Short answer: How much space a home sauna really needs, which timbers survive the thermal cycle, how ventilation is actually calculated and what a session costs to run.
· Wellness · 9 min
The home sauna is the wellness installation we most often find badly resolved. Not because it is difficult, but because it is bought like furniture and installed like furniture. A sauna is a thermal envelope with an 8 kW appliance inside it: treat it as a wardrobe and within two years the timber cracks, the ceiling of the adjacent bathroom stains and the cabin never quite reaches temperature.
This is the technical brief we hand clients before drawing anything.
The usual mistake is shrinking the depth. For two people seated and one lying down you need an upper bench 60 cm deep and 190 cm long, which fixes 1.8 × 1.6 m as a sensible minimum. Clear internal height belongs between 2.00 and 2.10 m: higher wastes energy heating air nobody uses, lower puts your head into the hottest stratum.
The upper bench must sit level with the top of the heater stones. That single rule is what delivers heat to your torso instead of only your feet.
Sauna timber cycles between 20 °C and 100 °C several times a week, so it must be low-conductivity, resin-free and dimensionally stable:
Internal surfaces are never varnished or lacquered. Varnish seals the pore, moisture is trapped and the boards cup. The correct finish is a specific paraffin oil, or nothing at all.
Behind the cladding sit 50-100 mm of mineral wool and, over it, an aluminium foil barrier with every joint taped, always on the warm side. Battens then leave a 10-15 mm ventilated cavity between barrier and timber. That cavity prevents interstitial condensation; it is also the first thing dropped to save money, and the reason stains appear on the ceiling next door.
A sauna needs 6 to 8 air changes per hour. An inlet under the heater roughly 10 cm off the floor and an outlet on the opposite wall, under the upper bench or 20 cm below the ceiling, establish the convective loop that distributes heat. With no outlet there is no circulation: just a scorching layer at head height and cold ankles.
The outlet must discharge outside or into a dedicated duct, never into the adjoining bathroom. And the door always opens outwards, with no lock.
In an urban home, electric: start at 1 kW per m³ of cabin and correct upwards for glazing or an external wall. A 6 m³ cabin asks for 6-8 kW, meaning a dedicated circuit and, above 8 kW, usually three-phase.
Wood-fired makes sense in a detached house with a certified flue, safety distances and its own combustion air supply. The heat is more enveloping and the ritual is different, but it demands maintenance and a serious flue strategy.
A well-insulated 8 kW cabin heats in 30-45 minutes and then cycles at 30-40 % duty. That means roughly 4-6 kWh per session. At three sessions a week the annual energy cost is modest next to an indoor pool or a spa. The real cost sits in the installation being right, not in using it.
Placing the cabin against an external wall without resolving the thermal bridge; putting a floor drain inside the cabin that dries out and returns odours; using halogen downlights instead of luminaires certified for high temperature; mounting the heater closer than the minimum clearance to the bench; forgetting the gully in the adjacent shower area.
We design the sauna as part of the whole wet suite: contrast shower, rest area, ventilation and acoustics are resolved together, not in pieces. We work the envelope with the installer before choosing timber, and we hand the construction detail to the trade so nobody improvises on site.
If you are weighing up a home sauna and want an honest technical read on your space, write to us and we will tell you whether the room you have can take it.
A comfortable two-person cabin starts at 1.8 × 1.6 m with 2.10 m clear height. Below 1.6 m depth you cannot fit a 60 cm upper bench and the session stops being comfortable. Allow another 10 cm all round for framing and insulation.
Aspen or thermally treated spruce for benches and backrests: they stay cool to the touch and carry no resin. Western red cedar for wall cladding if you want aroma. Avoid knotty pine on benches, where resin in the knots reaches scalding temperatures.
A well-insulated cabin with a 6-8 kW electric heater uses roughly 4-6 kWh per session: 30-45 minutes at full output to heat up, then cycling. From 8 kW upwards you normally need a three-phase or a properly sized dedicated supply.
A prefabricated cabin that touches no structure and only needs a new electrical circuit is usually a minor-works notification. Cutting ventilation openings in the façade, altering general services or moving a bathroom pushes the job into full consent with a technical project.