Specifying a Sauna or Steam Room in a London Renovation: Construction, Waterproofing, and Wellbeing Design

Sauna vs Steam Room: Key Differences

Sauna and steam room are often treated as interchangeable but they are fundamentally different environments with different construction requirements.

Sauna: A dry heat environment at 70–100°C with relative humidity of 10–20% (traditional Finnish sauna) or up to 50% (softer, lower-temperature sauna). The heat source is a sauna heater (kiuas) — either electric or wood-burning. The low humidity means that standard timber construction — tongue-and-groove softwood lining (Nordic spruce, aspen, or Canadian cedar) over a structural frame with insulation — is the correct approach. Waterproofing is not required in the same way as for a steam room: the environment is hot and dry, and the brief damp from löyly (water ladled over hot stones) evaporates rapidly.

Steam room: A wet heat environment at 40–55°C with relative humidity of 95–100% — essentially a cloud of water vapour. The steam generator (typically an electric unit of 3–9 kW for a residential steam room) continuously injects steam. Every surface is exposed to saturated water vapour and condensation; the floor is permanently wet. This requires complete waterproofing of all surfaces, drainage of the floor and bench areas, and materials that can tolerate continuous wetting and the thermal cycling that occurs each time the room heats and cools.

Many prime London wellness facilities include both — a dry sauna and a separate steam room — as they serve different physiological and experiential purposes. A combined sauna/steam unit (sometimes marketed as a "steam sauna") is a compromise and is generally less satisfactory than two purpose-built separate rooms.

Sauna Construction and Specification

A residential sauna of 2.4m × 2.0m × 2.1m high (a comfortable two-person sauna) requires:

Structure: The sauna cabin can be built as a freestanding timber structure within a larger room (a prefabricated sauna kit or bespoke carpentry), or as a room-within-a-room with a timber-lined interior. For basement installations, the latter approach — constructing the sauna volume within a concrete or block shell — is most common.

Insulation: The sauna must be well insulated to heat up quickly and efficiently. Mineral wool insulation (100mm minimum) between timber studwork, with a foil vapour barrier on the warm (interior) side to prevent moisture migration into the insulation. The foil barrier must be continuous and all joints taped; any gap will allow humid air into the insulation and cause condensation and deterioration.

Lining timber: The interior lining should be a non-resinous softwood — Nordic spruce (paanelu), aspen, or Canadian western red cedar. Avoid pine, which can release hot resin when heated. Aspen is very pale and smooth; cedar has a distinctive aroma and more visible grain. Tongue-and-groove boards at 90–120mm width, typically 15mm thick, fixed with concealed stainless clips to allow movement. All fixings in the sauna must be stainless steel — standard steel fixings will rust.

Heater specification: Electric sauna heaters are rated in kW relative to the room volume. As a rule of thumb: 1 kW per 1 m³ of room volume, with a minimum of 6 kW for a residential room. A 2.4m × 2.0m × 2.1m room (approximately 10 m³) requires a 9–12 kW heater for rapid heat-up. Quality Finnish manufacturers — Harvia, Helo, Tylö, Narvi — dominate the residential market. Stone quality matters: the kiuas stones should be large (fist-sized or larger), dense, and heat-retentive (olivine diabase or peridotite are the correct stone types). Small or poor-quality stones produce harsh, dry steam rather than soft löyly.

Ventilation: Fresh air supply to the sauna (through a low vent near the heater) and exhaust (through a high vent on the opposite wall) is essential both for comfort and for drying the room after use. After a sauna session, the room should be ventilated with the door open until the interior is dry; this prevents mould and extends the life of the timber lining.

Steam Room Construction and Specification

Steam room construction is considerably more demanding than sauna construction because of the continuous moisture exposure.

Waterproofing: Every surface of the steam room — floor, walls, ceiling — must be fully waterproofed before tiling. The correct system is a tanked wet room approach: cement board or tile-backer board substrate, a liquid-applied waterproofing membrane (Schluter Kerdi, Wedi, or Mapei Mapelastic) applied in two coats with fabric reinforcement at all junctions, then tiled with a porcelain or ceramic tile using a flexible adhesive and epoxy grout. The ceiling must also be tiled (not plastered) and the waterproofing must be continuous across ceiling, wall, and floor junctions. Any breach in the waterproofing will allow water vapour behind the tiles, leading to adhesive failure, tile delamination, and structural damage.

Gradient and drainage: The floor must fall to a drain at a minimum gradient of 1:80. The ceiling should be sloped (at least 15° from horizontal) to direct condensation to the walls rather than dripping on occupants. A concealed linear drain at the base of the back wall, combined with a central floor drain, is the preferred arrangement for a well-specified steam room.

Steam generator and inlet: The steam generator should be located in an accessible plant cupboard adjacent to the steam room — not inside the steam room where it would be subject to the same moisture conditions it creates. The steam inlet (a nozzle in the lower part of one wall) should be positioned to distribute steam evenly across the room; it must be stainless steel and must not be positioned where occupants will come into close contact with the live steam outlet.

Lighting: Lighting in a steam room must be IP68 rated (submersible standard) — not merely IP65. The combination of heat, condensation, and direct water exposure means that anything less than IP68 will fail. LED fibre optic ceiling lighting, with the light source outside the steam room, is the premium option and avoids any electrical components within the steam environment.

Benching: Benching in a steam room can be tiled (matching the walls and floor) or timber (typically teak or iroko, which are dense, naturally oily hardwoods that resist moisture absorption). Tiled benches are easier to keep clean and more hygienic; timber benches are more comfortable to sit on. Structural support for benching must be built into the waterproofed shell — fixing through completed waterproofing and tiles is a common cause of leaks.

Ventilation and Environment Control

Both sauna and steam room require dedicated mechanical ventilation that is separate from the general building ventilation system. The ventilation must:

• —Remove humid air from the steam room after sessions to allow drying
• —Provide fresh air make-up during use
• —Control humidity levels in the surrounding corridor or changing area to prevent condensation on adjacent surfaces

The pool hall and wellness area dehumidification system (if a pool is also present) should be designed to handle the latent heat load from both the pool surface and the sauna/steam room ventilation. These loads must be calculated by the M&E engineer as part of the building services design.

Budget Framework

A residential sauna (bespoke, not prefabricated kit): £25,000–£50,000 fully installed including heater, lining, benching, and controls. A prefabricated Finnish sauna cabin installed within an existing basement room: £8,000–£20,000.

A bespoke tiled steam room: £35,000–£70,000 fully installed including waterproofing, tiling, steam generator, drainage, and lighting. The wide range reflects tile specification and room size.

Both figures exclude building work (forming the room within the basement structure), ventilation and M&E connections, and any associated changing or relaxation area.