Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 19 RADICALRETROFIT at Trinity College, Cambridge JONATHAN TAYLOR T HE COLLEGE estates of Cambridge University include a high proportion of nationally important listed buildings by leading architects from every period since the 15th century, and the colleges have a continuing interest in showcasing the achievements of the architectural avant- garde. So, it is not surprising to find they have taken an interest in the greatest architectural challenge of the modern era – sustainability. Nevertheless, the scope of the interventions at New Court at Trinity College is astonishing. Despite being Grade I listed (the grade includes the top two per cent of all listed buildings in England), its retrofit is expected to achieve an 88 per cent reduction in carbon emissions. New Court was designed by William Wilkins to provide accommodation for students and has been in continuous use for this purpose since its completion in 1825. The building’s construction around a central court is relatively conventional, with pitched roofs of slate with dormer windows behind parapets, solid masonry walls, and single-glazed casement windows. The façades facing onto the court are of brick, originally rendered with Roman cement incorporating fine mouldings, much of which had been repaired or replaced with a cementitious render. Others are of bare-faced brick and, facing the river Cam, ashlar limestone. Refurbishment was required to meet fire officer requirements, to remove asbestos, to repair the fabric and to bring the existing accommodation up to a standard that would meet the needs of the college for the next 30 years. The accommodation includes 160 student rooms, some with en suite bathrooms, and a few teaching rooms and offices. Its listed status means that there was no requirement for an EPC, and that there was some flexibility under the requirements of the Building Regulations. But it was expensive to heat and its interior environment was poor. Penetrating damp meant that a high level of heating was required to keep the ground floor warm, while students on the floors above regularly had to keep their windows open to avoid over-heating. Fabric repairs and improvements were therefore essential, both for students’ comfort and to reduce heating bills, but the desire to improve the thermal performance of the building went further than this, driven by an ethical interest in reducing carbon emissions. As many buildings across the college’s estate face similar conflicts between heritage requirements and economic/ ethical requirements for conserving energy, the opportunity was taken to explore a radical approach which would challenge the flexibility of current heritage protection policy. This would provide a model for further improvements to buildings in other colleges. The sustainability measures included: • the repair and improvement of the external envelope to conserve historic fabric and to reduce uncontrolled heat loss through damp and drafts (windows, walls, doors and ceilings) • the addition of 60mm vapour permeable insulation to the inner face of external walls, accepting some limited thermal bridging through cross walls • reglazing the existing windows with 10mm thin double-glazed units • the introduction of underfloor heating beneath the original Georgian floor boards, to be warmed by ground source heat pumps at 36°C, and controlled by occupancy sensors in each room • mechanical ventilation using the existing chimney flues for air supply and venting stale air, with heat exchangers to pre-heat the intake • the installation of PV solar cells on south-facing roofs, accepting that they will be visible from other buildings. PRELIMINARY INVESTIGATION AND MONITORING A thorough understanding of the construction of the buildings and its defects was necessary in order to deal with its principal problems appropriately – damp at ground floor level and uncontrolled heat loss. Additionally, retrofitting to insulate the fabric and to control the air permeability of the external envelope has implications for moisture levels. In particular, insulating the interior faces of external walls and the underside of roof spaces leads to parts of the structure becoming Detail of the fine riverside elevation of New Court, Trinity College, completed in 1825 and now listed Grade I (Photo: Tim Soar)