Common facility operational issues are often blamed on the dehumidifier

Vapour barriers

Another misdiagnosed problem incorrectly associated with the dehumidifier is condensation on or inside a building structure. Instead, vapour barriers (i.e. vapour retarders)—a ubiquitous plastic sheeting that envelopes the entire pool room—could be to blame.

Indoor pools are warm environments with 50 to 60 per cent relative humidity (RH). This environment has a high pressure relative to the outdoors and cold weather. The vapour barrier is akin to a balloon keeping moisture contained in the environment. A well-sealed building with a good vapour barrier is a critical factor in preventing moisture penetration into the walls and roof. A vapour barrier that is not tight, due to a tear, allows moisture to migrate toward the low pressure of the outdoors. It is the contractor’s responsibility to install the vapour barrier undamaged and per the architect’s specifications. An improperly sealed or torn vapour barrier can allow moisture into areas where it will eventually damage the building envelope or even its infrastructure.

Further, condensation migration in subfreezing climates could produce ice on a natatorium’s exterior wall or roof, which is a serious issue that should be addressed immediately.

Condensation issues

Some facility managers accept natatorium condensation as an inherent nuisance of indoor pools. However, condensation is not acceptable on windows or walls and is many times mistakenly blamed on the dehumidifier.

Condensation can still occur at 30 or 40 per cent indoor RH levels. Window condensation typically points toward an air distribution problem; whereas wall condensation indicates the insulation is deficient or its R-value (i.e. its measure of thermal resistance) is too low. Wall condensation is the more serious problem of the two, as any amount of condensation must be addressed immediately to avoid serious building structure deterioration.

Condensation is predictable

Surfaces colder than the ambient dew point temperature will experience condensation, much like a cold can of soda on a summer day. Likewise, indoor pools typically have a high dew point temperature. For example, a 27.7 C (82 F) plus 50 per cent RH will produce a 16.6 C (62 F) dew point temperature. Windows and skylights are notorious for condensation because their low insulating R-values maintain a cold surface.

Architects and owners love a lot of windows in indoor pools, which should not pose a problem as long as designers understand the need to distribute proper airflow across their surfaces. Warming the window above the dew point temperature with air distribution eliminates condensation. Therefore, it is the HVAC engineer’s responsibility to assure every square inch of window space receives ample supply air.

Overhead or under-deck ventilation ductwork should span the entire width of windows. A common architectural mistake is the specification of windows with mullions that jut out on the interior rather than the exterior. Interior mullions block air flow across the entire window surface.

There are some control strategies that measure the inside surface temperature of the exterior window. As the window gets colder, the control activates more dehumidification to lower the space’s RH. This strategy drives up energy costs significantly and is ineffective in extremely cold climates. Dew points cannot be lowered enough in cold climates for this strategy to be effective. As such, the answer is effective air distribution across the window.