Sauna installation: what's involved and how to choose an installer

A full sauna installation is survey and siting, base preparation, delivery and access, assembly, insulation and vapour barrier, cladding and benches, heater and guard, the electrical supply with Part P certification, ventilation, first firing and handover. On a straightforward outdoor cabin with an existing base that is two to five days on site; with groundworks and a new cable run, usually two to three weeks end to end across several visits.

Two stages cause nearly all the problems that surface later: the vapour barrier and the ventilation. Both are invisible once the cabin is finished, and both are what separates a proper install from a cheap one.

Stage by stage

1. Survey and siting

A competent installer wants to see the plot, not a photo — checking access width along the whole route, ground conditions and levels, the route for the electrical run, boundary distances against the 2.5 m permitted development height trigger, and where the door, glass and any cold plunge should sit. Get this right and everything downstream is cheaper. See where to put a garden sauna, or run the plot through the fit checker first.

2. Base preparation

A loaded outdoor cabin runs comfortably over a tonne on a small footprint. Standard options are a compacted sub-base with paving slabs, ground screws, or a reinforced concrete pad — level to a few millimetres, load-bearing, and arranged so the floor is ventilated and the timber can dry. Concrete needs curing time, so this is a separate visit rather than day one of the build.

3. Delivery, access and assembly

Kerbside delivery is a pallet on the pavement; delivery to position is a different service at a different price. If the largest panel or an assembled barrel will not pass the narrowest point of the route, the cabin goes over the house or fence on a hiab — which needs reach, a legal place to park and occasionally a permit. Confirm the lift is feasible in writing before ordering, and agree who bears the cost if access fails on the day.

Kit builds then go together in sequence — floor, walls, roof, door, benches, heater — in one to four days for two people, depending on format. Bespoke builds are framed and clad on site or assembled from off-site modules: longer, more coordination, more scope for the design to be exactly right. See bespoke sauna for when that is worth it.

4. Insulation and the vapour barrier

This is the stage that decides whether the cabin is still sound in fifteen years. The correct build-up on a traditional sauna, from the outside in, is: external cladding, breather membrane, frame with mineral wool insulation (typically 50–100 mm in walls, more in the ceiling), a continuous aluminium foil vapour barrier on the warm side, battens forming a small ventilated cavity, then internal tongue-and-groove cladding.

The foil does two jobs. It stops the very high vapour pressure inside a sauna pushing moisture into the insulation, where it condenses, soaks the mineral wool and rots the frame. And it reflects radiant heat back into the room, so the cabin heats faster and holds temperature better.

Where self-builds and cut-price installs go wrong, in order:

• Omitting it entirely on the assumption that a sauna is “dry heat”. It is not — a ladle on the stones puts a great deal of moisture into the air.
• Substituting polythene DPM for foil. It does not reflect and is not specified for the temperatures involved.
• Discontinuous coverage — walls done, ceiling skipped. The ceiling is the hottest, wettest surface in the room.
• Unsealed laps and penetrations. Every seam needs lapping and taping with aluminium foil tape; every cable, vent and light penetration needs sealing.
• No cavity between foil and cladding, so the timber has no path to dry.

5. Cladding and benches

Internal cladding and benches want timber that stays cool to the touch, resists resin bleed and does not split — aspen, alder, thermally modified aspen, western red cedar or obeche. Benches sit on a frame with gaps between the boards for drainage and airflow, and fixings are hidden: a visible screw head on a bench at 85°C is a burn waiting to happen.

6. Heater, guard and stones

Heater sizing is roughly 1 kW per cubic metre of cabin volume for a well-insulated indoor room, with an uplift for glass, uninsulated walls and outdoor exposure — a glazed outdoor cabin can need half again as much. Undersize it and the room never gets properly hot; oversize it and you get scorching air over cold benches plus an electrical supply you did not need.

The heater is fixed at the manufacturer’s clearances to combustibles, with a timber guard rail against accidental contact, and stones are stacked loosely so air passes through them — packed stones choke the element. A wood-fired heater adds a hearth, a twin-wall insulated flue with correct clearances and flashing, and a dedicated combustion air supply.

