Indoor Saunas: What to Know Before You Buy

Buying an indoor sauna is one of the most significant wellness investments you can make for your home. It is also one of the most misunderstood. The market is flooded with cheap flat-pack units that look passable in photographs but deliver a mediocre experience, poor heat retention, and a limited lifespan. At the other end, you will find genuinely well-engineered saunas built to last decades — but telling the two apart requires knowing what to look for.

This guide covers everything you need to understand before committing to an indoor sauna: the practical differences between indoor and outdoor installations, the space and ventilation requirements that many buyers overlook, the electrical work involved, and the construction details that separate a quality sauna from a glorified wooden box.

At Wellness One, we supply premium saunas for indoor and outdoor homes. Our outdoor Connect, Lifestyle, and Therma ranges are built with 200mm insulated walls and thermo aspen interiors — the standard we hold for UK sauna construction. Whether you choose indoor or outdoor, this guide will help you make a properly informed decision.

Indoor vs Outdoor: The Fundamental Differences

The first decision is location, and it shapes everything that follows.

An outdoor sauna sits in your garden as a standalone structure. It has its own foundation, its own ventilation to the outside air, and its own weather-resistant exterior. The sauna experience tends to be more traditional — you step outside, heat the cabin, and the contrast between the cold outdoor air and the hot interior is part of the ritual. Outdoor saunas also benefit from natural ventilation and have no impact on your home's internal humidity or temperature.

An indoor sauna is installed inside your house — typically in a spare room, basement, garage conversion, loft, or purpose-built extension. The obvious advantage is convenience: you do not need to walk outside in January to use it. It is also easier to integrate into a daily routine, and you have immediate access to a shower.

But indoor installation introduces several complications that outdoor saunas avoid entirely:

Ventilation. An outdoor sauna ventilates naturally to the outside. An indoor sauna dumps heat and moisture into your home unless you engineer proper ventilation — and getting this wrong can cause serious structural damage over time.

Moisture management. A sauna session generates significant humidity. Outdoors, this dissipates harmlessly. Indoors, it can condense on walls, ceilings, and within the building fabric, leading to mould, rot, and damp problems.

Floor loading. A properly built sauna cabin is heavy. If you are installing in an upper floor or loft, you need to verify that the floor structure can handle the weight of the cabin, the heater, the stones, and the occupants.

Fire safety. A sauna heater reaches temperatures that demand proper clearances from combustible materials. In an enclosed indoor space, Building Regulations compliance becomes more critical.

Electrical supply. Both indoor and outdoor saunas need dedicated electrical circuits, but indoor installations sometimes have shorter cable runs, which can simplify the electrical work slightly.

Space Requirements

The minimum practical size for a sauna that seats two people comfortably is approximately 1.5 metres by 1.5 metres internally. This allows two people to sit side by side on a bench, with enough room for the heater and appropriate safety clearances.

For a family-sized sauna — seating four to six people with upper and lower benches — you are looking at an internal footprint of roughly 2 metres by 2 metres, or 2 metres by 2.5 metres if you want lounging benches.

But the room itself needs to be larger than the sauna cabin. You need clearance around the exterior for:

Access panels for maintenance and electrical connections
Air circulation around the cabin exterior (critical for preventing moisture buildup between the cabin walls and the room walls)
Door swing clearance
A minimum distance from the room's own walls to prevent heat transfer damage

As a practical rule, allow at least 100mm clearance on all sides of the cabin, and ideally 200mm or more at the rear where ventilation ducting may be routed.

Ceiling height matters too. A sauna needs a minimum internal ceiling height of around 2.1 metres to function properly — heat rises, and you want the hottest air at bench level, not trapped above your head. If your room ceiling is unusually high, the sauna will be less efficient because you are heating a larger air volume above the bathers.

Ventilation: The Detail Most Buyers Get Wrong

Ventilation is the single most important technical consideration for an indoor sauna, and it is the one most frequently botched.

A sauna needs fresh air intake and used air exhaust. Without proper airflow, the oxygen in the cabin depletes during use (making the experience unpleasant and potentially dangerous), moisture cannot escape, and the air becomes stale and heavy.

The standard arrangement is:

Fresh air inlet: positioned low on the wall, typically near or behind the heater. The heater warms the incoming air, which then rises naturally. This inlet should draw air from outside the building or from a well-ventilated adjacent space — not from a sealed room.

Exhaust outlet: positioned high on the opposite wall from the inlet, or on the ceiling. This allows the hottest, most moisture-laden air to escape. The exhaust should vent directly to the outside via ducting — not into an internal room, hallway, or roof space.

Some installations use a mechanical exhaust fan to ensure consistent airflow, particularly where natural convection is insufficient due to the duct routing. This is common in basement installations where the exhaust must travel upward and horizontally to reach an external wall.

Getting ventilation wrong does not just affect the sauna experience. Over months and years, inadequate moisture extraction from an indoor sauna can cause:

Black mould growth on adjacent walls and ceilings
Rot in timber framing within the building structure
Peeling paint and wallpaper in adjacent rooms
Condensation damage to electrical fittings
Musty odours that permeate the surrounding area

If your builder or sauna supplier cannot explain the ventilation strategy in detail, treat that as a serious warning sign.

