Condensation Control

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Condensation Control

  • Dew Point Temperature
  • Vapour Retarder
  • Window Design
  • Air Distribution

Natatorium Design Guide Manual

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Natatorium Design - Introduction   Bookmark and Share

The natatorium experience for a patron should be no different to any other room in a building. It should be comfortable, healthy and have good air quality. When designing a natatorium relative humidity, air temperature, pool water temperature, pool activity levels, air distribution, outdoor air, exhaust air and pool water treatment are all key aspects that must be addressed to provide a good environment.

While 50-60% relative humidity levels are ideal for bather comfort and health they can lead to condensation problems and serious damage to the building structure. If the building structure itself has not been properly designed for this higher humidity application catastrophic results may occur.

Condensation Control Bookmark and Share

You only have to enjoy a cold drink on a summer day to experience dew point and condensation firsthand. Condensation occurs because the surface temperature on your glass is below the ambient dew point temperature. While 50-60% relative humidity levels are ideal for bather comfort and health they much higher than people than what people are used to seeing in traditional spaces in winter. In northern climates it is very common to humidify in order to get the humidity levels up to 30-40%. An indoor pool and humidified space can experience condensation problems and serious damage to the building structure in cold weather if they are not designed properly.


Condensation is a major concern for all types of building construction. Condensation triggers a destruction process and allows mold and mildew to grow. If allowed to occur inside the building walls or roof, condensation will cause deterioration and can devastate the structure by freezing in winter.

As previously mentioned, off-gassed chloramines have a strong attraction to the airborne humidity and will combine with moisture in the air. Consequently any condensation in a pool, in addition to being destructive on its own, could also be corrosive.  It is critical that condensation be avoided at all costs.

The building design and construction must appropriate to house an indoor pool. The envelope design and construction must be suitable for 50% to 60% relative humidity year round.

A successful design will identify and blanket building elements low R-values (typically exterior windows) with warm supply air to prevent condensation. Window frames and emergency exit doors must also be thermally broken to avoid condensation.

Dew Point Temperature

The first step in condensation control is to establish the space dew point temperature based on the owners desired space conditions. With that the designer can establish potential condensation spots in the building.

A pool’s indoor design dew point will typically range from 62-69°F (82-84°F 50-60%RH). Contrast this to a typical space in winter that might be 70°F 40%RH which has a 45°F dew point.

Pools have a much higher likelihood of condensation because of both an elevated space temperature and slightly higher relative humidity adding up to a very high dew point.

These are building elements with low R-values that will have an inside surface temperature below the dew point at winter design condition. Most importantly, the dew point also establishes where to locate the vapor retarder in the wall. Figure 6 shows that a typical pool design of 82°F 50% RH has a dew point of 62°F. Therefore, any surface with a temperature below 62°F will condense moisture.

Figure 6 - Dew Point Temperature
Figure 6 - Dew Point Temperature

Vapour Retarder

A vapor retarder is a material that restricts the rate of water vapor diffusion through the ceilings and walls of a building when below dew point temperature occurs. Figure 7 illustrates how failure to install the vapor retarder in the proper location will result in condensation within the structure. Condensation in your walls or roof can lead to structural failure. A vapor retarder should be sealed at all seams.

Figure 7 - Do Not Build an Indoor Pool Without a Vapour Retarder
Figure 7 - Do Not Build an Indoor Pool Without a Vapour Retarder

Ensure the entire pool enclosure design (walls and ceilings) has a vapor retarder in the correct location. Care must be taken where walls and roof and walls and floor meet to ensure there is no breach in the vapor barrier.

A properly located and installed vapor retarder is the only means of protecting a building structure from vapor migration that becomes moisture damage.

Figure 8 is an example of a wall detail with its temperature gradient. This exercise allows the designer to identify the dew point temperature in the wall and where the vapor retarder must be installed.

Figure 8 - Install Vapour Retarder on the Warm Side of the Dew Point Location
Figure 8 - Install Vapour Retarder on the Warm Side of the Dew Point Location

Window Design

Windows have a relatively low R-value and as a result will have surface temperatures below the pool room dew point when it gets cool outside. Exterior windows will develop condensation on the first cold day unless measures are taken. The solution to the condensation problem is to fully blanket every part of the window with supply air from the HVAC system. It is critical that no section be missed or it will get cold and condense.

The solution exterior window condensation is simple: fully blanket them with supply air from the HVAC system.

Figure 9 - Window Design
Figure 9 - Window Design

Air Distribution

Since exterior windows and exterior doors are a primary condensation concern it is extremely important that the supply air is focused there. The warm air from the dehumidifier will keep the window surface temperature above the dew point temperature and this in turn ensures the windows and exterior doors remain condensation free.

There are five basic steps to laying out the ductwork:

  1. Supply air to exterior windows and doors.
  2. Supply air to the breathing zone at the deck level and water surface.
  3. Supply air to the remainder of the room to ensuring there are no stagnant areas
  4. Locate the return duct where it will optimize the entire airflow pattern.
  5. Prevent air short-circuiting by avoiding supply air diffusers near the return grille.

The following sample duct diagrams illustrate good air distribution practices:

Figure 10 - Perimeter Duct Layout
Figure 10 - Perimeter Duct Layout
Figure 11 - Perimeter Below-Grade Duct Layout
Figure 11 - Perimeter Below-Grade Duct Layout

All air distribution systems should:

  • Satisfy ASHRAE design requirements and local codes.
  • Supply at least 4-6 volumetric air changes per hour.
  • Blanket exterior windows, exterior surfaces and other areas prone to condensation with supply air. A good rule of thumb is 3 - 5 CFM per ft2 of exterior glass.
  • Locate the return grille to enhance the overall air pattern within the room.
  • Select grilles, registers and diffusers that deliver the required throw distance, and the specified CFM rating.
  • Introduced outdoor air per local codes and/or ASHRAE Standard 62-2004.
  • Maintain a negative pressure in the space with an exhaust fan.

General Recommendations:

  • Galvanized sheet metal ducts are acceptable in most installations. A below-grade duct system should use PVC or plastic-coated galvanized spiral pipe to avoid deterioration.
  • Fabric duct is an excellent choice of duct material for a Natatorium. The duct material should not allow air to leak. The location of supply grilles and overall duct layout should be exactly as you would with metal duct.
  • Ductwork that passes through an unconditioned area should be insulated on the exterior.
  • When applicable, locate exhaust fan air intakes as close to the whirlpool as possible.
  • To prevent excessive vibration noise, install neoprene flex connectors when attaching ductwork to the dehumidifier. Acoustic insulation on the duct close to the unit may also be a consideration.
  • Skylights require significant airflow to avoid condensation on their surfaces.