Heritage Retrofit

20 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION cooler, potentially inviting interstitial condensation. When combined with a decrease in air movement due to draft exclusion, there is a real risk that dry exterior walls above ground floor level could become damp. Proposals for the retrofit were therefore preceded by a three-year programme of investigation, monitoring and modelling to develop a clear picture of the hygrothermal performance of the spaces and fabric that could be most affected, and to provide a benchmark for assessing subsequent performance, from one season to another. A modified form of WUFI software was used by building physics engineers Max Fordham to explore how the materials would be affected. WUFI (an acronym of wärme und feuchte instationär – heat and moisture transiency) tends to underestimate the thermal performance of traditional materials. Old bricks, for example, tend to be less well fired than modern ones and the clays are less uniform so they do not conduct heat as well. Nevertheless, WUFI provides a useful model for assessing the relative performance of insulation measures and the effects of cold bridging, particularly when combined with real data from monitoring the performance of the existing structures, and by material analysis. Samples of brick, stone and render were therefore sent for testing by Glasgow Caledonian University, and probes were installed by Archimetrics in 2011 to record real time variations in moisture and temperature at four depths through walls of different orientation and material. A weather station was also installed so the WUFI model could be calibrated according to the local environmental conditions and the U-values of the walls recorded by Archimetrics. In addition to the technical impact on the performance of historic fabric, the insulation of walls and windows has a substantial design impact, and all aspects of the retrofit would affect the historic and architectural significance of the building. Before any proposals were put forward, the building was thoroughly surveyed by the architects and Beacon Planning to identify how the building had evolved, what alterations had been made in the past, and what fabric was original. At New Court, exterior insulation was clearly out of the question. Interiors, on the other hand, were generally quite plain and had been affected by alterations over the course of 185 years of student occupation, particularly in the 1970s when extensive repairs were required for dry rot. From a design perspective, phenolic foam insulation offers the least intrusive solution as it gives the highest insulation levels for the least thickness, but the material is impermeable. The WUFI modelling indicated that this could cause problems on those elevations most exposed to driving rain, and a permeable solution which allowed evaporation from both interior and exterior surfaces would be necessary, particularly on north- and west-facing walls. The exception was in rooms where high levels of humidity would be expected, such as bathrooms. The solution was to locate en suite showers and bathrooms away from external walls and ventilate them thoroughly. Only two bathrooms could not easily be moved. In these cases the design of the ventilation was particularly important to ensure that the interior vapour pressure remains within acceptable levels, and moisture levels in these walls will be monitored carefully for years to come. INTERIOR WALL INSULATION Most rooms had been subjected to extensive repairs in the past, particularly the exterior walls, due to defective gutters and outbreaks of dry rot. Few retained original plasterwork. The exterior walls were stripped of their plaster finishes and refinished with a lime plaster base coat to ensure that all gaps were sealed, particularly where penetrated by structural timbers and joinery. As well as being essential for air-tightness, this would also help to draw moisture away from joist ends and other vulnerable timbers. The Originally the façades facing the courtyard were all rendered with Roman cement, later repairs were executed in cement, and they have now been re-rendered using a more permeable hydraulic lime painted with limewash (above). One of the windows facing the courtyard (left) at the start of the project, and (right), the same image modified to show the architect’s proposals for re-colouring the walls and window frames, following surviving evidence of the original colour scheme (All photos: Tim Soar)