via solar collectors for the heating, hot water and photovoltaic systems. 
Light House, HTA Architects' entry for the Home for the Future competition, is a passive house that is envisaged as a barn-like building where as many wood-based products are used as possible, along with high levels of insulation and phase change materials to store energy. A phase change material is matter that undergoes a phase change at a specific temperature and gives off or takes in large amounts of energy in the process. For example, when water changes from liquid to steam it takes some energy to do this but the steam is still at the same temperature. Phase change materials are usually engineered in order to achieve a certain amount of temperature consistency within a system. In construction they are typically added to another material to give that material the ability to store heat above a certain temperature. These materials then warm up to a temperature, of say 25 degrees Celcius, and then stop warming up; when the temperature drops below 25 degrees Celcius the material cools down and, over a period, releases the heat stored.
Phase change materials have the advantage that they can be added to plasterboard type materials and are lightweight. A one-centimetre thick piece of board containing phase-change material is the equivalent in heat storage terms to nine centimetres of concrete. This allows a house built with lightweight construction like timber-frame or light-gauge steel to behave in a similar way to heavyweight traditional construction, but it can be prefabricated offsite and will require much smaller foundations.
The external envelope of the house is built first, then sealed and pressure-tested for insulation. The timber-frame is clad with boards of compressed straw. The windows are triple glazed, except for one large south-facing window in the living room that will be double-glazed to allow only the sun's warmth to permeate.
Photovoltaic and solar water heating panels will be sited on the roof. Inside, a single biomass stove within the living space will be used on extreme days in the middle of winter to contribute to the space and water heating needs.
Olav Skretteberg, managing director of Broxwood, the Scottish-based timber windows and doors supplier, emphasises that passive house technology depends largely on good building practice. "The Passive approach requires that windows and doors are fitted well, minimising heat loss from drafts, thermal bridges and air leakage; common sense but not common practice.
Skretteberg makes it clear that one of the most crucial factors in a successful passive house is the design and type of window used, as glazed external surfaces have a major impact on the energy efficiency of the building envelope.
"It is a fact that heat loss and gain in a well-insulated home occurs mostly through the windows. In summer, each square metre of glass in direct sun can allow as much heat in as would be produced by a single bar radiator. In the winter months, losses from a window can be ten or more times the losses through the same area of insulated wall. With a good passive design - large, south-facing windows are typical - windows can trap warmth in winter and repel summer heat and admit cooling breezes for warmer days and exclude cold winter winds on others."
The most important point to remember about passive housing is that, while it is a construction standard, it involves a holistic design approach rather than a single method or style of building. Consequently, passive homes can be created using a variety of technologies, designs and materials, and factors such as location, climate, materials, structure and ventilation all affect the resulting passive house design.
First published in Show House Magazine October 2007.
The greatest care has been taken to ensure accuracy but some information contained within this article may have changed since it was first published.
Posted by Roger Hunt
in BRE Trust, Broxwood, HTA, PassivHaus Institut, Sustainability on Mon 1 Oct 2007
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