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Canadian Building Digest

CBD-83. Indoor Swimming Pools

Originally published November 1966.

G.K. Garden

It is generally assumed that through modern mechanical engineering technology any indoor physical environment can be produced and maintained. There are, however, many indications that there is a lack of appreciation of the fact that the enclosing construction must be appropriate if the desired conditions are to be realized and serious building problems avoided.

Indoor swimming pools and other occupancies that produce or require high humidity need careful consideration of both the interior environment and the building design. Unfortunately it has become common practice to accept the many problems that develop in cold weather as an inevitable part of a swimming pool facility. This is not correct, however, nor is it necessary to accept the loss of amenity, curtailment of function, nuisance problems, disfigurement and accelerated deterioration of the building as part of the price one pays for an indoor swimming pool.

Although swimming pools must provide environmental conditions conducive to human comfort, the conditions differ from one class of user to another. A swimmer's comfort is determined by his activity and the water temperature. Out of the water, air motion, evaporation from the skin, and radiation become important factors, but their effects on the bather and the spectator are not the same.

Water evaporation from the surface of a heated pool tends to increase the humidity in the air over it, influencing user comfort and producing severe problems in the building fabric during cold weather. The humidity can be reduced in several ways, but proper operation of the facilities is required.

Compromises may have to be made in establishing the indoor conditions that will be maintained in swimming pools as well as in the design of the enclosure. Only if the choices are faced at the outset can an over-all optimum solution be achieved. The considerations involved are the subject of this Digest.

User Requirements

The main user requirement is for a reasonable level of comfort, which may be related to many things - in this Digest, specifically, to thermal sensations. Comfort may be regarded both physically and psychologically as a condition of thermal neutrality under which the body need not strain to reduce or increase heat loss.

The level of physical activity is a major factor in comfort, since it determines the rate of heat generation within the body. In its attempt to maintain a constant temperature, the body must dispose of excess heat to the medium surrounding it. In swimming the skin is in intimate contact with water, and because of the efficiency of the conductive heat transfer the water temperature must be reasonably close to the tolerable skin temperature. Because of the high heat output from the body during competitive or active swimming, a water temperature of 72 to 75°F is acceptable. With a lower level of activity, a water temperature of 75 to 80°F may be desirable. Older people and those who relax in a pool may even prefer 85°F or higher.

A swimmer, out of the water, could be comfortable at an air temperature lower than that of the pool water except for the increased convective cooling effect of moving air, cooling from evaporation of water on the skin, and the radiative transfer of heat to surrounding cool surfaces. To provide comfort, any one of these conditions can be modified, within limits, to compensate for the others.

A swimmer who is very active out of the water will be reasonably comfortable with an air temperature of 75°F and a fairly high relative humidity unless there is exceptional air movement or radiation cooling. Less active bathers will require an air temperature of 80°F or more for reasonable comfort, but when the humidity is low bathers who are wet may be less comfortable than those who are dry.

Spectators require the usual conditions of about 70°F and a relative humidity between 30 and 70 per cent for optimum comfort. Fortunately, these conditions do not conflict greatly with those acceptable for competitive swimming. A spectator can be very uncomfortable, however, when exposed to the higher air temperature required by less active swimmers, especially if the humidity is also high. When there are conflicts between spectator comfort and pool use, it is possible to consider separating spectators and swimmers with transparent walls so that different conditions may be provided for each occupancy.

The Pool and the Building

The water surface presented by a pool must be regarded as a large humidifier capable of adding water to the space at relatively high rates whenever the dew-point of the air is less than the water surface temperature. The thermal characteristics of walls and windows, as normally constructed, usually limit the relative humidity that can be carried in winter to about 35 per cent at 73°F. This corresponds to a dew-point temperature of 43°F and will produce condensation on surfaces lower than that temperature. In pools, which are always well above this temperature, evaporation will take place continuously, with consequent increasing relative humidity in the space above until the level is reached at which evaporation is balanced by loss of moisture from condensation and ventilation.

In this way, condensation will always take place in winter on windows and walls of normal construction enclosing pool areas unless high rates of ventilation with relatively dry air are provided to carry off moisture at the appropriate rate to balance evaporation. When such air is drawn from outside in winter, it must first be heated to indoor air temperature. When no ventilation is provided, evaporation will continue and condensation will be produced on all enclosing surfaces that are below air or pool temperature, whichever is the lower. The condition that exists when the pool is warmer than the air will be recognized as particularly critical. In general, pool spaces must be ventilated continuously in order to prevent high humidity and the possibility of excessive condensation on enclosing surfaces.

