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How to control humidity in an indoor pool hall

Updated 23 October 2026 · SEO Dons Editorial

What conditions do UK guides set for a pool hall?

The published design reference is Sport England's Swimming Pools Design Guidance Note, whose Appendix 2 covers servicing the building. It says air temperatures are usually kept at one degree above the pool water temperature to minimise evaporation, and gives target conditions for the hall. HSE's pool guidance is not the place to look: its fourth edition says it no longer contains detailed information on swimming pool design.

SourceWhat it gives for the pool hall
Sport England, Appendix 2 (2013)Air about 1°C above the water, approximately 30°C where the water is 29°C. Relative humidity 60% (the appendix gives a band around it). Minimum fresh air of 4 to 10 air changes an hour, with air velocity of approximately 0.1 m/s in occupied areas
CIBSE Journal (2019), one UK Passivhaus pool designHall held at 64% relative humidity
CIBSE Journal (2021), a modelled leisure centreOperation at 50%, 60% and 70% compared; at lower outdoor temperatures the humidity would need to be lower

These are design targets and not a single legal limit. The right set-point for a given hall depends on how well its fabric resists condensation.

Why does the humidity set-point change the energy use?

Because ventilation or dehumidification removes the moisture, and the drier the target, the more air has to be treated. The 2019 CIBSE Journal design article reports that raising the relative humidity set-point by just 5% cut the peak airflow needed for dehumidification by 30%.

The 2021 article makes the link to the building. It says better insulation lets a pool run at a much higher relative humidity, with a much lower ventilation rate, without causing condensation. The optimum ventilation level is the one just sufficient to prevent condensation, and it varies with pool activity and outside temperature: lower overnight and higher in winter, when the walls are colder.

That article also describes a common failure. In many halls the ventilation ran flat out all year, day and night, because nobody understood the controls or wanted to risk changing them. Variable-speed ventilation, controlled by hall humidity and outdoor temperature, is its recommendation.

What drives evaporation in the first place?

The 2021 article draws on research from the United States that related evaporation to the number of people in the pool, the speed and humidity of air moving over the surface, air temperature and other factors. A busy session therefore makes a very different moisture load from a quiet one, which is why a fixed airflow is a poor match.

Water temperature belongs on the list. A CIBSE case study of an Olympic pool names, among measures an operator could consider, lowering the pool water from 30°C to 28°C for leisure use and lowering the poolside air temperature accordingly. The air follows the water, as the Sport England note implies.

Ventilation-led or dehumidifier-led: how do they differ?

In a ventilation-led design, humidity is held by changing how much outside air is used. The CIBSE Olympic pool case study above is an example: its air handling units adjust the fresh air ratio to control the space humidity, with plate heat exchangers giving heat recovery of up to 84%.

A dehumidifier-led unit cools air below its dew point so that moisture condenses, then reheats it. A CIBSE Journal CPD module calls pool halls a common application for condensing dehumidifiers, which suit moderate to warm air, typically 15 to 36°C, and puts their typical energy use at 0.5 to 1.5 kWh per kilogram of moisture removed. It also notes that the heat is rejected at the condenser and may need to be exhausted externally.

The choice shifts with the weather. Dantherm, a dehumidifier maker, notes that hot months are the worst case for outside air because hot air holds more water, which weakens dilution by ventilation in summer. The commercial dehumidification guide covers the technologies and sizing, and the pool air handling unit page covers the equipment. For the opposite problem, adding moisture to dry air, see the steam versus adiabatic humidifier guide.

What do pool covers and out-of-hours control change?

PWTAG's technical note TN48 (July 2020) says a cover can give savings of up to 20 per cent, but may be inappropriate if heating and ventilation must run constantly. The 2021 CIBSE Journal article found covers unused at some sites it visited. For the energy saving to arrive, the plant has to change its behaviour too. PWTAG says to run ventilation and heating for 30 minutes once the cover is on, then let them stay off or run reduced.

The restart matters as much as the shutdown. About 30 minutes before the cover comes off, PWTAG says heating and ventilation should be restored to bring the hall and its internal surfaces close to the pool water temperature. It adds that this needs enough insulation to prevent condensation, and that cold weather may need some background heating and ventilation. For a hall without a cover, the point in the 2021 article still stands: the overnight load is far lower than the peak.

Where does condensation get into the building, and how is it found?

CIBSE Journal describes older halls with the ruinous impact of condensation on structural steelwork, and says new buildings need to be very well insulated with no thermal bridges, exposed steelwork or crevices where condensation can collect. A CIBSE case study of an Olympic pool added a trench heater system around the perimeter to limit condensation, and sensors that monitor roof void temperature and humidity to hold it above dew point at all times.

Sport England's appendix asks for effective moisture vapour barriers, sealed gaps where services pass through walls, and a negative pressure between the hall and adjoining areas such as changing rooms to help contain the pool environment. PWTAG's troubleshooting table gives a quick check from the other direction: chlorinous smells with high air humidity point to ineffective air circulation in the hall, so check air handling, dampers, filters, automatic controls and the amount of fresh air.

Do the ecodesign rules apply to a pool unit?

Commission Regulation (EU) 1253/2014, as published on legislation.gov.uk, applies to ventilation units. Article 1 excludes, among others, units that include a heat exchanger and a heat pump for heat recovery or heat transfer additional to the heat recovery system, so a pool unit with a built-in heat pump may fall outside it. It also excludes units specified exclusively for toxic, highly corrosive or flammable environments. Which exclusion a given unit relies on, if any, is for the maker to state.

Where a unit is in scope, Annex III asks for a multi-speed or variable-speed drive, heat recovery on bidirectional units with a thermal by-pass, and from 1 January 2018 a minimum heat recovery efficiency of 73%, or 68% for run-around systems.

Sources

Common questions

Is 70% relative humidity too high for a pool hall?

Not by itself. The 2021 CIBSE Journal article modelled the saving from running a hall at 70% rather than 50% and tied higher humidity to well-insulated fabric, while warning that lower outdoor temperatures need lower humidity. It describes condensation damaging structural steelwork in older, poorly insulated halls.

Does HSE set a humidity limit for pool halls?

HSE's HSG179 says it no longer contains detailed information on swimming pool design or pool water treatment. The design conditions in this guide come from Sport England's note and CIBSE Journal instead.

Should the ventilation be switched off when the pool is closed?

Not entirely. PWTAG says that with a cover on, ventilation and heating can stay off or run reduced after the first 30 minutes, then be restored about 30 minutes before uncovering. In cold weather it may be necessary to keep some background heating and ventilation running.

Does a chlorine smell mean the hall needs a bigger dehumidifier?

Not necessarily. PWTAG links chlorinous smells with high air humidity to ineffective air circulation in the pool hall and suggests checking air handling, dampers, filters, automatic controls and fresh air before assuming capacity is short.

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