Green Building Store is now 21°

Retrofit for the Future project, Balham

Robert Prewett, Director at Prewett Bizley Architects, recently completed a ‘Retrofit for the Future’ funded project in Balham, south-east London, refurbishing a terraced Edwardian home, to near Passivhaus standards. The house was a 100-year-old, three-bed, solid-walled housing association property. The project has achieved impressive energy savings, with a space heating requirement of just over 25 kWh/m2/year. To achieve this, the house had to be highly insulated and airtight; the team has managed to achieve an airtightness figure of 1.2 m3/hr/m2 – that’s over 8 times better than the Building Regulations demand for new build.

The project used a combination of internal insulation to the front of the building to avoid impinging on the attractive and well-built streetscape, with external insulation to the rear of the building, where maintaining the pre-existing external appearance of the property was less of an issue. The internal dimensions of the rooms within the rear part of the house were also much tighter than at the front, so taking any space away for insulation would have affected the usability of those spaces. The use of external render at the rear of the building was not without its challenges however and the design team had to develop innovative solutions for relocating the soil stack on the rear elevation and also how to seal the external insulation at the top of the wall (developing a super wide rainwater gutter) to avoid having to replace the roof.

MVHR ventilation
For mechanical ventilation with heat recovery (MVHR), while one can find a host of suppliers of kit nowadays, there are very few designers willing to work on one off projects. I should note a debt of gratitude to the 21° who supplied the components for our project and to their MVHR Technical Sales Manager Andrew Farr in particular who has engineered our system. It might sound rather obvious to say it but for systems like this to work effectively it is important that they are designed and installed by competent persons. Andrew is one of few such people in the UK and we consider his input an absolute essential rather than an optional extra.

Rob Prewett

High performance timber windows

At the front of the house, double-glazed replica sliding sash windows were specified, to fit in with the nearby conservation area. However 21°s Ecopassiv (since superseded by ULTRA range) high performance triple glazed windows were specified for the rear elevation of the property. In addition, two PERFORMANCE triple glazed outward opening windows (whole window U value = 0.9 W/m2K) from 21° were also specified.

There are now a number of triple glazed windows systems available in the UK. Our choice was based on a performance to cost rating and we chose 21°’s Ecopassiv range. The glazing units have a U-value of 0.6W/m2K, low conductivity glazing bars and thermally broken frames, giving an overall U-value of 0.8W/m2K-0.9W/m2K, putting them in real Passivhaus territory. Importantly, the opening casements have double seals that seal tight when the multipoint espagnolette is locked down. The windows open inwards European style and use a tilt/turn mechanism that allows them to operate as a convention hinged casement or be tilted inwards from the top for night ventilation. We also used two outward opening units from 21°s PERFORMANCE range, where internal planning issues made this more practical. The units perform nearly as well as the Ecopassiv range, although the thermal bridge where the window is installed is marginally worse. Generally we prefer inward openers for improved performance but having the option to ‘mix and match’ as it were from one supplier was an advantage.

Rob Prewett

The triple glazed windows have been installed within plywood box linings that were fitted within the structural openings, offering a number of advantages during construction. The relationship between the window and the insulation was also considered. In line with Passivhaus detailing, the design team lapped the insulation over the timber frame of the triple glazed windows to minimise thermal bridging, but also splayed the reveal to maximise daylight and sightlines.

More information

Passivhaus Refurb DiariesRob Prewett’s regular blogs during the refurbishment process

Project details


February 2011

Life changing homes