1930s Bristol I property reborn thanks to AECB Silver retrofit This challenging energy upgrade and remodelling turned an old industrial unit into a unique modern home. Words by Mischa Hewitt, Earthwise
Clients: James & Poppy Walker Designer: James Walker Structural engineer: Milner Associates Energy modelling: Earthwise (www.earth-wise.co.uk) Contractor: Earthwise Construction (www.earthwiseconstruction.co.uk).
20 | passivehouseplus.co.uk | Issue 21
n 2007, James and Poppy Walker bought a 1930s light industrial live/work unit in Bristol. They were immediately drawn to the big open spaces with high ceilings and set about making it their home. A few year ago, motivated by the need for more living space to accommodate their growing family, they decided to extend and refurbish the building while reducing their energy consumption by aiming for the AECB Silver energy standard. As a structural engineer specialising in timber construction, James designed the conversion and retrofit himself, with energy calculations done by Earthwise. The AECB Silver standard is a low energy standard based on the passive house standard. The aim of AECB Silver is to drive better building design with a ‘fabric first’ approach that focuses on insulation, thermal bridging, high performance glazing, airtightness and mechanical ventilation with heat recovery (MVHR). The standard is less demanding than passive house and has a self-certification process, which is attractive to self-builders and those on a tight budget as it is cheaper than passive house certification, while maintaining technical rigour. AECB Silver is recognised by the Ecology Building Society and is eligible for discount on its C-Change mortgage. In the UK, 27 projects have been certified to the standard so far. Wellington Lane is in a conservation area in central Bristol and is a narrow cul-de-sac. The building occupies the whole plot apart from a small strip of hard standing to the front, making the logistics of the build a challenge. The most complicated element of this project was the installation of the glue laminated (glulam) roof beams and insulated roof panels. This required careful planning, as the only way to install the beams was with a crane, meaning that the road had to be closed for two days. The building was carried out by Earthwise Construction and took five months. With no outdoor space and limited room to extend the house sideways, the new development added a second storey and roof terrace. The downstairs is characterised by a generous entrance hallway, and the upstairs by a wide curved roof and large open plan living area. The front terrace extends the living area and is finished with flagstone tiles and vertical larch cladding. Other materials included wooden floors, stone tiles, exposed brick, smooth plaster and industrial roofing sheets. The eaves overhanging at the front
of the building give good shading in summer to prevent overheating, but allow the low winter sun in. Exposed structural timber features prominently in the new building, reflecting James’s expertise in timber engineering and Earthwise Construction’s preference for natural materials. Bespoke curved glulam timber beams were designed as the main load bearing roof structure. Glulam beams are small pieces of timber, glued and laminated for increased strength, and can be used to replace steel as load bearing elements. The use of curved glulam beams maximised the height of the new storey without overshadowing neighbouring gardens. High performance phenolic foam insulation was used throughout, and the walls, roof and floor have all been insulated beyond current UK building regulations. It was a challenge to integrate some of the new building elements with the existing ones, particularly where the original structure penetrates the new insulation layer. Earthwise Construction worked closely with James to ensure continuous insulation could be maintained where the new structure meets the original building. Once the building had been highly insulated, the next step was to reduce draughts by sealing the building. A pro clima Intello Plus membrane was wrapped around the whole building and was taped and sealed at every junction. Parts of the existing structure that were retained were challenging to seal. There is currently no requirement to pressure test existing buildings for airtightness when they are retrofitted in the UK, but the AECB Silver standard requires it. The project beat the AECB Silver airtightness criteria achieving an air permeability of 0.81 m3/hr/m2, which is over 10 times better than the building regulations require. Heating and hot water is provided by a highly efficient Worcester Greenstar 12i ErP system boiler range with 250 litre unvented cylinder. The ventilation system is a Paul Focus 200 MVHR system supplied by Green Building Store. There is low energy LED lighting throughout, and a 4 kWp solar photovoltaic array. The project was completed in summer 2015 and the measured primary energy demand is 80 kWh/ m2/yr, which is 30% better than predicted by the PHPP model. The project was the winner of the best self-build prize at the Structural Timber Awards last year. For more information on the AECB Silver standard see www.aecb.net. •