Interserve Construction regional office, Leicestershire

Project: Interserve Construction regional office, Leicestershire

The East Midlands regional office for Interserve Construction Limited in Syston, Leicestershire is the first Zero Carbon office in the UK to be fully certified to Passivhaus standard.

In air tightness tests, it achieved an air change rate of just 0.44 per hour @ 50 Pa, well within the Passivhaus limit of 0.6.

Much of this air tightness can be attributed to the use of Passivhaus certified REHAU GENEO® triple glazed windows and external doors throughout the building, fabricated and installed by Solar Windows.

27 windows and four doors were fabricated for the £1.5m, 680m² office, the majority of which were installed across its south facing aspect to increase solar gain in winter and minimise the requirement to generate additional heat. 

The new 86mm REHAU GENEO® window is manufactured using the 100% recyclable RAU-FIPRO® fibre composite material, which is similar to the kinds of fire composites already used extensively in aircraft construction and Formula 1 design.

The inherent strength of this material enables the six chambered REHAU GENEO® system to perform without any requirement for steel in standard sized windows, eliminating the thermal bridges which can occur in conventionally reinforced PVC-U and which result in energy being lost.

Triple glazed and Krypton filled at Interserve, the windows achieve Uw values as low as 0.73W/m²K.  They can also be fabricated double glazed and Argon filled to achieve Uw values of 1.1W/m²K.


The Interserve building uses some of the most advanced green building solutions available to achieve its Passivhaus status, including REHAU’s own AWADUKT Thermo ground – air heat exchanger system which draws ventilation air into the building through a network of underground pipes so that it is pre-warmed or pre-cooled depending on the season using the natural temperature of the earth.

To achieve its targeted reduction of 90% in energy usage and costs compared with a traditional office, the building has no gas fired space heating system, but instead relies on the solar gain achieved by installing the REHAU GENEO® windows combined with AWADUKT Thermo, a MVHR system and a small heat pump.

As well as using it as a showcase for Passivhaus design, Interserve is also now evaluating the effectiveness of the various design solutions which also include highly insulated blocks with concrete infil, pre-cast concrete beams to provide some additional tempering, and timber cladding to increase the insulation performance.