Electrical Requirements

Every sauna heater requires a dedicated electrical supply. This is not optional — you cannot run a sauna heater from a standard plug socket.

For a small to medium sauna (internal volume up to about 8 cubic metres), you will typically need a heater rated between 6kW and 9kW. This requires a dedicated 32A or 40A circuit, run directly from your consumer unit (fuse board) to the sauna heater via appropriate cable — typically 6mm or 10mm twin and earth, depending on the cable run length and the heater rating.

For a larger sauna, heaters of 10kW to 15kW or more may be needed. These often require a three-phase electrical supply, which most domestic properties in the UK do not have as standard. If you are considering a larger indoor sauna and your property is on a single-phase supply, you will need to factor in the cost of upgrading — which can be significant and requires application to your District Network Operator.

All sauna electrical work must be carried out by a qualified electrician registered with a competent person scheme (such as NICEIC or NAPIT). The installation must comply with BS 7671 (the IET Wiring Regulations), and there are specific requirements for electrical installations in sauna rooms — including the zones within the sauna where different types of cable and equipment are permitted.

Key points:

The heater must have its own dedicated circuit with appropriate RCD protection
All wiring within the sauna cabin must be heat-resistant (typically silicone-sheathed cable rated to at least 170 degrees Celsius)
The heater control unit is usually mounted outside the sauna cabin
Light fittings inside the sauna must be suitable for the operating temperature
No standard plug sockets are permitted inside the sauna cabin

Heater Types

There are three main categories of sauna heater, each with distinct characteristics.

Traditional electric heaters are the most common choice for domestic saunas. They heat sauna stones, and you pour water over the stones to generate steam (loyly). They are reliable, relatively affordable, and available in a wide range of sizes. Quality brands include Harvia, Helo, and Narvi. Look for heaters with a good stone capacity — more stones means more thermal mass, smoother heat, and better steam production.

Wood-burning stoves provide the most traditional sauna experience. The heat quality is widely considered superior — softer, more enveloping, with a distinctive character. However, wood burners are rarely practical for indoor installation in the UK. They require a flue that must be routed through the building structure and roof, they need a hearth and appropriate fire-resistant surroundings, and they produce emissions that may fall foul of Clean Air Act regulations in smoke control areas. For indoor saunas, electric is almost always the practical choice.

Infrared panels are sometimes marketed as saunas, but they work on a fundamentally different principle. They heat your body directly with infrared radiation rather than heating the air in the cabin. The air temperature in an infrared cabin is much lower (typically 40-60 degrees Celsius versus 70-100 degrees in a traditional sauna), and you cannot produce steam. Whether an infrared cabin qualifies as a sauna is debatable — many sauna purists would say it does not. Infrared units do have lower electrical requirements and produce less moisture, which makes them simpler to install indoors, but the experience is not comparable to a traditional sauna.

It is worth noting that sauna use is not suitable for everyone. People with unstable cardiovascular conditions, very low blood pressure, or who are pregnant should consult their doctor before using a sauna. Alcohol and sauna use should never be combined.

Construction Quality: What Separates Good from Bad

This is where most buyers go wrong, because most buyers cannot see inside the walls of a sauna cabin. The visible interior — the benches, the panels, the heater — is only part of the story. The parts you cannot see determine whether the sauna will still be performing well in 15 years or whether it will be rotting from the inside out within five.

Wall thickness and insulation. This is the single biggest differentiator. A cheap flat-pack sauna might have walls of 40mm to 60mm with minimal or no dedicated insulation. A properly engineered sauna has substantially thicker walls with high-performance insulation.

At Wellness One, our outdoor saunas are built with 200mm walls incorporating 100mm of dedicated insulation. This is the standard we consider the benchmark, and it is worth understanding why: thicker, better-insulated walls heat up faster, hold temperature more consistently, use less energy to maintain temperature, and resist moisture penetration far more effectively. When you are evaluating indoor sauna options, ask the manufacturer what the total wall thickness is and what insulation is used. If they cannot answer clearly, or if the total wall construction is under 80mm, the thermal performance will be compromised.

Interior wood species. The wood used on the interior surfaces — walls, ceiling, and especially benches — matters enormously. It must handle extreme temperature cycling without warping, cracking, or becoming dangerously hot to sit on. It must resist moisture absorption and the resulting fungal growth. And ideally, it should be pleasant to touch, look at, and smell.

Thermo aspen is widely regarded as the finest interior sauna wood available. The thermal modification process — heating the wood to high temperatures in a controlled, oxygen-free environment — fundamentally changes its cellular structure. This dramatically reduces its moisture absorption, improves its dimensional stability (meaning it resists warping and movement), increases its resistance to decay, and gives it a beautiful, warm colour. Thermo aspen also has a notably low thermal conductivity, meaning the bench surfaces remain comfortable to sit on even at high sauna temperatures.

