Quantifying the Influence of a Pool Cover on Water Evaporation in Indoor Swimming Halls

Water evaporation is the largest source of energy loss in indoor swimming pools. Besides increasing heating demand, evaporation raises indoor humidity, requiring larger ventilation and dehumidification capacities. A 2025 Norwegian study investigated whether a movable pool floor, when positioned at the water surface, can function as an effective pool cover and significantly reduce evaporation.

Why evaporation matters

Evaporation removes heat from the pool water while simultaneously adding moisture to the air. This increases the load on HVAC systems, which must remove excess humidity to maintain thermal comfort, prevent condensation and minimize corrosion within the building.

For indoor swimming facilities, controlling evaporation is therefore one of the most important factors affecting both operational costs and indoor climate.

Traditional pool covers versus movable floors

Conventional pool covers typically consist of a diffusion-tight plastic cover that is rolled over the pool during idle periods. While these virtually eliminate evaporation, they require additional equipment, maintenance and operational handling.

Modern movable pool floors offer an alternative solution. Besides allowing adjustable water depth for different user groups, they can also be raised to the water surface outside operating hours, acting as a pool cover while remaining permanently integrated into the pool structure.

Study objective

The researchers investigated how much evaporation is reduced when a movable pool floor is used as a pool cover.

The study was performed in a Norwegian educational swimming pool with:

• Pool surface area: 100 m²
• Pool length: 12.5 m
• Water temperature: 32°C
• Air temperature: 31.5°C
• Relative humidity: 55%
• Measurement period: 215 hours of validated operational data

The evaporation rate was calculated using a moisture mass balance for the swimming hall and compared with the widely used ASHRAE evaporation model.

Key findings

Evaporation reduced by up to 70%

When the movable floor was positioned directly above the water surface, the average ASHRAE correction factor decreased from 0.50 (standard unoccupied pool) to approximately 0.18.

Under optimal operating conditions, the correction factor further decreased to 0.15, corresponding to an evaporation reduction of approximately 70% compared with an uncovered pool.

Measured performance

The study reported:

These results indicate that a movable floor substantially suppresses evaporation, although ventilation strategy continues to influence performance.

Effect of ventilation mode

The study identified two distinct operating conditions.

Idle mode

During night-time operation:

• Lower airflow
• Air recirculation with dehumidification
• Stable humidity control

Average correction factor:

0.15

This represented the highest evaporation reduction.

Bathing mode

During daytime operation:

• Higher fresh-air ventilation
• Lower indoor relative humidity
• Larger vapour pressure difference

Average correction factor:

0.23

Although still substantially lower than an uncovered pool, evaporation nearly doubled compared with idle mode because drier air increased moisture transfer from the water surface.

Why the movable floor does not completely stop evaporation

Unlike a sealed plastic cover, a movable floor is not airtight.

Small openings between the floor panels and the perimeter gap allow limited air circulation beneath the floor. This creates an air cavity where moisture can still evaporate, although at a much lower rate than from an exposed water surface.

Importance of floor positioning

One of the most important practical observations was that the movable floor was frequently positioned slightly above the intended water level.

Because the floor is manually operated, operators often stop lifting too early, leaving a small air gap between the water surface and the floor.

This gap increases natural air circulation beneath the floor through the stack effect, allowing additional evaporation. The authors suggest that automatic positioning sensors or feedback systems could further reduce evaporation by ensuring the floor consistently reaches the optimal position.

Practical implications for pool design

The findings have important consequences for designers and operators of therapeutic and public swimming pools.

Benefits include:

• Reduced pool heating demand
• Lower dehumidification load
• Reduced HVAC energy consumption
• Lower operating costs
• Improved accuracy of building energy simulations
• Better sustainability of swimming facilities

For designers using the ASHRAE evaporation equation, the authors recommend reducing the correction factor from 0.50 to 0.15 when a movable floor is used as an effective pool cover under optimal conditions.

Relevance for hydrotherapy pools

Hydrotherapy pools frequently include movable floors to accommodate different patient groups and treatment techniques.

This study demonstrates that these floors provide a second important benefit beyond accessibility:

• Adjustable therapy depth
• Improved accessibility
• Reduced evaporation
• Lower energy consumption
• Reduced HVAC capacity requirements
• Improved sustainability throughout the facility lifecycle

For hospitals and rehabilitation centres where pools remain unused overnight, raising the movable floor after operating hours can substantially reduce energy consumption without requiring a separate pool cover.

Limitations

The study was conducted in a single Norwegian indoor swimming facility during winter conditions.

Although the measured reduction in evaporation was substantial, the authors recommend additional studies across different climates, pool types and operating conditions to confirm the general applicability of the findings.

Conclusion

This study demonstrates that a movable pool floor can function as an effective pool cover, reducing water evaporation by up to 70% under optimal conditions. By lowering evaporation, facilities can significantly reduce heating and ventilation energy demand while improving indoor climate control. The research also highlights that correct positioning of the movable floor and appropriate ventilation management are essential to achieve maximum energy savings. These findings support the integration of movable floors as both an accessibility solution and an energy-efficiency measure in modern hydrotherapy and public swimming facilities.