Timber floors usually give good service - typically well over 50 years. Many floors last a hundred years or more, but where decay occurs this is often owing to excessive dampness. Poor design may be to blame in many cases. Timber in modern floors will probably be chemically treated, but good design is still advisable if not essential.

Suspended timber floors in the UK are usually ventilated with outdoor air, via air-bricks This helps ensure a low moisture content, but cannot compensate for timber being laid in contact with damp ground. Joist ends can be most vulnerable because in older houses they were either laid directly into the wall or sometimes onto slates, which acted as a damp-proof layer. Often the end grain of the timber was unprotected.

Devon and Cornwall are amongst the wettest parts of the UK and many floor joists are on the verge of going rotten. Thus, a small change in conditions may be disastrous. Local repair of joists can be effected by building new sleeper walls and resting the still sound sections upon these. More serious problems can occur where the whole floor is decayed.

Traditionally, air-bricks have been provided beneath floor level and on two or more opposite sides of the house so as to encourage cross-ventilation. It is common for these to become restricted, either deliberately or inadvertently. In Devon and Cornwall fewer air-bricks were often installed, because of the strength of prevailing winds. Indeed, generous provision especially of single sided ventilation can result in carpets lifting.

In older houses - those built prior to 1985 when the Building Regulations required over site concrete - timber floors are often still exposed directly to underlying earth which can be quite damp. However, provided underfloor humidity is kept low by ventilation, and unless joist ends are at risk owing to poor design, the floor may still be quite safe from decay. Probably hundreds of thousands of old floors are likely to give good service well into the next century.

Vapour barriers and thermal insulation.

It is not recommended practice to install a vapour barrier over suspended timber floors. This is despite the almost universal provision of such barriers in 'equivalent' situations at roof level. Even with insulated floor designs the vapour barrier is omitted, again apparently at variance with roofing and walling technology.

A possible reason for this divergence of advice is the predominant pressure differences across various building elements. Owing to wind, and with a stack effect operating for most of the year also, floors may have outdoor air drawn through them, whilst walls and roofs may have warm moist indoor air forced into them, thus increasing the risk of interstitial condensation.

With this understanding of how timber floors may operate, the influence of radon systems may be predicted.

In the early days of radon remediation in the UK, it was thought that houses with timber floors were easy to cure. Either the number of air-bricks could simply be increased, or a small fan could be fitted in place of one air-brick to increase air flow and, so it was thought, decrease the radon concentration underfloor.

Indeed, much advice was issued on the basis of little evidence, so confident were some radon scientists in their understanding of building behaviour. The truth is more complicated.

The behaviour of real floors.

The first mistake is to assume that pressure reversal can be achieved. It has been the experience in the UK, and it is a standard result from the USA also that not only may the radon level underfloor increase because of greater air flow from the exposed ground, but that air can be drawn down cavity walls into the underfloor space, in some cases to such an extent that pressure reversal is not achieved even at very high air flow rates.

A few houses have been treated commercially by using several fans, the idea being apparently that if enough air from somewhere is drawn under the floor the radon level will decrease to a low level. However, under some conditions of 'success' for radon control the floor may be subject to higher humidity levels than previously, despite any greater total flow rate.

Views of how timber floors work in practice should take account of experimental evidence. If there is significant depressurisation of the house caused perhaps by use of an open fire, and if the windows and doors have been sealed, a substantial proportion of ventilation air for the house may enter through the air bricks and then through the cracks and gaps in and around the floor. This mode of ventilation is encouraged by the stack effect within the building and can be responsible for high radon entry rates, albeit at moderate concentrations. This is even more likely to be the case where there is single sided ventilation of the underfloor space.

In all cases, even with the presence of over site concrete, radon can quite readily gain access to the underfloor space and from there into the house. It is a matter of simple experimentation with tracer gas that sealing a timber floor with a plastic sheet can have a dramatic effect on the flow rates. However, results in terms of indoor radon level are sometimes less good.

The reason for this is partially that the reduction in ventilation of the underfloor space, brought about by sealing the floor, leads to a much higher radon concentration under the floor. Also, the slight depressurisation induced by a fan may draw more radon from underground, and radon levels under sealed timber floors may be tens of thousands of Bq/m³ despite the fan assisted ventilation. Even if on average pressure reversal or neutrality is achieved, room radon levels may still be moderate because of pulsed entry during windy conditions.

Another cause of exasperation is that in many old houses especially, sealing the floor does not address all of the possible radon entry routes. Many older houses in the country districts of Devon, Cornwall, Northamptonshire and Derbyshire have thick granite, sandstone or limestone walls, often with rubble or other highly permeable cores.

The other possible (and potentially catastrophic) effect of sealing timber floors in this way may be to increase their average moisture content, since what may have been a principal route for ventilation air has now been blocked off. This has been investigated in a few houses and schools, but measurements of average moisture content take time.

In some cases installation of radon measures has been blamed for decay where it seemed highly likely that defective drains were responsible for increasing local moisture levels. It is essential to understand the reasons for decay before embarking on heroic and perhaps unnecessarily expensive remedial works.

Equipment is available to monitor and report moisture levels in timber: the author may be consulted for details of application.

Wood block floors.

Wood block is usually laid above concrete, and set in tar or similar material. Plastic sheets or other vapour impermeable material should never be laid over these floors to help cure either radon or dampness. The consequence in only a few months may be severe and irreversible damage to the blocks. The author may be consulted for advice.

In the USA there has been much good experience with 'sub poly' systems used in crawl spaces. Here, a plastic sheet is laid not over the timber floor but over the underlying ground or over site concrete. A fan is installed to draw air from beneath the plastic, which should be sealed as well as possible to the walls. However, in typical 'difficult' UK houses it could be expensive and highly disruptive to install such a system because access is so limited.

The systems are suited to some houses having deep voids - typically those built on steeply sloping sites - but even here there may be easier solutions. Sub poly systems can also help to reduce underfloor moisture levels, but depending much upon the source of the moisture.

Other overseas work.

In Sweden, timber floors are common, and often affected by radon and dampness. Some research interest has centred upon allergies in occupants owing to mould growth underfloor.

Studies have shown that laying a plastic sheet over the ground can have a marked effect on the average yearly moisture content of the timber floor. However, simply laying an over site sheet is unlikely to affect radon levels.

Mould was a problem in around 15% of floors inspected in one study, but in only one case was timber decay sufficiently bad to warrant replacement. This may be a consequence of timber treatments or simply a reflection of the age of the houses, all of which were less than 10 years old. Relative humidity levels in the underfloor air were recorded over a year. During times when the levels were over 80%, the wood could more readily support mould growth, and would be at greater risk of decay.

Effects on heating systems.

Another crucial factor for suspended floor houses is whether pipe-work for household water supply or central heating is located underfloor. If so, an extensive sealing project should be completed only after pipe-work has been inspected and properly insulated: it might be difficult to repair faults later without compromising some of the sealing.

Frost damage to pipe-work is a severe problem in some areas of the USA where crawl space ventilation has been used. In Devon and Cornwall pipe-work is unlikely to freeze beneath uninsulated floors except in severe winters and with high air flow rates. Radon fans may be switched off during unoccupied periods.

In more northern parts of the UK, pipe-work may be at severe risk from frost damage if located in the air stream from a fan or air-brick. Suitable insulation or relocation of pipe-work should be ensured if high flow rates underfloor are envisaged.

Summary for the UK.

Installing a fan to ventilate under a suspended timber floor can increase or decrease radon levels.

Houses are already known to present quite different symptoms depending upon the dampness of the underlying ground and (crucially) upon any local defects in rainwater drainage arrangements.

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