Heritage Retrofit

22 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION this was considered too intrusive for most elevations. Increased ventilation can also increase fluctuations in humidity levels in unheated voids, by admitting warm, moist air. A modern closed-cell insulation system with vapour barrier on the warm side was therefore introduced as the most practical option. However, Historic England (then English Heritage) objected to the approach as it relies on the long- term integrity of the vapour barrier. Any defects which arise would leave timbers in the insulation layer vulnerable to condensation and decay. As many of the attic timbers are completely inaccessible, risk management relies on specialist monitoring technology. GROUND FLOOR In most areas the suspended timber floor had been replaced with concrete in the 1970s and walls had been replastered with a waterproof plaster. Externally, ground levels had risen and original lime based renders had been replaced with cement. As a result, there was evidence of rising damp from ground and surface water and from leaking drains, exacerbated by damp from wall surfaces and window sills above. External ground levels were therefore reduced, drainage improved, and vapour permeability was restored to the walls and these were insulated. However, a non- traditional solution was adopted for the floor, with a conventional modern slab laid on rigid foam insulation, and isolated from the walls by perimeter insulation. This replaced both the existing slab and any surviving, but decayed timber floors. HEATING AND VENTILATION The carbon footprint of the building is being further reduced by the use of renewables, with photovoltaic panels to be installed on the south facing roof slopes over Garret Hostel Lane, where they are screened from street views by adjacent college buildings, and by using heat extracted from boreholes in the central court with ground source heat pumps. Underfloor heating was therefore used throughout, including beneath the floor boards of the upper floors, with sound insulation below. Absence detectors are used to reduce the heating level if a room is unoccupied for more than 24 hours, and window detectors will turn down the heating if the window is open for more than 10 minutes during the colder months. Chimney flues were used to provide controlled ventilation. Two ducts were installed, one supplying fresh air to the students’ room, and the other extracting waste air from the shower rooms and loos. In the roof a heat exchanger was incorporated into the system to extract almost 80 per cent of the heat from the exhaust, which is used to preheat the incoming fresh air supply. The system can be reversed in the summer to provide cooled air. LEGACY – A MODEL SCHEME? The extent of the interventions made at New Court is extraordinary for a Grade I listed building. Key changes such as the stripping of plaster from interior wall faces, the replacement of single glazing, and the replacement of the remaining timber floors at ground level with insulated concrete slabs, are all irreversible. However, the level of alteration is the product of its own unique circumstances, which include cumulative alterations to the building over many decades, the damage caused by dry rot in the past, the simplicity of its interiors, and the necessity for change to meet the requirements of health, safety, preservation and use. Perhaps above all it was the scientific approach to the issues which enabled the college to gain listed building consent for the proposals, despite objections raised by the conservation authorities. The level of change may not, as a result, provide a model for other colleges to follow, but the level of investigation and analysis certainly does. Beneath the romantic gothic embellishments, this is a common structure of bricks and mortar. Lessons learnt from monitoring its hygrothermal performance are equally applicable to countless solid wall buildings throughout the UK, from the retrofit of Victorian terraced housing to the conversion of industrial buildings for office, residential and other uses. While the project has been based on the very best available expertise, our understanding of the long- term effects of such changes remains incomplete. That is why the decision to monitor New Court for the next seven years is so important, and it also why the legacy of this project is so valuable. THE AUTHOR: this article was prepared by editor Jonathan Taylor with the help of architect Oliver Smith , 5th Studio (oliver@5thstudio.co.uk ). Window shutters and architraves (left) were brought forward slightly to accommodate secondary glazing, security fixings and wall insulation. Bookcases and other fittings on either side were designed to accommodate services and (right) to hide an en suite shower. All showers were located away from exterior walls to avoid contributing to the moisture load. (All photos: Tim Soar)