Watermead Business Park, Leicester

In 2011, the UK’s first PassivHaus accredited office building was completed at Watermead Business Park, Leicester. The 560m² building was constructed and occupied by Interserve Construction Ltd as its regional headquarters. The project was funded by the developer Raynsway properties, who are also leasing the building to Interserve.

The central objective of a PassivHaus design is to achieve a total heating energy demand of less than 15 kWh/m²/y and a primary energy demand below 120 kWh/m²/y. A combination of design features have contributed towards this goal, including very high levels of insulation, optimised building orientation, exemplary airtightness, high efficiency mechanical ventilation, effective solar shading and the use of thermal mass to enhance the building’s overall passive performance.

Insulation is provided to a standard roughly twice that of current Building Regulations, with an envelope U-value of 0.12 W/m²K and triple glazed windows with a U-value of 0.8 W/m²K. An exceptional air tightness level of 0.9 m³/(h.m²) was achieved. This required considerable attention to architectural detailing and led CPMG to develop a series of practical and economical details that were approved by PassivHaus assessors. A close working relationship between the designers, supply chain and on-site operatives ensured that the performance standards were met.

Mechanical ventilation is used in combination with an air source heat pump, which preheats the air in winter. It is also warmed by heat recovered from the extract air via a high efficiency plate exchanger. The fresh air supply comes from an array of plastic pipes buried 1.5m below the car park. This uses the stable ground temperature to warm the air by around 6 degrees during the winter and cool it by a similar amount during the summer.

The external walls have been built using Durisol blocks, which are made from recycled wood that is chipped and mixed with a small amount of concrete that acts as the binder. The blocks are cellular i.e. hollow, which enables insulation to be incorporated along with in-situ concrete.

This is poured once all the blocks have been laid, resulting in a very robust, load bearing wall that supports an exposed concrete upper floor, adding significantly to the building’s thermal mass. The internal insulation comprises 50mm of Celotex PIR, with an additional layer applied to the exterior surface of the blocks to achieve the extremely low U-value the project required.

With the help of 130m² of photovoltaic panels on the south facing roof, the new headquarters for Interserve Building is expected to save the company around £20,000.