Indoor Pool, Spa and Hot Tub Room Humidity Control

Short answer: an indoor pool, spa or hot tub room should sit at 50 to 60% relative humidity, with the air held about 1 to 2°C above the water temperature. Let it drift above 70% and you are no longer managing comfort — you are watching the building corrode.

Portable dehumidifiers are what you hire when the fixed plant fails, when a new pool is being commissioned, or when a spa room never had a system in the first place. Industrial units from £39.99 a day, delivered across London.

50–60%Target relative humidity in a pool hall

+1 to 2°CAir temperature above the water temperature

355 LWater a 46 m² pool can evaporate in a single day

60%RH above which ferrous metal starts corroding fast

70%RH at which mould germinates on surfaces

28–32°CTypical UK indoor pool water temperature

A pool room is the only part of a building deliberately designed around a large, warm, open tank of water. Everything else in the structure — the steel, the timber, the plaster, the glazing, the insulation — is being asked to live next to it. Get the air right and it lasts decades. Get it wrong and the damage is quiet, continuous and expensive, and it usually shows up in the roof void long before anyone notices a problem in the room.

This guide covers the numbers that actually matter, what goes wrong when they slip, when a hired dehumidifier is the right answer and when it is not, and what the kit costs.

On this page

The three numbers that run a pool room
Why a pool room sweats
What uncontrolled humidity destroys
When hiring a dehumidifier is the right call
Find your building
How much extraction you need
What it costs to hire
Six mistakes that cost buildings
Pool and spa humidity questions

The three numbers that run a pool room

Almost every pool room problem traces back to one of these three being wrong. They work together — fixing one while ignoring the others rarely helps.

1. Relative humidity: 50 to 60%

Below 50% the water evaporates faster, swimmers feel cold the moment they step out, and you waste heat replacing pool water. Above 60% you are into corrosion territory, and above 70% mould will grow on surfaces. The band is narrower than people expect.

2. Air temperature: 1 to 2°C above the water

Warmer air holds more moisture, which reduces evaporation off the pool surface, and it keeps swimmers comfortable when they get out. Air cooler than the water is the classic mistake — it drives evaporation up and dumps the moisture on every cold surface in the room.

3. Dew point versus surface temperature

Condensation does not care about the room average. It forms wherever a surface is colder than the dew point of the air — a window reveal, a steel purlin, an uninsulated lintel. This is why the damage usually starts somewhere nobody is looking.

Why a pool room sweats

Evaporation never stops, so the moisture load is constant. It condenses wherever a surface is colder than the dew point — usually the glazing and the steel in the roof.

A 46 m² pool can lose around 355 litres of water a day to evaporation alone. That is not a leak — that is the pool doing what warm open water does, and it is 355 litres a day that has to leave the building as vapour or condense somewhere inside it. There is no third option.

What uncontrolled humidity destroys

Pool air is not just damp. It is damp and chlorinated, and that combination is far more aggressive than moisture on its own. Damage runs in this order, roughly.

Structural steel and fixings

Ferrous metal corrodes quickly above 60% RH, and chlorinated moisture accelerates it. Roof fixings and hidden brackets are the ones that matter, because nobody inspects them until something moves.

Timber — roof, joinery, decking

Wood absorbs moisture, swells, and eventually rots. Ceiling timbers above a pool hall live in the worst air in the building and dry out the slowest.

Plaster, paint and render

Blown plaster, peeling paint, salt bloom. Redecorating a pool room that has not had its humidity fixed simply buys you the same job again in two years.

Glazing, frames and seals

Condensation running down glass rots frames, degrades seals and stains sills. In a domestic pool room this is usually the first visible symptom.

Insulation

Wet insulation stops insulating. The wall gets colder, the surface drops below dew point more easily, and the problem accelerates itself.

Air quality and comfort

High humidity concentrates chloramines, which is the smell people wrongly call “too much chlorine”. It irritates eyes and airways, and floors around the pool get slippery with condensate.

The quiet version of this problem is the expensive one. Most pool room damage is discovered during unrelated work — a roof repair, a re-roof, a survey before a sale. By then the humidity has been wrong for years. If your glass runs with water in winter, treat it as a reading, not a nuisance.

When hiring a dehumidifier is the right call

Straight answer, because it saves everyone time: a permanently used pool hall should have a fixed dehumidification or air handling system sized for the water surface area. Portable units are not a substitute for that, and we will tell you so on the phone.

What portable industrial dehumidifiers are genuinely good at is everything around that system.

Six jobs a hire unit does well

The fixed plant has failed and parts are two weeks away. A hire unit keeps the building safe in the meantime.
A new pool is being commissioned and the room has to be dried and stabilised before handover.
A spa, hot tub or plunge pool room that never had a system, because nobody expected a domestic room to produce that much vapour.
Seasonal or occasional use — a pool covered for winter still evaporates unless the cover is genuinely sealed.
Refurbishment — screed, tanking, plaster and adhesive all have to dry before tiling, in a room already fighting humidity.
Proving the problem before you spend on fixed plant. A month of readings with a hire unit tells you the real moisture load.

