Proper design reduces the risk of damage to swimming pool buildings

Structural air leaks can increase the risk of moisture damage in indoor swimming pool buildings. The height of the pool area affects the moisture load in the upper structures. Temperature differences can cause indoor over-pressure, which increases the risk of moisture accumulation in air leaky structures. Design, implementation and the use of technical systems can have an impact on moisture loads and the risks they pose to buildings. Air tightness of structures and indoor air relative humidity levels are the key issues for moisture safety.

Temperature differences between inside and outside air tends to cause overpressure in the upper parts of swimming pool buildings which have a high inside open air space. This allows warm and humid indoor air to flow out via the air leakage routes along the ceiling and upper walls, causing moisture from the inside air to condense in structures.

The long-term build-up of moisture causes various problems in structures, such as the growth of mould or, at worst, structural weakening. Air leakages into structures tend to be at their greatest during winter, when the indoor air humidity level is greatest compared to outside, in turn creating the greatest risk of moisture accumulating.


Figure 1. The indoor air pressure conditions in swimming pools, caused by indoor air conditions and the height of the premises, pose challenges to the moisture performance of structures.


‘Rain’ from the roof into the interior may appear in swimming halls with poor airtightness of the structure. During cold periods, indoor moisture condenses and freezes in structures via air leakage routes and when the weather turns milder, the melted water runs into the inside air space via air leaks. In cases where the airtightness is this poor, there is an elevated risk of structural damage and it is very likely that impurities within the structures can enter the indoor air as the direction of the air leakage flows changes.

The pressure conditions in a high and heated space cannot be fully controlled by ventilation. Even if a typical level of under-pressure is maintained at floor level, long periods of overpressure can occur in the upper areas. For example, if the ventilation maintains a constant under-pressure of 10 Pa at floor level in a 9-metre high hall, overpressure will occur in the upper areas for almost half of the year (Figure 2).

Figure 2 The duration of the pressure difference between indoor and outdoor air in the upper parts of indoor swimming pools of different heights, when the indoor and outdoor air pressure is the same at floor level. Evaluation performed for a one year period in Helsinki climate conditions (nominal heating year).


The study conducted by VTT presents the principles of design, implementation and use of technical system, which can have an effect on the moisture loads of structures and the risks they pose. The key issue is to have sufficient airtightness of structures. In addition, the relative humidity indoors is set to the lower limits of the comfort zone during winter, around 40% RH.

Due to the high humidity of the indoor air, the risk of indoor air leakages through the structures is elevated in indoor swimming pools. A similar risk can be associated with other high hall structures, even if they typically have lower indoor air humidity than swimming pools.

The results presented are part of the ’Uimahallien yläpohjarakenteiden kosteustekniikka ja paloturvalliset PU-lämmöneristeiset hallirakenteet’ (Moisture Performance of the Roof Structures of Indoor Swimming Pools and Fire Safe Hall Structures with PU thermal insulation) project, which was conducted from 21 September 2015 to 31 December 2017. The study involved examining research data, guidelines and regulations related to the indoor air and structural moisture load of indoor swimming pools, and compiling expert views on the subject. In addition, the duration of pressure differences in high halls in the Finnish climate were analysed.

You can read the publicly available customer report here


Tuomo Ojanen VTT

Tuomo Ojanen
Senior Scientist, VTT

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