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A happy mediation: Diggs Field renovation by 5th Studio

5th Studio has reduced the space heating demand of a 1960s Peter Aldington residence by 90 per cent without undermining its architectural qualities, writes Felix Mara. Photography by Mike Jacob

The UK’s building stock benefits from healthy confrontation between two poles: on the one hand the defenders and advocates of high environmental performance standards, and on the other the conservationists. At the furthest extremities with gloves removed, technocrats brandish chunky uPVC TGU window frames in one corner while in the other the heritage faithful, screaming ‘Philistines’, clamour for north-facing single glazing.

Grade II-listed Modernist buildings are no longer neutral territory if, like Peter Aldington’s Diggs Field in Haddenham, they attract the attention of English Heritage and the 20th Century Society and cannot be sustained by their own fabric. As stated on the website of architect 5th Studio, which renovated this near-derelict residence, Diggs Field represented ‘an earlier generation of environmental design’.

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Diggs Field before renovation

For all its quality and quirky detail, Diggs Field, designed between 1967 and 1969, before Aldington teamed up with John Craig in 1970 and later Paul Collinge, predated the 1973 oil crisis. At its core, and integral to the brief to house a man, his wife and a single woman, was a double-height, glass-walled, south-facing sun room, where their zones overlapped, which relied on active solar gain control. The 110,000BThU/hr oil-fired warm air system itemised on Aldington’s hand-drawn imperial scale plan haemorrhaged fuel expenditure as energy costs spiralled and as Diggs Field’s underperforming envelope deteriorated.

The current owners, who bought Diggs Fields in 2010, would have been alive to its quality and pedigree: an early work by a future partner of one of the defining British practices of its era informed by the Modernism of Wright, Mies and Kahn, but also grounded in an awareness of site, brief, landscape and traditional construction, combining technical inventiveness with design finesse and versatility. But the client also wanted a working residence, adapted to the needs of contemporary family life, structurally sound, asbestos-free, and, above all, with high environmental performance, close to Passivhaus fabric-first standards.

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Working with specialist consultant Trunk Low Energy Building, 5th Studio aimed at high performance standards for all building elements to minimise fabric damage and discomfort, but was thwarted by heritage bodies, which objected to the visual impact of proposed increases to the external wall thickness that would have achieved the target U-values for these elements. Although the U-value of these walls was improved by 88 per cent, floor, roof and window heat transfer coefficients had to be lowered to achieve the design team’s overall targets as measured by the Passive House Planning Package (PHPP), which allows different building elements’ thermal performances to be balanced against one another.

Heat loss through floors was reduced by 70 per cent, with U-values lowered from 0.50 to 0.09W/m2K and no impact on building appearance. The U-value of the roof was lowered from 0.64 to 0.20W/m2K by introducing insulation above and between joists, where it is most beneficial. This has a big impact on overall performance because the roof surface area is so large. The design team also persuaded English Heritage that rotten window frames along with old blown DGUs and single glazing should be replaced, generally with high-performance TGUs, reducing U-values for this element from 2.98 to 0.79W/m2K.

‘We carried out TAS (Thermal Analysis Software) dynamic thermal modelling to assess a range of options for reducing overheating,’ says Trunk director Mike Jacob. ‘The one we settled on involves externally mounted solar control blinds, which can be seen through from inside and have the option of thermostatic controls, although the client decided not to invest in this.’

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Using the CIBSE Guide A limit of 21 hours a year exceeding 28 degrees C, thermal modelling confirmed a reduction in overheating from 714 to 18 hours: a 97 per cent improvement.

The tested airtightness level of 0.94 m3/m2/hour at 50Pa is below the Energy Saving Trust’s ‘Best Practice’ threshold and Approved Document L1A’s required 5m3/m2/hour for new dwellings. These levels were achieved by comprehensive training, inspection and testing from the early stages of the project, using equipment hired from the Building Services Research and Information Association.

After losing a planning appeal for consent to remove and reinstate existing brick walls, the design team took alternative steps to address concerns that this part of the fabric had mild steel wall ties and possibly cavities full of debris. This involved opening cavities at top and bottom, removing a brickwork course at the level of the floor insulation zone and replacing it with load-bearing Foamglas Perinsul insulating blocks after debris was removed. The cavities were also pumped full of EPS beads, freeing them from potential cold bridges.

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The endothermic roof collectors, a departure from the passive strategy, involve a long story recounted by Jacob, who notes that the roof had been leaking for 40 years. An area of roof had been covered in slate, probably to rectify this problem. This area continued to leak and the slates’ pitch was too shallow. English Heritage sanctioned a watertight replacement, but asked for a material which resembled slate. The client wanted to replace it with an integrated solar zinc roof system which would take advantage of the roof’s orientation, which English Heritage was happy with. When the manufacturer said the roof area was too small for its system to be effective, an extruded aluminium alternative was substituted, which has identical appearance but connects to a water source heat pump with two large stores in the ground.

‘The building’s construction and detailing is incredibly simple, with white-painted brick walls running into cedar cladding, inside and out,’ says Jacob. ‘There are no plastered surfaces or boarding to conceal a continuous airtightness layer on the brick sections and we had to rely on the sand lime bricks themselves being airtight, with several layers of masonry paint.’

The weatherproofing strategy therefore involved achieving continuity of the airtight envelope without relying on typical details, although here there was judicious use of high-performance airtightness products and materials such as Pro-Clima tapes, sealants and membranes. Compriband was used in locations where tapes and sheet materials could not be returned, such as butt joints between brick and timber, which were both structural and finished elements. On the wall-to-roof junction of brick cavity sections, continuous strips of Intello membrane were fixed on the void’s inside face with Orcon F sealant with Tescon tape, wedged into place with tight-fitting Celotex insulation. This was then carefully lapped to 18mm T&G SmartPly on the roof, itself taped and sealed at every joint.
The story ends on a positive note of collaboration. ‘It was airtightness by stealth, based upon the fact that we had to deliver an end-result that did not differ from the original on heritage grounds,’ says Jacob. ‘However, English Heritage was well aware and supportive of our objectives.’

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