Indoor vs garden cold plunge: where should you put it?

Put a cold plunge outdoors if you can. A garden install is cheaper, simpler, gives the chiller free cooling and unlimited airflow, and moves the noise and the moisture outside the building envelope. Indoors makes sense when weather, privacy, neighbours or year-round convenience outweigh that — but it converts an easy job into a building problem, because you now have to solve floor loading, condensation, waterproofing, drainage, ventilation and chiller heat rejection before the tub arrives, not after.

The single most common indoor mistake is treating the tub as furniture. It is closer to installing a small water tank in a room that also needs mechanical ventilation.

The decision in one table

	Garden	Indoor
Base	Slab, paving or engineered deck	Existing floor — must be checked
Typical extra cost	£300–£2,500 groundworks	£1,000–£6,000+ for floor, tanking, drainage, ventilation
Chiller efficiency	Best — free ambient cooling most of the year	Worse — rejects heat into the room it is cooling
Noise	Outside, but audible to neighbours	Inside, and reflected off hard surfaces
Condensation risk	None	Significant, needs designing for
Drainage	Gully, soakaway, or hose to a drain	Must be provided, often the hardest part
Usable in bad weather	Cold, wet walk both ways	Always
Planning implications	Only if enclosed or sunken	Only if it’s a new building or a conversion

The garden case

Most home installs go outside, and the reasons are practical rather than aesthetic.

A chiller in a UK garden is working in ambient air that is below the tub’s target temperature for a meaningful part of the year. That is why running costs collapse in winter — the numbers are on the running costs page. The condenser has unlimited air, waste heat disappears, and nothing you do to the tub can raise the ambient temperature it is fighting.

What to get right outdoors:

• A level, load-bearing base. Paving on a compacted sub-base, a concrete pad, or a deck specifically designed for the load. Not lawn, and not a deck built for a table and chairs.
• Chiller siting and noise. A compressor and fan run at roughly the level of an air conditioning unit — often quoted around 45–55 dB at a metre. It will run at unsociable hours in summer. Keep it off a shared boundary and off a wall that faces a neighbour’s bedroom, and give it clear air on all sides.
• Where the water goes. A few hundred litres, several times a year. A gully, a soakaway or a hose run to a drain — sanitised water should not simply be tipped into a border.
• Frost. At a 10°C target the water will not freeze, but if you shut the system down over a cold snap, exposed pipework and a pump can. Either keep circulation running or drain down.
• Shade. Free running-cost savings. A dark tub in full sun absorbs solar gain all day.

Full detail sits on cold plunge installation.

The indoor case — what actually has to be solved

Floor loading

This is the one that turns a purchase into an engineering question, and it is routinely skipped.

A 400-litre tub holds 400 kg of water. Add 50–80 kg of shell and 80–90 kg of bather and you are at roughly 550 kg over a footprint of about one square metre — around 5 kN/m². UK domestic floors are designed to an imposed load of 1.5 kN/m² under the relevant loading standard. A 700-litre tub pushes past 8 kN/m².

• Ground floor, ground-bearing concrete slab — normally fine. This is why garages and ground-floor utilities are the usual answer.
• Suspended timber floor, ground or upper storey — needs a structural engineer to check joist size, span, condition and how the load reaches a wall or beam. Sometimes it is a matter of running the tub across joists rather than along one; sometimes it needs new bearers or props. Do not eyeball it.
• Basement slab — usually fine structurally; the problem there is drainage.

Where structural work is needed, that is Building Regulations territory — covered on the planning permission and electrics page.

Condensation, not humidity

A cold plunge behaves in the opposite way to a hot tub, and people plan for the wrong problem.

Cold water holds the tub shell, the pipework and any uninsulated chiller lines below the room’s dew point. Warm room air touching those surfaces condenses onto them. In a 20°C room at 60% relative humidity, the dew point is around 12°C — so a 10°C tub is a condensation surface, permanently, whether or not anyone uses it. Meanwhile splash-out and wet floors evaporate moisture into the room.

The result in an unventilated garage or utility is damp under and around the tub, corrosion on fixings, musty smell and mould on cold wall surfaces within a season.

What fixes it: insulate every cold surface including pipe runs, provide mechanical extraction rather than relying on an open window, keep the room modestly warm rather than cold and damp, and finish walls and floor in materials that tolerate wetting.

Chiller heat rejection

A chiller does not destroy heat, it moves it. Whatever it removes from the water, plus the compressor’s own electrical input, comes out of the condenser as warm air — commonly 1.5–3 kW while running.

Put that in a closed 15 m² garage and you have a fan heater fighting the machine that produced it. Room temperature rises, the water gains heat faster, the chiller runs longer, running costs climb, and the compressor works in hot intake air, which shortens its life.

The fixes, in order of preference: a split unit with the condenser outdoors; ducting the condenser discharge through an external wall; or, at minimum, a generously ventilated room with a real air path in and out. A trickle vent is not a solution.

Waterproofing and drainage

• Floor finish. Tanked and tiled, sealed resin, or a welded vinyl safety floor with coved skirtings. Bare concrete stains and dusts; laminate and engineered timber will not survive.
• Falls and a gully. Ideally the floor falls to a gully connected to foul drainage.
• Below drain level — a basement, typically — means a sump and pump, or accepting that every water change is a submersible pump and a hose to an outside gully.
• Splash zone. Allow a metre of wet floor around the tub, and think about how someone gets in and out safely on a wet hard surface.

Room by room

Garage — usually the best answer. Slab floor, tolerant finishes, already cool, often a fall to the door. Needs ventilation, drainage and a compliant supply. If the garage is attached and you are converting it, fire separation and Building Regulations come into play.

Utility room — plausible if it is ground floor on a slab and already has drainage and extraction. Check the floor construction first; many are suspended timber.

Basement — structurally easy, environmentally the worst. No natural ventilation, no gravity drainage, and condensation with nowhere to go. Workable, but it needs mechanical ventilation and a pumped drain designed in.

Bathroom — already tanked and drained, which solves two problems. Floor loading is usually the blocker, and BS 7671 treats a room containing a bath or shower as a special location with its own electrical zoning rules.

Garden room — the popular compromise: outside the house, but enclosed. It only works if the building was specified for it — a slab or an engineered floor deck, waterproof finishes, ventilation, and the chiller sited outside the structure. It is also the case most likely to raise a planning question.

Which way to go

Choose the garden if you have a suitable base, somewhere for water to go, and a chiller position that will not annoy anyone. It is cheaper, more efficient and reversible.

Choose indoors if weather or privacy would otherwise stop you using it, and you are willing to treat it as a small building project — with the floor checked, ventilation specified and drainage designed before anything is ordered.

Our fit checker walks through the space, access, base and supply questions for a specific spot, and the cold plunge cost calculator will price the difference between the two routes. If you are still deciding between a chilled tub and something simpler, start with cold plunge vs ice bath.