Site Lines: Designing envelopes for Passivhaus townhouses

Architect Gavin Finnan describes the design of Passivhaus townhouses in a south London regeneration project, focusing on how their external envelope was fundamental to performance.

South Gardens is the first phase of Elephant Park, a major regeneration project that sits on the site of the former Heygate Estate in Southwark, which when complete, will deliver
360 new homes. A total of 15 townhouses within South Gardens have been designed as ‘future homes’, i.e. aspiring to a raft of sustainable innovations which include Passivhaus standards, use of CLT (cross laminated timber), green roofs, additional renewables and local community initiatives.

The townhouses are composed of three- and four-bed house types, forming part of a terrace. The primary aim is to make homes in the best tradition of London architecture that allow inhabitation in a clearly defined manner. It is also important that the materials age gracefully and are low maintenance, in order to ensure that the area will continue to look better over time.

The design of the terrace has been informed by the architecture of the traditional Victorian era housing which is found within the nearby conservation area. The new townhouses have projecting bay windows and their materials will sit comfortably with the existing buildings.

The simple brick terraces have elegant, vertically proportioned windows with vertical opening casements recessed behind a brickwork facade. All external rainwater goods are attached to
the rear of the terraces to provide a clean, crisp, uncluttered facade to the street. Recessed timber lined entrances and projecting bay windows break up the elevation at lower level, and give a scale and rhythm to the street. Front doors have fixed glazed panels to the sides, and in some case have a glazed light above.

The structure of the townhouses is formed from cross-laminated timber (CLT), a highly sustainable material as trees absorb carbon dioxide while growing. In addition, the CLT panels can be re-used at the end of a building’s life. Walls, floors and roofs have been constructed from pre-fabricated panels. Off-site production has meant less on-site waste, a shorter build programme, and whole-life cost savings. A lighter frame and reduced weight has ensured a lighter building and smaller foundations. The ultra-low energy buildings require very little energy for space heating or cooling, thus reducing their ecological footprint.

Passivhaus standards are rigorous and result in ultra-low energy buildings that require little energy input in use. A range of passive design measures have been incorporated in the project to ensure energy demand and associated CO2 emissions are minimised, including clean and zero or low carbon energy technologies are proposed, including connection to the CHP-powered Heygate Masterplan District Heating Network.

Design challenge & solution

The larger the thermal envelope, the more challenging it becomes to keep the heat in the building. Projections and recesses like bay windows increase the thermal envelope of a building which in turn increases the potential for it to lose heat. This is why they are generally discouraged in Passivhaus design. As a result, on this project higher target U-values were set when compared to a standard Passivhaus to compensate for the increased envelope, and particular care had to be taken to avoid cold bridges.

The houses demonstrate that good passive design does not have to be an attachment or supplement to architectural design, but can be a process that is integrated with it and which is free to respond to its context.

Each bay window measures 3200 mm tall x 3000 mm wide x 950 mm deep, and is divided into three glazed sections comprising two side opening windows and one fixed window to the front. The unit comprises of a European manufactured composite wood and aluminium thermally broken frame with high performance triple glazed units.

To adhere to Passivhaus requirements for reduced thermal bridging, the ‘future home’ bay window is formed from two independent steel sub frames. An inner frame is mechanically fixed onto the thermally broken concrete upstand and secured back to the CLT structure; this supports the composite window frames and supports an insulated timber roof cassette. A 240 mm layer of rigid Alumasc insulation is laid on top of the roof cassette and is then waterproofed, creating a warm roof that forms a sealed, airtight and watertight enclosure. Independent of this, a second, outer sub frame is mechanically fixed to the concrete footing which supports the reconstituted stone and forms the cold part of the structure.

It’s also worth mentioning here that the external brickwork to the elevations of the townhouses are restrained to the CLT structure using wall ties. Typically, when building a brickwork cavity wall the inner leaf is tied to the outer leaf. However this method creates too great a cold bridge and wouldn’t comply with Passivhaus. The solution involved using Basalt fibre wall ties, which have a lower conductivity and can be drilled into the CLT inner wall, therefore tying together the inner and outer leaf to provide stability.

The reconstituted stone pillars that sit between the three sections of the window come to site in several pieces and are supported off the reconstituted stone plinth which rests on the concrete footing. The stone mullion is tied back to the outer steel frame along with the lower and upper panels and attached with stainless steel bracketry and pins. Once the stone is embedded into place the joints are mortared.

The reconstituted stone lid or roof to the bay is the largest, arriving on site in one single piece and providing the ground surface to the balcony above. It acts as the primary water stopping layer draining any water falling on top of the bay. The reconstituted stone mullions, positioned on either side of the window, are profiled with narrow vertical flutes to direct the water down to the ground to reduce staining.

In contrast, a conventional bay would have more connections made through it and would probably incorporate one frame. An inner skin would probably be made from the CLT and smaller individual steel brackets would be fixed through the insulation and the reconstituted stone would be hung off the CLT.

Gavin Finnan is an associate director at MaccreanorLavington