The glass sauna: one wall of view, or two meeting at a corner

Why glass, and what it costs you in heat

Glass does three things timber cannot: it shows the garden, it makes a small cabin feel twice its size, and in a club or hotel it lets staff see the room without opening the door. It also loses heat quickly. Single toughened glass has a U-value of roughly 5.7 W per square metre per kelvin; an insulated timber wall is closer to 0.4. At 90 °C inside and 20 °C outside, a 2 m² glass wall gives up about 800 W, which is a lot of heater for a surface you cannot sit against.

So the heater is sized from the cabin volume and then raised for the glass, using the maker's tables. It is one of the details that separates a glass sauna that reaches 90 °C in about forty minutes from one that never quite gets there. The glass-and-energy note has the arithmetic.

One glass wall with the door, or two at a corner

• One full glass wall with the door. The sober version: the door and a fixed panel in one frameless plane, three solid timber walls behind you. Best against a wall, in a spa room, or where the neighbours are close.
• Two glass walls meeting at a corner. Our usual drawing. The corner opens the cabin to a diagonal view and takes the room's darkest angle and turns it into its brightest. Best on a terrace and at the edge of a garden.
• Glass door only. A cheaper step for cabins inside a bathroom or basement, where there is nothing to look at. Still a glass door, never a solid one.

From one seat to twelve

Up to six seats is a household. From eight seats the project is usually a business, and the heater is a three-phase machine: see villa mains or three-phase before you choose a cabin for a club.

What is behind the glass

Two levels of slatted benches in pale cedar or thermally modified ash, with aspen or ash backrests that stay comfortable to touch. A Harvia heater with a stone basket, a bucket and a ladle, a thermometer and a hygrometer on the wall, and low lighting behind the benches. The door is toughened glass in a frameless leaf. It opens outward and has no lock a bather can be trapped behind: by convention and by common sense, that is how a sauna door works.

Specifying the glass: five lines to ask for in any quote

1. Glass type and thickness, and that it is toughened safety glass. Ask for the declaration of conformity.
2. Edge and corner treatment. Polished edges, and a corner joint that is sealed and supported rather than just butted.
3. Hinges, handle and fixings in stainless steel, marine grade if the cabin is within a few kilometres of the sea.
4. Door seal and threshold. The door should close on a seal that survives heat and does not trap water.
5. The swing. A drawing showing which way the door opens and what it clears. A sauna door opens outward.

Looking after glass in a sauna

The glass fogs a little when the cabin cools and clears as it heats; that is physics, not a fault. Clean it with a microfibre cloth and a mild cleaner, never with abrasive pads, and wipe the seals now and then. Near the sea, check the hinges and fixings once a year: they should be stainless steel of marine grade. Salt air, timber, glass and steel lists what to look at.

Each size has its range in the price list; the kit builder shows what a cabin and a plunge pool come to together, with the heater and the chiller already sized.