7. Electrical supply and Part P

A traditional heater needs a dedicated radial circuit from the consumer unit, sized to the heater. A 6 kW single-phase heater typically means 6 mm² cable on a 32 A protective device; 8–9 kW pushes towards 10 mm² and 40 A, and on some older domestic supplies that requires a load assessment first. Outdoor runs use steel wire armoured cable, buried at the appropriate depth with warning tape and terminated at an external isolator.

Inside the cabin, only the heater, its sensor, the light and the controller belong in the hot zone, wired in heat-resistant cable; the controller normally sits outside. A room containing a sauna heater is a special location under BS 7671, and installing a new circuit is notifiable under Part P of the Building Regulations in England and Wales. Expect an electrical installation certificate and a building control notification — see home sauna electrical requirements.

8. Ventilation

Small, cheap, and the thing most often skipped. The conventional gravity arrangement is an inlet vent low, near or underneath the heater, so incoming cool air is drawn straight up through the heat, and an outlet vent high on the opposite wall, with the outlet at least as large as the inlet and commonly around twice the area. That sets up a circulation loop across the room instead of a stagnant hot layer at the ceiling. Some installations use mechanical extract with the outlet low on the opposite wall; either works, a random hole does not.

Get it wrong and the cabin is stuffy, hot at head height and cold at the feet, unpleasant after twenty minutes and slow to dry. A closable outlet, plus the habit of leaving the door ajar afterwards, is what keeps the timber dry between sessions.

9. First firing, curing and handover

New heaters carry manufacturing residues on the elements, so the first run is done hot, empty and well ventilated for an hour or two before anyone uses the room. Handover should cover heat-up times, ventilation settings, cleaning, and the warranty position on cabin, heater and electrical work separately.

Typical timescales

Job	On site	End to end
Indoor kit in a prepared room	1–2 days	1–3 weeks including electrics
Outdoor barrel on an existing base	1–2 days	2–4 weeks including delivery lead time
Panel-built insulated outdoor cabin	2–5 days	4–10 weeks including base and electrics
Bespoke build	1–3 weeks	8–20 weeks from design sign-off

Lead times, not labour, dominate: most of the calendar is manufacture and delivery.

Choosing an installer, and what a quote should say

There is no single sauna-specific licence in the UK, so you are assessing a combination of trades. Look for sauna-specific experience evidenced by photographs and callable references; a registered electrician on the job — NICEIC, NAPIT, ELECSA or equivalent — who will issue the certificate; HETAS registration if a wood-fired heater and flue are involved; public liability insurance confirmed in writing; and a clear division of responsibility where supplier, installer and electrician are three different companies. Ask directly how the heater was sized for your cabin, where the vents will go, and what happens if the cabin will not fit through the access on the day.

A quote that is one line and one number cannot be compared with anything. A proper one itemises:

Line	What to look for
Base	Specification and thickness, or explicitly excluded
Cabin	Footprint, internal height, wall build-up, insulation thickness
Vapour barrier	Material named — foil, not “membrane”
Cladding and benches	Species and thickness
Glazing	Toughened, area, door only or panels
Heater	Make, model, kW, kilograms of stone
Ventilation	Inlet and outlet positions and sizes
Electrical	Circuit, cable size, isolator, certification, who issues it
Delivery and labour	Kerbside or to position, hiab, days and crew
Warranty and payment	Scope by element, deposit, stage payments, VAT

Red flags in a sauna installation quote

• A large deposit — more than 25–30% — with no stage payments and no protection.
• No mention of the vapour barrier anywhere in the specification.
• “You’ll just need to get a sparky in”, with no certificate in scope.
• A quote assuming kerbside delivery without anyone having looked at your access.
• A heater sized by footprint alone, ignoring ceiling height and glazing.
• Pressure to decide on the visit, or cash only, no VAT — which also means no paper trail.

Getting a price you can rely on

Costs vary far more with access, ground conditions and cable run than with the cabin itself, which is why an honest number needs your specifics. Put your size, format, access route and distance from the consumer unit into the sauna cost calculator for an itemised estimate, then use it as the baseline when you get quotes from installers — three, against a single written specification. Price bands are on home sauna cost, formats on barrel vs cabin, and consent on garden sauna planning permission. If a cold plunge is going in alongside, plan both together — cold plunge installation shares the trench and often the base.