Industrial dehumidifier and air mover hire package set up for drying in a London property — What actually turns up: an industrial dehumidifier with pump-out drainage and air movers, delivered as one matched package from our Greenford depot.

Find your building

Open the one that matches. Each gives the kit, the daily hire price and the thing most people get wrong in that situation.

Domestic indoor pool room

Usually a glazed extension or garden building around a 6 to 12 m pool. The glazing is the weak point and the roof void is the hidden one.

Kit: 63L LGR dehumidifier at £59.99 a day, with pump-out so it can run unattended. Add an air mover from £15.49 a day to stop dead air in corners.

Watch for: running the room cooler than the water to save on heating. It feels sensible and it makes the condensation worse.

Hot tub, spa or plunge pool room

The most common call we get, and the most underestimated. A 40°C spa in a small unventilated room produces a serious vapour load in a space with none of a pool hall’s design allowances.

Kit: 43L Dri-Eaz Cube at £39.99 a day — compact enough for a small room, industrial enough to hold the humidity down.

Watch for: a domestic plug-in dehumidifier rated for a bedroom. It will run continuously and still lose.

Commercial pool with failed plant

Gym, hotel, school or leisure centre where the AHU or fixed dehumidifier is down and the building is still open.

Kit: 95L building dryer at £77.99 a day, or two units for a large hall. Pump-out to the nearest gully, positioned away from walkways.

Watch for: waiting for the repair. Every week at 75% RH is a week of corrosion you are paying for later.

New pool build or commissioning

Screed, tanking, adhesive and grout all curing in a room that is about to be filled with warm water.

Kit: 55L DrizAir 1200 at £49.99 a day with air movers. Same logic as drying new plaster — do not tile onto a wet substrate.

Pool room refurbishment

Old tiles off, new screed down, decorating to follow, and the pool often still full.

Kit: an LGR unit plus two air movers. Cover the pool properly while you work — an uncovered pool is a humidifier fighting your dehumidifier.

Pool hall closed for the winter

Heating turned down, plant switched off, cover on, doors shut. This is where buildings quietly rot.

Kit: a 38L unit at £29.99 a day on a humidistat is often enough to hold a closed room under 60% RH. Bear in mind a cold room slows a refrigerant unit down — see why dehumidifiers stall below 15°C.

Changing rooms, showers and plant room

Not the pool itself, but the rooms attached to it, which are often worse because they have no dedicated ventilation at all.

Kit: a 20L compact unit at £19.99 a day per room, or one larger unit with the doors left open between them.

Not sure whether it is the pool or the building

If the damp is on external walls away from the pool, or at low level, it may not be pool humidity at all. Rising damp, condensation or penetrating damp tells them apart before you spend money on the wrong fix.

How much extraction you need

Sizing a pool room is different from sizing a bedroom, because the moisture load is continuous rather than a fixed amount of water you are removing once. Two things drive it: the water surface area and the temperature difference between air and water.

Rough starting points for portable hire units
Space	Water surface	Start with	Per day
Hot tub or spa room	Up to 6 m²	43L Dri-Eaz Cube	£39.99
Small domestic pool room	10 to 25 m²	55L DrizAir 1200	£49.99
Domestic pool hall	25 to 45 m²	63L LGR	£59.99
Large or commercial hall	45 m² and up	95L building dryer, often two	£77.99 each
Refurbishment, pool covered	Any	55L or 63L plus air movers	from £49.99

These are starting points for a hire unit holding a room, not a design calculation for permanent plant. If the room has been wet for a long time the fabric itself is saturated, and the first two or three weeks are spent pulling water out of the building rather than out of the air. Our guides to sizing a dehumidifier and how many units you need cover the arithmetic in more detail.

Blue industrial dehumidifier with pump-out drainage set up in a London commercial space — Pump-out drainage matters more than tank size in a pool room — nobody is going to empty a bucket twice a day for six weeks.

Four setup details that decide whether it works

Drain it, do not empty it

Every unit we send for this kind of work can run to a drain or gully. A tank that fills in four hours is a machine that stops for twenty.

Keep the room closed

Doors to the rest of the house shut. Otherwise you are exporting pool humidity into bedrooms and hallways, which is how a pool room damages the house around it.

Cover the pool when you can

A properly fitted cover cuts evaporation dramatically. During refurbishment it is the cheapest single thing you can do.

Measure, do not guess

A £15 hygrometer left in the room tells you whether you are at 55% or 75%. Without it you are hiring on a feeling.

What it costs to hire

Daily hire from our Greenford UB6 depot, delivered across Greater London with same and next day delivery available. A refundable deposit applies and comes back in full when the equipment returns undamaged.

