Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 21 walls were then lined using a vapour- permeable Pavatex fibreboard insulation system, 60 mm thick, and plastered to give a U-value of 0.25W/m 2 /°C. WUFI and other static thermal modelling indicated that leaving the cross walls and even cornices exposed without insulation would have surprisingly little impact on heat loss, due to the relatively low thermal conductivity of the bricks. It was concluded that cold-bridging would be insufficient to allow condensation to reach a level where mould growth could occur. Where rooms still retained original cornices, it was decided to terminate the insulation (and the replastering) just below the cornice. This would result in a rather curious detail, with the cornice running around three sides of a room as normal, before diving into a recess on the fourth side. However, the scheme developed by the architects, 5th Studio, demonstrated that the impact of this detail on the character of the interior would be significantly reduced when combined with modern furnishings and fittings, and it was seen as an ‘honest’ approach to the retrofit. WINDOWS AND SHUTTERS The original window shutters and their housing had to be removed for several reasons; first, repairs were best carried out in the workshop; second, it enabled their position to be modified so their relationship with the face of the wall, which was now insulated, could be maintained; and third, it allowed the insulation to be run behind them into the reveal. After conservation and repair, the joinery was reinstated in its new position. This created a slight gap between the shutters and the face of the windows, providing the opportunity to improve security with window locks. Although the original windows had been replaced in the late 19th century, upgrading the glazing remains a controversial decision. In listed building terms, all alterations are considered to be part of a building’s history and the conservation authorities are rarely in favour of the replacement of single glazing with sealed units. However, many factors may be taken into consideration when assessing the significance of a later alteration, and in this case it was concluded that the replacement was acceptable in principle. The original glass was saved for use in the repair of windows on the estate, and modern sealed units with a thickness of just 10mm were chosen from the Holloseal range. For the outer pane machine-drawn cylinder glass was used to produce an uneven reflection similar to that of early glass. The timber casements were also draft stripped, and contacts were added to the casements to detect when they were opened, automatically turning off the heating. ROOF INSULATION Rooms on the top floor were partially within the roof, with no insulation in the sections of the ceilings formed against the rafters, nor in the dormer windows. All the original lath and plaster had been replaced in the past with gypsum and metal lath, so from a listed building perspective these ceilings had little significance, allowing them to be remade with rigid insulation bats between the rafters and below, retaining a generous ventilation channel between the top and the underside of the roof covering. The roof slates had been relayed in the past over an impervious roofing felt. A vapour permeable insulation system at ceiling level would allow moisture to enter a cooler space, increasing the risk of condensation on the underside of the felt. The usual solution to this problem is to introduce additional ventilators, but Computer modelling indicated that the cooling effect of a 60mm-lining of vapour permeable insulation would not cause an unacceptable increase in moisture in the masonry, but long-term monitoring was essential. The diagram shows an external weather station and probes installed at different depths in the masonry to monitor changes in temperature and relative humidity. (Image: 5th Studio) Thermal image showing the cooling effect of condensation and poor insulation in one of the attic bedrooms: the ceilings (modern gypsum plaster on metal lath) were stripped out and replaced with insulation below and between the rafters to achieve a U-value of 0.15w/m 2 /°C and air-tightness of 3.0m 3 /h/m 2 @50Pa. (Image: ArchiMetrics)