Short answer: An indoor pool is not a pool with a roof: it is a wet room 365 days a year. What has to be resolved before anything is excavated.
· Wellness · 10 min
Technically, an indoor pool is the most demanding space in a luxury home. More than a professional kitchen, more than a private cinema. Because it does not fail suddenly: it fails slowly, across five winters, until the first corrosion appears on fixings and the timber ceiling starts to move.
Everything that follows is decided before the tank is excavated.
A 40 m² pool at 28 °C evaporates in the order of 8-12 litres of water per hour at rest, and considerably more with bathers and a disturbed surface. Those litres are vapour that must be removed from the air continuously, or they will condense on the coldest surface in the room: the glass, the thermal bridge at a beam, the junction with the roof.
Which is why the principal engineering item is not the filtration plant. It is the dehumidifier.
Two strategies. A heat-pump dehumidifier with recovery returns extracted heat to the room and is usually the sensible option in a Mediterranean climate. An outside-air system with enthalpy recovery works very well where outdoor air is dry and cold, but is penalised in humid climates.
In both cases supply air is directed to wash the glazing from top to bottom. That is the only way to prevent condensation on the frames: it is not beaten with capacity, it is beaten with air direction.
The whole enclosure needs a continuous internal vapour barrier, insulation free of thermal bridges, and thermally broken frames with low-emissivity glass. A single thermal bridge — a slab edge, a through fixing — becomes a permanent condensation point and, eventually, a defect.
With timber roof structures, the barrier and its sealing are literally the difference between a healthy structure and a replacement in ten years.
The characteristic smell of covered pools is chloramines, heavier than air and concentrated just above the water. They are reduced with extraction at water level, compulsory pre-swim showering, UV or ozone supporting the disinfection, and tight pH control between 7.2 and 7.4. Salt water is no exemption: an electrolyser still generates chlorine, and therefore chloramines.
A water hall with hard surfaces reaches reverberation times of 3 seconds or more. It is a beautiful space where nobody can hold a conversation. Correction means absorption on the ceiling and the upper wall bands, in humidity-resistant materials: mineral wool panels with hydrophobic tissue, treated micro-perforated timber, or perforated metal ceilings with acoustic backing.
Going from 3 to 1.5 seconds transforms how the space feels and is one of the best cost-to-result interventions in the whole project.
For a 30-40 m² pool in a well-insulated private house: €3,000 to €7,000 a year, with the thermal cover as the single most profitable measure — it cuts night-time evaporation by 50-70 %. An indoor pool with no cover and no scheduling can double that figure.
Sizing the dehumidifier by room volume instead of water surface area and temperature; placing diffusers on the wall opposite the glazing; specifying AISI 304 stainless steel in a chlorinated atmosphere where 316L or 1.4571 belongs; forgetting access for plant; failing to plan a plant room with its own ventilation and drainage.
We coordinate the HVAC engineer, the pool contractor and the finishes team from day one, because in an indoor pool the aesthetic decisions and the technical ones are the same decision. We fix set points, calculate evaporation and check condensation risk at every junction before a single material is chosen.
If you are weighing up an indoor pool, or enclosing an existing one, write to us for an honest first read on feasibility and cost.
For a 30-40 m² pool in a private house, annual running cost typically lands between €3,000 and €7,000 depending on insulation, plant strategy and whether a thermal cover is used. Dehumidification and air heating weigh more than heating the water itself.
Air should sit 1-2 °C above the water, with relative humidity between 55 % and 65 %. With water at 28 °C, air runs at 29-30 °C. Air colder than water sends evaporation up sharply and everything condenses.
That smell is not chlorine but chloramines, formed when chlorine reacts with organic matter. They collect in the air layer just above the water and are only removed by extraction at that level, compulsory showering before bathing and correct oxidation of the water.
It can, but it becomes a different technical problem. Enclosing it creates a permanently wet room that requires dehumidification, a vapour-controlled envelope and condensation control at the glazing. Enclosing without solving that wrecks the structure within a few winters.