Sport facilities are invariably humid. Here, Barbour Index and Scott Brownrigg explain how to stop moisture in the air turning into water vapour that can damage the fabric and finishes

Sport and leisure buildings invariably contain facilities that generate high humidity, which leads to moisture vapour. The specifier must be aware of the consequences of moisture vapour and its effect on the building fabric.

Moisture vapour exists all around us and is expressed as relative humidity. Warm air will hold more water vapour than cold air so if the temperature falls, water will condense. Any cool surface will act as a seeding ground and beads of water will form readily. If the water is allowed to settle – and bearing in mind that this may happen over many cycles – significant damage can occur to the surrounding fabric. Rotting mould, insect attack and chemical breakdown can be caused by continual wetting in this manner. The amount of vapour present can also produce a pressure that will drive moisture through a building’s fabric.

Moisture vapour can lead to premature degradation of the finishes, which will require constant cleaning and maintenance. If degradation occurs undetected within the fabric, catastrophic failure may eventually follow. To minimise the risk of moisture vapour, the specifier should carry out the following checks.

Assess vapour risk

Buildings are rated for humidity under BS 5250:2002 as classes 1 to 5 of moisture generation. Sport and leisure facilities are placed in the highest risk groups. A large number of people exercising and the showers in the changing rooms generate large quantities of water vapour. Intermittent use is also a factor: high levels of activity alternate with low occupancy, which results in large swings in both heat and humidity. The specifier should ensure the design is fully appropriate for the building’s use. Make sure the use is clearly understood, including the occupancy rate, times when changing rooms are busy and so on. These, with the general design of the fabric, will give a clear idea of the vapour risk, BS 5250 recommends a risk assessment.

Control water vapour

For most leisure and sport facilities, the fabric should be designed to control or dissipate water vapour. If all surfaces are kept warm and ventilation rates high, condensation should not occur. To comply with the latest Building Regulations, insulation thickness has increased and ventilation has decreased, which means there will be fewer cold surfaces than before. This should result in less condensation, but the ability to remove it will also be diminished. The specifier must carefully control the volume of air flow in the building and reduce uncontrollable air flow (infiltration), which will make an air seal layer almost essential.


Specify a vapour control layer to prevent vapour finding its way in to any vulnerable fabric. Some materials with low resistance to vapour can cope with a flow of water vapour through them and are designed with this in mind. However, a strategy of full protection should be considered in most circumstances. The vapour control layer may also be the air seal but not necessarily.

There are materials available that provide nearly every combination of air, vapour and water resistance required. Care is required, however, to specify correctly for the location. Safe specification should always be the starting point. If there is any chance of high humidity, vapour control and avoidance of any cold bridges is vital. Insist on a good-quality vapour control layer that has a resistance of more than 1000 g/kg m. Some manufacturers offer vapour control layers with much lower performance and these should be considered carefully.


Ensure that any vapour control layer can be practically installed. Consider all the building junctions and check the condensation potential. Ensure that the layer is specified to be full lapped and sealed, particularly at material junctions and complex areas. Ensure also that the fabric is matched to the performance of any mechanical plant so the extraction process is efficient and minimises energy loss.

Careful detailing is required at the junctions. There are now several proprietary products that can be used around windows and doors built in to the jamb junctions and then independently sealed to the membrane. It is often very difficult to give vapour control due prominence in the procurement and construction phases of the project.

The specifier should make clear reference to previous experience to encourage adequate provision and installation quality. Quality control examples of joints and junctions should be established as work commences on site. These can be reviewed as the work progresses. With current technology, it is very difficult to check performance after construction is complete and any defects may only appear over a long-time span.

Lifetime costs

Bear in mind that the common methods of vapour control and condensation analysis are based on constant conditions. This is rarely the case, particularly in sport and leisure buildings. Varying conditions will test the specification to the limit so it is prudent to allow for the worst-case scenario. Do not allow others to alter the specification because of previous experience. The effects of condensation may be very long term and hidden until substantial failure occurs. Despite research, this is not an exact science and the specifier should beware.

Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit

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