The surfaces of large windows are always quite cold in winter and will always experience condensation unless the relative humidity is held to a low level by ventilation. They also produce cooling by radiation, which may be felt by the occupants. Curtains over large glass areas to provide privacy reduce the radiative cooling effect, but they further reduce the surface temperature of the glass as well. In such a situation, condensation will occur on the glass under less severe conditions and over longer periods of time.

It is quite apparent that a swimming pool room must be considered a high humidity space and the design of the enclosure and mechanical services must take full recognition of the nature and probability of condensation.

Minimizing the Problems

Condensation of water vapour, the cause of most problems associated with pools, occurs when the dew-point temperature of the air is higher than the temperature of any surface it may contact. To prevent surface condensation, it is necessary to keep the dew-point temperature sufficiently low or keep all surfaces warmer than the maximum dew-point temperature. It has become common practice to select interior surface materials that are not severely affected by some condensation. Run-off from these surfaces can, however, damage other surfaces or enter the wall and window materials to produce a variety of problems.

Air leakage, a very prominent mechanism operating in most buildings, transports water vapour into walls and roofs, producing interstitial condensation. This concealed condensation is responsible for much building deterioration in the form of corrosion, masonry failures and efflorescence, wet freezing damage, paint blistering and failure of roofing systems. Reduction in the effectiveness of many insulation materials also occurs, increasing heat loss and further reducing interior surface temperatures.

Air movement from a high humidity space through a building can also cause these problems at points remote from the swimming pool, especially at upper floors. Because of this, it is important to prevent air movement from the swimming pool room to the remainder of the building. It is also necessary to provide a separate air handling system for the pool area. All enclosing walls and roofs must incorporate barriers to the passage of air and vapour from the inside or must be arranged so that contact with the pool atmosphere is prevented in order to avoid condensation within them.

To Reduce Humidity

To reduce evaporation of water from the pool surface, which is the main source of high humidity, the water should be kept as cool as possible. Because evaporation goes on at all times, it is recommended that designers and owners consider covering the pool when it is not in use. A light plastic film can be floated on the water surface, being placed mechanically or manually, depending upon size and convenience. This would allow a reduction in the rate and thus the cost of ventilation when the pool is not in use; although mechanical ventilation or dehumidification must always be provided to keep the relative humidity within reasonable limits. Mechanical ventilation can be quite successful in winter since the specific humidity of the make-up air is extremely low, but it is considerably less effective in hot, humid weather. Condensation problems, however, are far less probable when the exterior temperature is high.

Design of a Satisfactory Enclosure

The design of building enclosures for high humidity conditions has been discussed in many previous Digests. It should be recalled that maintaining high interior surface temperatures demands sufficient insulation so positioned that it will not be subjected to wetting by condensation from vapour diffusion or air leakage. Air leakage through the construction and around window and door frames, ducts, electrical fixtures and conduits must be prevented.

Improved glass surface temperatures require proper frame design and the use of multiple glazing units. Reflective glasses having a low emissivity can also be used to advantage in gaining higher interior surface temperatures.

The best enclosure design may be one in which the exterior enclosure elements are not exposed to the severe environment of the swimming pool space. This can be accomplished by incorporating a heated space between the interior surfaces and the main enclosing elements and ventilating it with low-humidity air. In this case the inner layers of material will be required to resist the passage of air and vapour, but will not be subjected to a temperature gradient. Any moisture that might leak into the space will not raise the humidity appreciably because it will be removed by the ventilation process. This space can be as little as is required for proper air movement or it can be wide enough to perform as a corridor. It could in fact, be wide enough to provide the entire space required for spectators or for other facilities.


It is important to realize that a wide range of solutions to the total problem of swimming pool design can be found, depending upon the compromises that can be accepted. If a simple, conventional wall and roof system is to be used, the humidity level must be kept low, using a high level of ventilation or mechanical dehumidification and recognizing the high operating costs inherent in equipment and heating. The use of wall and roof designs that can tolerate the swimming pool environment may involve slightly higher initial cost for building, but a reduced expenditure for mechanical equipment and operation. The implications of a shut-down or improper operation of mechanical equipment must also be considered in the selection of building type.

It should be emphasized that some ventilation of the swimming pool space is essential regardless of building design. The owners and operators of swimming pools must be made aware of the fact that mechanical equipment is an essential part of the over-all design and that it must be properly operated if serious building problems are to be avoided.

Date Published: 1966-11-01
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