C O N T E X T 1 3 4 : M A Y 2 0 1 4 17 BRIAN RIDOUT Ventilation and timber decay English Heritage is studying how timber roofs and floors respond to loads that might trigger condensation, or result in deterioration, including the role played by ventilation. English Heritage has been investigating the complex interactions between timber, moisture and ventilation for many years. It is now turning its attention to the effects of adding insulation to roof voids and suspended timber ground floors. Fears that retrofitted insulation might lead to decay because of moisture accumulation in building fabric are based on assumptions about the way the building envelope responds to heat and moisture. Current advice on mitigating the perceived risks is also largely based on supposition, in particular the emphasis given to ensuring ‘adequate’ ventilation. The benefits of ventilation in maintaining a healthy environment for building occupants is not disputed. But how important is it in preventing decay? Is it necessarily beneficial?This article looks at the history behind the assumptions, and outlines the investigations English Heritage is carrying out to test them. Belief in the benefits of ventilation is well established in building professionals.At first sight this does not seem to be unreasonable.‘Hygienists’ of the 19th century showed that ventilation dispelled condensation and invigorated a building’s occupants, while mycologists in the 19th and 20th centuries claimed that air movement discouraged fungus growth – particularly the dreadful dry rot. A ventilated building was a healthy building. In his book The House: health lectures for the people, published in 1878, Henry Simpson wrote: ‘Spaces must be left under the floors, on the ground level, if they are of wood, or they will soon decay; and they ought to be well ventilated with some of the various contrivances as ventilating bricks etc that are now so common.’ The ventilating bricks he is referring to seem to have been first patented by John Burridge, whose book with the somewhat ponderous title: ‘Improvements in Civil Architecture proving the necessity, utility and importance of ventilation to render wood equally durable as walls’ was published in 1825. Burridge, in turn, acknowledged and developed ideas first expounded by Ralph Dodd in Practical Observations on the Dry Rot inTimber, published in 1815. Our modern belief that ventilation protects timber, therefore, has a pedigree that dates back nearly 200 years. But in the days of Dodd and Burridge there was no general acceptance that ‘dry rot’ was caused by organisms – fungi – that required water to survive. The important point was that ventilation circulated air around the timber, not that it kept the timber dry. Burridge writes: ‘The durability of wainscots (whether against dry or wet walls) would always be increased by ventilation, and this might easily be done by boring small ornamental holes at regular intervals, a little above floors and near ceilings in cornice borders, which would allow a gentle current of air.’ However, by the end of the 19th century, it was firmly accepted that ‘dry rot’ was caused by fungi requiring water, and by the mid-20th century it was well known that the source of water had to be abundant and continuous. To have worked effectively, the floor vents advocated by Simpson, and the little holes advocated by Burridge, would have needed to supply air in sufficient quantities, and at suitable humidity, to dry the subfloor void or wall cavity (if it was wet enough for decay) and keep it dry.This expectationwas unrealistic, but the belief in ventilationwas so entrenched that nobody thought to question it. Later in the 20th century, researchers in Dundee found that small air movements actually stimulated the growth of ‘dry rot’ (Serpula lacrymans), challenging the acceptedwisdom that a little air movement was better than none at all. Building professionals are wary of not ventilating voids and cavities.However, surprisingly little practical research has been done to investigate the way air flows in these situations, or whether air exchange is necessarily desirable. For instance, the environment in a traditional enclosed roof space can be very stable, due to the humidity-buffering capacity of the timber.The large surface area of wood can easily absorb excess moisture vapour rising from the rooms below.The moisture content of the timber might rise by a percent or two, but this could not cause decay under normal circumstances. In the UK’s climate, introducing ventilation to the exterior will admit moist air into the structure for most of the year. At best this would achieve nothing; at worst it could cause environmental instability in a structure that survived safely for several hundred years.Vents can also allow easy access for pest creatures, including furniture beetles, that will easily colonise modern timbers used for repairs. If A sub-floor ventilator in Surrey. In the UK’s climate, introducing ventilation will admit moist air into the structure for most of the year. Vents can also allow easy access for pest creatures.
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