BCD 2018

C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 143 PROTECTION & REMEDIAL TREATMENT 4.1 CONDENSATION in HISTORIC ROOFS IAIN McCAIG and BRIAN RIDOUT A HOUSEHOLDER IN the West Midlands was anxious to upgrade the thermal performance of the pitched roof on his terraced house. The glass fibre insulation between the ceiling joists had sunk to about 50mm thickness so a further 200mm-thick layer was added. When the weather became colder and the house was heated he noticed condensation streaming down the underside of the bituminous felt underlay. Also, items that had been stored in the loft safely for years became mouldy. Somehow, he had destabilised the roof environment. Another correspondent who added an extra 200mm of insulation at ceiling level reported a similar result. In this case a dehumidifier had to be used to dry out the insulation. Although this appears to be an increasingly common problem, the reasons for it remain unclear. The purpose of the added insulation in a roof is to further reduce heat loss through the ceiling from the rooms below. If this is successful then the roof environment must become colder, but why should this increase condensation? If a dry house with a dry roof is left empty and unheated for a few weeks in winter the roof does not begin to run with moisture. There are several possibilities, and the starting point for discussion must be a consideration of how a normal roof environment works. THE DRY ROOF If the air moisture (measured as g/m 3 or as vapour pressure) outside the building and within the roof space are monitored, the outlines of the graph curves obtained will be very similar (compare the green and blue traces in Figure 1). The environment in the roof space responds to external moisture fluctuations, although muted to some extent by the properties of the construction materials. The data shown in Figure 1 (overleaf), recorded at a semi-detached house in the south of England, also shows that the next most important factor influencing the roof environment is solar gain (compare the red and blue traces). Ventilation, despite popular belief, does not have a significant effect because the air entering the roof has a similar moisture content to the air already there. Condensation may form on impermeable surfaces such as roofing felt (even those described as vapour permeable) when the temperature of those surfaces dips below dew point (the point of air saturation where condensation occurs). But this is generally a transient event, and moisture deposited during a cold night is re-absorbed by the air when the temperature rises again in the daytime. If the air moisture content within a roof space is basically tracking external fluctuations and condensation occurs only briefly, then moisture to destabilise the roof environment must come from the rooms below. MOISTURE FROM OCCUPANCY It is generally accepted that moisture from occupancy rises into the roof. The Building Research Establishment has calculated that about 20 per cent of the moisture entering a loft comes from within the building. Most of this is thought to come from air leakage through cracks, gaps around service pipes, or loft hatches. Some may be the result of vapour diffusion through the ceiling, but moisture will tend to be retained within the pore structure of the plaster. This has been demonstrated in monitoring carried out by Historic England which has not yet been able to demonstrate that normal usage of a bathroom will raise the moisture content of the air immediately above the ceiling. Georgian Bath: increasing levels of insulation could lead to a rash of roof vents across the historic roofscape, but would they reduce condensations? (Photo: Jonathan Taylor)