Dehumidifier and air mover hire prices
Equipment	Per day	Extraction	Best for
20L compact dehumidifier	£19.99	20 L/day	Changing rooms, small shower rooms, holding a dry room dry
38L dehumidifier	£29.99	38 L/day	A closed pool hall over winter, quiet enough for occupied buildings
the 43 litre Dri-Eaz Cube	£39.99	43 L/day	Hot tub and spa rooms, tight access, stackable
55L Dri-Eaz DrizAir 1200	£49.99	55 L/day	Small pool rooms and commissioning, pump-out with a 12.2 m hose
63L LGR industrial dehumidifier	£59.99	63 L/day	Domestic pool halls — keeps working once the air is already dry
a 95 litre building dryer	£77.99	95 L/day	Commercial halls and failed plant cover
Air mover	from £15.49	—	Moving air onto cold surfaces and into dead corners

For longer jobs, weekly rates and trade accounts are available — how tool hire trade accounts work explains the terms. Running cost is covered in what it costs to run a hired dehumidifier.

Six mistakes that cost buildings

Running the air cooler than the water

The single most common error. Cool air evaporates more water off the pool and holds less of it, so everything cold in the room starts dripping. Hold the air 1 to 2°C above the water.

Leaving the pool uncovered overnight

An uncovered pool evaporates all night, into a room nobody is heating or ventilating. A cover is the cheapest humidity control there is.

Opening the doors to the house

It moves the moisture rather than removing it, and it takes the problem into rooms with soft furnishings and no drainage.

Treating it as a decorating problem

Repainting a wall that is wet from the inside buys you two years. The paint is not failing — the air is.

Sizing on floor area, not water surface

Two rooms with the same footprint but different pool sizes have completely different moisture loads.

Ignoring the roof void

Warm moist air rises into it, condenses on cold steel and timber, and does its damage where nobody looks until the ceiling stains.

Pool and spa humidity questions

What humidity should an indoor swimming pool be?

50 to 60% relative humidity. Below 50% evaporation increases, swimmers feel cold leaving the water and you lose more pool water to the air. Above 60% ferrous metal starts corroding quickly, and above 70% mould will grow on surfaces. The band is narrow and it needs holding, not just reaching.

What temperature should the air be in a pool room?

About 1 to 2°C above the water temperature. UK indoor pools typically run at 28°C for fitness swimming up to 30 to 32°C for leisure and children’s pools, so the air usually sits somewhere between 29 and 33°C. Air cooler than the water is the classic mistake: it increases evaporation and drops the dew point onto every cold surface.

How much water evaporates from an indoor pool?

More than people expect. A pool of around 46 m² can lose roughly 355 litres a day to evaporation alone, before anyone swims in it. That water has to be removed from the air continuously or it condenses inside the structure.

Can a portable dehumidifier control an indoor pool?

For a permanently used pool hall, no — that needs fixed dehumidification or air handling sized to the water surface. Portable industrial units are the right answer for covering failed plant, commissioning a new pool, drying a refurbishment, holding a closed pool over winter, and for spa and hot tub rooms that never had a system at all.

My hot tub room is soaking wet. What do I need?

A 43L industrial unit is the usual answer — a domestic plug-in dehumidifier will run permanently and still lose. A spa at 40°C in a small room produces a large vapour load in a space with none of a pool hall’s design allowances. Keep the lid on when it is not in use and keep the door to the rest of the house shut. Ours is £39.99 a day.

Why is condensation running down my pool room windows?

Because the glass is colder than the dew point of the air. It tells you the humidity is too high, the glazing is under-performing, or both. It is a reading rather than a nuisance — the same condensation is forming on colder surfaces you cannot see, including roof steel and fixings. The domestic version of the same physics is covered in how to stop condensation on windows and walls.

Does chlorine make the damage worse?

Yes. Chlorinated moisture attacks building materials more aggressively than water alone — timber rots, metal corrodes and mould still grows. It is also why high humidity concentrates chloramines, the smell people wrongly blame on too much chlorine. Controlling the air fixes both problems at once.

Should I just ventilate instead?

Ventilation alone means dumping heated, treated air outside and pulling cold outdoor air in — expensive, and in a British winter that outdoor air often arrives at 85% RH itself. Ventilation has a role in a designed system; on its own it is an expensive way to make the room cold and still damp.

Can I run a dehumidifier while people are swimming?

Yes, but position it sensibly: away from walkways, cable routed so nobody trips, drain hose run to a gully and not across the floor, and on an RCD-protected supply. If the room is in constant public use, the low-profile options and a longer drain run are worth asking about when you book.

Can you deliver to a pool or leisure site in London today?

Yes — same and next day delivery is available across Greater London from our Greenford UB6 depot. Tell us the water surface area, the room temperature and whether there is a gully or drain nearby, and we will send the right unit with enough hose. Call 020 3375 4048 or message us on WhatsApp.

Pool, spa or hot tub room out of control?

Industrial dehumidifiers with pump-out drainage from £39.99 a day, delivered across Greater London.

See dehumidifier hire

Tell us the water surface area and the room temperature and we will tell you exactly what to take. Call 020 3375 4048 or message us on WhatsApp.