Other acceptable interior woods include alder, which is traditional in Scandinavian saunas and has good properties but is less dimensionally stable than thermo-treated wood, and Western red cedar, which is aromatic and naturally rot-resistant but can be resinous and is not as thermally stable as thermo aspen.

Avoid saunas with interior surfaces made from spruce or pine that has not been thermally treated. These softwoods absorb moisture readily, are prone to resin bleeding at high temperatures, and will warp and crack over time.

Frame construction. The structural frame of the sauna cabin should be solid, well-joined, and designed to support the insulation without compression (compressed insulation loses its effectiveness). Look for frames made from kiln-dried timber with proper jointing rather than cheap metal brackets.

Vapour barrier. A critical and often overlooked component. The vapour barrier sits behind the interior panelling, on the warm side of the insulation. Its job is to prevent moisture from the sauna interior penetrating into the insulation and wall structure. Without an effective vapour barrier, moisture will condense within the wall construction, saturate the insulation, and eventually cause structural damage. Aluminium foil vapour barriers are the standard in quality sauna construction.

Door. The sauna door should be made from tempered glass (safety glass that breaks into small, harmless pieces rather than dangerous shards) with a wooden frame. Glass doors allow light into the cabin and make the space feel less claustrophobic. The door must open outwards — this is a safety requirement so that a person who feels faint can fall against the door and it will open rather than trap them inside.

Benches. Bench construction should use the same quality timber as the wall panels, with a slatted design that allows air circulation. Benches should be properly supported on wall-mounted brackets with adequate load-bearing capacity. The bench surface should be smooth and free from any metal fixings that could become dangerously hot.

What to Ask Before You Buy

Whether you are considering an indoor sauna from a specialist manufacturer or a more accessible flat-pack option, these are the questions that will tell you what you are actually getting:

What is the total wall thickness, and what insulation material and thickness is used?
What wood species is used for the interior, and has it been thermally treated?
Is there a dedicated vapour barrier, and what material is it?
What is the heater brand and wattage, and what is its stone capacity?
How is ventilation managed — where are the inlet and outlet, and what is the recommended exhaust routing?
Is the door tempered glass, and does it open outwards?
What guarantee is provided on the structure, the heater, and the electronics?
Can the manufacturer provide references from UK installations?
What are the total electrical requirements, including whether three-phase power is needed?
Does the quoted price include delivery, installation, and electrical connection, or are these additional?

Running Costs

The running cost of an indoor sauna depends on the heater size, the insulation quality, how often you use it, and your electricity tariff.

A typical domestic sauna with a 6kW to 9kW heater takes 30 to 45 minutes to reach operating temperature. A well-insulated sauna at the smaller end of this range might use around 6kW for 45 minutes of heating (4.5 kWh) plus perhaps 3kW average during a one-hour bathing session (another 3 kWh) — roughly 7.5 kWh per session. At current UK electricity rates of approximately 24p per kWh, that is around £1.80 per session.

A poorly insulated sauna will take longer to heat, lose heat faster during use, and cost substantially more per session. This is another reason why wall thickness and insulation quality are not just construction details — they directly affect your ongoing costs for the entire life of the sauna.

Indoor Sauna Installation Checklist

Before committing to an indoor sauna, verify the following:

The floor can support the weight (sauna cabin plus heater, stones, and occupants — typically 500kg to over 1,000kg depending on size)
There is a viable route to vent moisture-laden exhaust air to the outside
Your electrical supply can accommodate a dedicated circuit of the required rating
The room has adequate clearance around the sauna for maintenance access and air circulation
You have confirmed any Building Regulations requirements with your local authority
The room has a drain or is waterproofed to handle occasional water spillage
You have planned for a shower either in the same room or immediately adjacent

Why Outdoor Remains the Gold Standard

While indoor saunas offer undeniable convenience, there are good reasons why the traditional outdoor sauna remains the preferred choice for serious sauna enthusiasts and wellness practitioners.

The contrast experience — stepping from a hot sauna into cold outdoor air, or into a cold plunge pool — is central to the Scandinavian sauna tradition. This contrast is difficult to replicate with an indoor sauna.

Outdoor saunas eliminate all the moisture management and ventilation challenges of indoor installation. They can be larger without consuming valuable indoor living space. And there is something psychologically valuable about having a dedicated wellness space separate from the main house — a place you go to specifically for that purpose, which creates a ritual and a mental separation from daily life.

At Wellness One, our outdoor saunas — across the Connect, Lifestyle, and Therma ranges — are built to a standard that reflects over a decade of experience in the UK outdoor wellness market. Every cabin features 200mm insulated walls with 100mm of dedicated insulation, thermo aspen interior surfaces throughout, and thermo wood exteriors that handle the British climate year-round. These are not adapted indoor products placed outdoors — they are purpose-built for outdoor use from the ground up.

Whether you are leaning toward indoor or outdoor, we are happy to talk through your specific situation and help you understand what will work best for your space, your budget, and the experience you are looking for. You can browse our full range and get in touch at shop.wellnessonegroup.co.uk.