UK’s 1st Enerphit, six years on At Passive House Plus we write about some of the most energy efficient buildings in the world, and our writers often complain about the frustration of reporting on ultra-comfortable buildings while not having the luxury of living or working in one. Equally most passive house architects we meet don’t live or work in super low energy buildings, so they might share our frustration. One architect who does live in a passive house is Andy Simmonds of Simmonds Mills, one of the UK’s leading passive house practices. He lives at Grove cottage, a detached house in Hereford originally built in 1869, but the subject of a deep energy retrofit in 2008. The aim was to bring the house as close-as-possible to the passive house standard, and the dwelling was later certified as meeting the Passive House Institute’s Enerphit standard for retrofit. This makes Grove Cottage a valuable source of information for the passive house community. Not only is it an ultra low energy retrofit, it’s one inhabited by a passive house architect who can report on the minutiae of its performance. Grove Cottage is of solid brick construction, and though it’s completely detached, one gable
wall is a mere 25mm away from the next door neighbour’s house. Not surprisingly given its age, the house was hard to heat and draughty, with uneven temperatures inside, despite its high energy bills. In 2008, Simmonds set out to upgrade the 90 square metre cottage, and to build a 45 square metre extension to the rear. Planning restrictions on the extension, and the desire to add new south-facing glazing, meant the form of the upgraded dwelling would be quite complex — and more surface area means more heat loss. To offset this, onerous low U-values were required from each of the building elements. Most of the existing brick walls were insulated externally with 250mm of EPS insulation, while insulated timber studwork (Larsen truss method) was used externally to insulate part of an old two-storey extension at the back. A new, singlestorey kitchen extension was also built into the garden, comprising externally insulated blockwork on an insulated, reinforced concrete raft. Meanwhile the gap between Grove Cottage and the neighbour’s house was filled with polyurethane to provide airtightness, and to reduce heat loss and air movement between the buildings.
Grove Cottage was completely re-roofed, with the airtightness and insulation layer placed over the rafters to create a habitable attic space. The main axis of the house is east-west, but new high-level south-facing windows were built into the extension and the old part of the house to provide daylighting and winter solar gain. Naturally given its passive house ambitions, triple-glazing was installed throughout. The basement was thermally isolated from the rest of the house. The airtight, vapour control membrane was installed under the joists, with wool insulation between and below the joists. And to turn the basement into a more useful (yet still unheated) space, a new DPM and concrete slab replaced the earthen floor. Vapour control membranes form the airtight layer for high-level timber frame wall and roof elements, while an external cement parge coat provides this function around the masonry of the original house and the new kitchen extension. Careful detailing ensures these two systems connect up to form a continuous airtight layer. There’s also mechanical ventilation with heat recovery, while space heating and hot water is supplied by a small natural gas boiler delivering
Photos: Christopher Kenworthy
Six years after it was completed, Passive House Plus takes a look at a pioneering low energy upgrade that went on to become the UK’s first certified Enerphit project, to find out how it has performed — and what lessons have been learned. Words: Lenny Antonelli