BCD 2018

PROTECTION & REMEDIAL TREATMENT 4.1 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 8 145 insect attack might be a possibility, but there are other controlling factors to consider. Neither furniture beetle (woodworm) nor deathwatch beetle can attack the heartwood of oak or pine (normal construction materials) unless its chemistry has been modified by fungus. The beetles would only be able to infest any residual sapwood and, in a historic building, this will probably have been damaged decades or centuries before (the old carpenters used to call this ‘wormed-out’). This is the reason why the precautionary treatment of old timbers is generally pointless, except to meet the requirement for a guarantee. Decay, caused by fungus, will only occur when liquid water starts to form and the relative humidity is too high to measure accurately. If the building and its roof are basically dry then this will only occur adjacent to a fault that is allowing direct water penetration. VENTILATION AND THE STABLE ENVIRONMENT Ventilation will achieve nothing useful in a dry roof because the air that enters will have the same moisture content as the air that is already there. Increasing the air flow will not protect the wood. The vents may only change the roof’s appearance and provide more hibernation opportunities for a range of insects. This is important because the popular concept of ‘breathing buildings’ is easily misapplied. Most historic roofs, even those covered with lead, have survived well without anybody making holes in them. The idea that ventilation somehow prevents decay comes from a 19th-century lack of understanding about its causes. Many people will still look at fungus growing under a floor or in a roof and say the problem was lack of ventilation. This is almost certainly wrong unless the problem is caused entirely by condensation. The cause of the problem would be a large and prolonged source of water, and ventilation would have no effect unless the air introduced was dry enough and moving fast enough to absorb and remove this water. This also assumes that the moisture supply is finite, which it generally isn’t unless the causal fault has been rectified. Therefore, the only valid reason to add extra ventilation is to prevent condensation. But condensation does not occur in the roofs of most historic buildings, provided they are normally dry. Furthermore, increasing the rate of air change within a roof can also increase the migration of air and moisture from the rooms below. Increasing the air flow through an unventilated void when there is moisture rising from below might have unexpected consequences. VENTILATION AND THE UNSTABLE ENVIRONMENT Unfortunately, interventions in pursuit of energy efficiency mean that roofs may not remain dry and the only solution to a consequent condensation problem may be air exchange via extra ventilation. The external air might sometimes have a high relative humidity but not for the induced period caused by the intervention so that air exchange should have a drying effect. The combination which commonly triggers this instability seems to be roofing felt, thick insulation and no air exchange. The latter may be because there never was any or, perhaps, eaves vents have become blocked by added insulation. The amount of moisture permeating up into the roof space does not need to be excessive. In an effort to gain a better understanding of induced condensation and other problems in historic roofs, Historic England has been monitoring the hygrothermal behaviour of a number of pitched roofs with insulation at ceiling level. These roofs have either impermeable, permeable or no underlay. Meteorological data has also been collected from weather stations set up near the roofs. (This project is part of a wider programme of research about the energy performance of historic buildings and the effects of measures to increase energy efficiency.) The monitoring programme has yielded a large quantity of data which is currently the subject of statistical analysis. The findings will demonstrate how particular roof environments react to changing internal and external environmental loads, and may challenge some of the assumptions commonly made about the performance of roofs, in particular the role of ventilation. IAIN McCAIG DipArch IHBC is senior architectural conservator at Historic England. He studied architecture before specialising in building conservation and has many years of experience working within both statutory conservation bodies and private practice. BRIAN RIDOUT MA PhD is managing director of Ridout Associates, a small team of independent consultants specialising in the scientific assessment of timber decay, environmental monitoring and other damp- related problems in historic buildings. Figure 3. Wood sorption curve Monitoring the interior environment of the roof of a typical semi-detached house showed that its humidity levels naturally followed conditions outside: contrary to popular belief, increasing ventilation would not affect its ability to dry. (Photo: Brian Ridout) 30 25 20 15 10 5 0 Minimum moisture content for fungal decay Minimum moisture content for successful colonisation by furniture beetle Minimum moisture content for survival of furniture beetle 0 20 40 60 80 100% RELATIVE HUMIDIT Y MOISTURE CONTENT %