Steam Sauna Rooms: A UK Buying Guide to Home Steam Rooms

Steam sauna rooms are the hardest of the home heat options to get right, and the one most often bought on the assumption it is a sauna that happens to be wet. It is not. A sauna is a well-insulated wooden box that gets hot. A steam room is a sealed, waterproofed enclosure running near 100% humidity at around 40°C to 45°C, and everything difficult about it follows from the water. This guide covers the decisions that decide whether yours works: generator sizing, the material multiplier nobody mentions in the showroom, tanking, and extraction.

If you are still deciding between the two experiences, start with sauna vs steam room. If you have settled on dry heat, our home sauna buying guide covers that path. This page assumes you want steam.

The four ways to have steam at home

A bespoke tiled steam room. Built in place: waterproofed, tiled, generator hidden in a cupboard nearby. The best result and the most expensive, with the most that can go wrong. This is a building job, not a delivery.

A steam shower cabin. A self-contained acrylic pod with the generator built in. Plumbing and power in, and it works. Far more forgiving because the manufacturer solved the waterproofing for you. Less beautiful, dramatically less risky.

A combi cabin (sauna plus steam). A wooden cabin with a heater that also produces steam. Attractive on paper. In practice it is a compromise: you get a sauna that can be humid rather than a real steam room, because a wooden cabin cannot hold near-100% humidity without eventually paying for it, and the temperatures the two need are different. Buy one if you want a sauna with generous löyly. Do not buy one expecting a steam room.

A portable steam pod. A zip-up tent with a small generator. Cheap and genuinely usable for a few sessions a week if you have no room and no budget. Nobody pretends it is the same thing.

Sizing the generator: the multiplier that catches people

Steam generators are rated in kW and sized to the volume of the room. The starting point is simple arithmetic: length x width x height in metres. A room 2m x 1.5m x 1.8m is 5.4m³.

Then comes the part that gets skipped. Hard, dense surfaces absorb heat, so you size against an adjusted volume, not the real one. The standard multipliers applied to your cubic metre figure:

Look at what that means. That 5.4m³ room in acrylic needs a generator for 5.4m³. The identical room lined in thick natural stone needs one sized for 12.15m³, more than double. This is the single most common reason a home steam room underperforms: someone specified the beautiful stone and sized the generator off the tape measure.

Typical UK domestic generator coverage runs roughly:

• 3kW: 2 to 3m³
• 7.5kW: 4 to 9m³
• 10.5kW: 9 to 11m³
• 12kW: 11 to 14m³
• 15kW: 14 to 16m³
• 18kW: 16 to 22m³

Adjust upward again if the room sits somewhere colder than about 18°C ambient, which in a British garage or outbuilding it usually does. Steamist publish their own sizing guidance if you want a manufacturer’s version to check yours against.

The electrical reality

This is where domestic steam plans meet the consumer unit. A 12kW generator at 230V draws roughly 52A. That is more than a typical electric shower circuit, and it is nowhere near a ring main. A 15kW or 18kW unit is heading toward a supply upgrade.

The practical consequence: the stone-lined room you wanted may not be an expensive choice, it may be an impossible one on your existing supply. Get an electrician to look at your incoming supply and board before you choose surfaces, not after. Working backwards from what your electrics can carry to what surface you can afford to heat is the cheaper order to do this in.

Tanking and extraction: the two jobs that protect your house

Near-100% humidity finds every gap. A steam room that is not properly sealed does not fail dramatically; it quietly pushes moisture into the structure around it, and you find out via mould in the room next door a year later.

Tanking. The enclosure needs a continuous waterproof layer, not just tiles. Wall tanking and insulation board kits exist for exactly this, and the insulation is not optional either: an uninsulated tanked wall is a cold surface that condenses steam as fast as you make it, which costs you both comfort and running cost.

The floor. A mortar bed falling at a minimum 2% toward a centre or linear drain, with a flexible vapour membrane bonded over the mortar before the tile goes on. The fall matters. Standing water in a steam room is a smell you will not fix later.

Extraction. After a session the room has to dry. Without extraction the enclosure stays wet between uses and the grout goes black. This is the difference between a steam room that is a pleasure in year five and one you stop using.

If you are not confident specifying all three, that is the honest argument for a steam shower cabin. The manufacturer has already solved the waterproofing in a factory, and a factory does it better than most bathroom fitters.

Running costs and reality

Steam is cheaper to run per session than people fear, because 40°C to 45°C is not a hard target. The generator works hardest at the start and then holds. The costs that bite are the ones around it: descaling in a hard water area (a steam generator boils water constantly, so limescale is not a risk, it is a schedule), and the cost of getting the build wrong.

Hard water is worth flagging. If you are in a hard water area, ask specifically about descaling access and whether the unit has an automatic drain. A generator you cannot descale easily is a generator you will replace early.

Frequently asked questions

What is the difference between a steam room and a sauna? A sauna is dry heat, typically 70°C to 90°C, in an insulated wooden cabin. A steam room runs near 100% humidity at around 40°C to 45°C in a sealed, waterproofed enclosure. The lower temperature makes steam feel gentler on the airways for many people, while the humidity makes it far more demanding to build.

What size steam generator do I need? Measure the room in cubic metres, then multiply by a factor for the surface: 1.0 for acrylic, 1.30 for ceramic tile, 1.60 for porcelain, and up to 2.25 for thick natural stone. Size the generator to that adjusted figure. A 5.4m³ room needs roughly 5.4m³ of capacity in acrylic but over 12m³ of capacity in thick stone.

Can I put a steam room in an existing bathroom? Only if it is a sealed enclosure with its own tanking, drainage fall and extraction. You cannot simply add a steam generator to a normal bathroom, because the humidity needs somewhere to be contained and then removed. A steam shower cabin is the usual answer for retrofitting into an existing bathroom.

Are combi sauna and steam cabins any good? As saunas that can run humid, yes. As steam rooms, no. A wooden cabin is not built to hold near-100% humidity, and the two experiences want different temperatures. Buy a combi if you mainly want a sauna and like plenty of steam on the stones. Do not buy one expecting a proper steam room.

Do steam rooms cause mould? A properly tanked, insulated and extracted one does not. A poorly built one absolutely does, and the mould usually shows up in the room next door rather than in the steam room itself. Extraction is what dries the enclosure between sessions and it is the component most often left out.

How much electrical supply does a home steam room need? More than most people expect. A 12kW generator draws roughly 52A at 230V, which exceeds a typical electric shower circuit. Have an electrician check your incoming supply and consumer unit before you choose surface materials, since the surface decides the generator size.

Does hard water damage a steam generator? It scales one, inevitably, because the unit boils water every session. In a hard water area treat descaling as routine maintenance rather than a fault. Ask about descaling access and whether the generator has an automatic drain before buying.