Brian Edwards visits a prototype house in Denmark that suggests an alternative approach to the PassivHaus low-energy concept – and one that that could have greater relevance in the UK.
The Active House (or Home for Life) in Aarhus, Denmark, is the first of eight prototype low-energy houses being developed by VKR Holdings, the parent company of Velux and Velfac. Like the Barratt and Kingspan houses at the BRE in Watford, Home for Life is the result of research and design development aimed at securing a foothold in volume house construction in an anticipated low-carbon future. Designed by AART architects and engineered by Esbensen Consulting, the prototype builds on the principles of PassivHaus but adapts them for the conditions of northern Europe. In this sense the Active House may provide a better model for the UK than the much publicised PassivHaus.
Whereas PassivHaus relies on reducing heat loss and the use of passive solar and accidental heat gains from lighting and equipment to provide satisfactory winter conditions, Active House uses underfloor heating. The lack of solar gain in the grey north and resulting higher humidity levels limit PassivHaus technology. However, while both Active House and PassivHaus use whole-house mechanical ventilation with heat recovery and are heavily dependent upon super-insulation and high airtightness, Active House incorporates underfloor coil heating supplied from a heat pump in combination with solar heating and solar cells. Therefore Active House can be built anywhere and is independent of the local district heating system.
In terms of space heating, Active House is planned to operate at around 15kWh/m2/yr, compared to 8-12 for PassivHaus, 19 for Kingspan Lighthouse (using the PHPP calculation tool) and just over 15 for the new Barratt Green House (using the SAP method). However, as these figures are based on different methodologies, direct comparisons can be unreliable.
Energy efficiency depends upon both the building and user behaviour. To test the interaction between the two, Active House is being occupied by a family to determine actual consumption over a typical year. Even in the highly engineered PassivHaus, many occupants supplement the frugal lifestyle with additional heating appliances, so the question of user reaction is critical.
In energy terms, Active House is close to the UK Code for Sustainable Homes Level 6. It is designed to meet the building regulations of the future. It will test the Danish regulations of 2015 which under Class One set a target of 15kWh/m2/yr, with an anticipated reduction to 12kWh/m2/yr by 2020. However space heating is only part of the picture. Where Active House departs from general practice is the amount of daylight provided (with consequent heating disadvantages). By maximising daylight, the designers believe they will reduce demand for electricity in the form of lighting while also helping to combat the Scandinavian ailment of ‘winter depression’ which is believed to be related to daylight deprivation. In fact, Home for Life has a window area of 40 per cent of the floor area, with windows facing in all four directions.
In order to solve the dilemma implicit in the daylight/heat-loss relationship, Velux and Velfac have developed a new generation of energy-efficient windows and facade systems for this and other projects. Using integral solar collectors, Sonnenkraft solar heat pumps and solar cells, the building can generate more than 60kWh/m2/yr of heat and power. This supplements imported energy and gives legitimacy to the claim that, over the building’s lifetime, the full life cycle carbon equation is neutral (allowing for construction and use over 40 years).
Key aspects of the house concern comfort and control. Active House is designed for people rather than engineered into a soulless energy-efficient machine. Space, light and choice are fundamental, and the aesthetics of the building designed to appeal to ordinary Danes. As such, the model adopted is the standard one-and-a-half storey detached house that makes up the bulk of modern suburbia in Denmark and Sweden. VKR has ambition to make Active House the consumers’ first choice in the post-petroleum age.
The construction is relatively traditional with timber framing (laminated and softwood) above a concrete raft. The lack of thermal and acoustic capacity is offset by the flexibility provided by the timber stud walls. U-values are 0.10 for the walls and 0.07 for the floor and roof. The building is clad in slate fixed to timber battens, the floor tiles are mosaic made from recycled glass, and the windows incorporate new energy-saving glass technology, intelligent shading systems and thermal breaks in the frames. Ventilation, which is seen as key to consumer satisfaction, is computer operated (with manual over-ride) using Velux’s WindowMaster and incorporated into Velfac’s Helo active facade system. In summer the building is naturally ventilated while the heat recovery system operates with 85 per cent efficiency in winter and incorporates sensors to adjust to external relative humidity levels.
Home for Life is the first of eight Active Houses planned over the next two years with others earmarked for Germany, France, Austria and the UK, each designed by a different architect and shaped by local climatic and cultural practices. Unlike PassivHaus, which can become rather stereotypical in form and construction, Active House hopes to create diversity of approach around a set of core principles – energy efficiency, comfort, health and aesthetics. In the UK, for instance, the Active Houses planned will be a pair of semis. Like with the Barratt eco-house and Bill Dunster’s terraces at RuralZed, this Danish iniative is intent on making sustainable living mainstream.
Brian Edwards, author of a number of books on sustainable design, teaches in the architecture school at the Royal Danish Academy of Fine Arts in Copenhagen.
• Sonnenkraft solar collectors of 6.7 square metres cover 50-60 per cent of the annual cost of heating water while boosting room heating via the solar heating pump. Energy production for heated utility water: 2,100 kWh/annum.
Energy production for home heat (solar heating pump): 4,200 kWh/annum.
• A high-efficiency solar heat pump utilises the energy from the solar collectors even when the weather is cold and overcast and supplies hot water to the floor heating system. Energy production: 4,200 kWh/annum.
• The floor heating system covers the remaining home heating requirement not covered by the passive solar heat gain from the energy-optimised windows.
• A hot water tank stores water heated by the solar collectors and solar heating pumps and supplies it for washing, bathing and underfloor heating.
• A solar cell system (polycrystalline) generates electricity for installations, household and lighting – a total of 5,500 kWh/annum. Of this, the family uses only 2,700 kWh/annum.
• Indoor climate control – a centralised system controls the house in such a way that electricity and heating consumption are kept to a minimum. The system controls natural and mechanical ventilation and interior and exterior sunscreening, and ensures that lights are switched off when a room is vacated.
• Natural ventilation is by io-homecontrol and WindowMaster solutions.
• Roof windows _- energy-optimised Velux windows with three-layer low-energy glazing with a U-value of 1.0W/m2K. The window area in the house covers 40 per cent of the wall area (as against the normal 20-25 per cent). Linings reduce thermal transmittance and allow daylight to penetrate deep into the interior. The windows also aid natural ventilation.
• Vertical windows – energy-optimised Velfac Helo windows with slender profiles and three-layer low-energy glazing with a U value of 0.9W/m2K.
• Solar radiation, summer – the large eaves shield against hot summer sun and reduce the need for air conditioning. Solar radiation, winter – approximately 50 per cent of the room heating requirement is covered by the passive solar heat gain from the energy-optimised windows.
• The natural ventilation system provides fresh air throughout the summer and is controlled by sensors. This gives significant energy savings over a mechanical ventilation system.
• Mechanical ventilation with heat recovery – in winter, fresh air is supplied by an on-demand system which recycles heat from the exhaust air. Low-speed venting into the interior helps avoid draughts.
• Wall insulation is optimised, and cold bridges kept to a minimum. U-value of exterior walls: 0.1W/m2K (395mm insulation); U-value of roof: 0.07W/m2K (540 mm insulation); U-value of exterior walls: 0.07 W/m2K (500mm insulation).
• Interior sunscreening provides night insulation and enhances the window’s heat-transmitting properties, while providing privacy.
• Exterior sunscreening forms part of the active facade which automatically controls the amount of light and heat transmitted through the windows. The sunscreening reduces passive solar heat gain which can over-heat the living space.
Further details www.vkractivehouse.com
ET21/November 2009 p10