Sauna glass and doors: design, safety and heat

The short answer

• Glass opens up a sauna visually and architecturally, but it is never free: it costs heater capacity, heat-up time and design effort.
• Size the heater for the glass. Harvia adds about 1.2 m³ to the design volume for every square metre of glass or other uninsulated surface. A fully glazed front can push a small room up one or more heater sizes.
• Use toughened safety glass made for the purpose. In Australia, glazing is governed by AS 1288:2021; the glazier and certifier confirm it.
• Doors open outward and never lock. Magnetic, friction or spring catches only.
• The door gap is part of the ventilation. Its size depends on how the room is ventilated, per the heater’s documentation.
• Large glass walls need planning: structure, airtightness, hot outer surfaces, sun and glare, privacy and cleaning.

What glass does for a sauna

A sauna has traditionally been a closed timber box. Glass changes that.

• Openness. A glass door or front lets light in and views out, so even a small sauna feels larger and less enclosed. Some people find a closed room confining; glass helps.
• Architecture. A glazed sauna becomes part of the bathroom, pool house or spa. The timber interior and its lighting are on show, so the sauna can be a feature rather than a cupboard.
• Supervision. Glass in or beside the door lets people see in. That matters with children at home, and it matters in commercial rooms where staff need to check on bathers.
• Connection. In a pool house or garden setting, glass connects the sauna to its surroundings, a view of the water or garden while you sit.

Harvia describes glass walls as an increasingly common option that brings spaciousness and a modern feel, and notes that a professional must assess whether a glass wall can be installed at the demolition stage of a renovation.

Privacy

Clear glass is not always wanted, especially where a sauna opens off a bedroom, ensuite or shared space. Options include:

• Satin (frosted) glass, which lets light through but prevents visibility. Harvia offers satin glass doors for this reason.
• Tinted glass, such as bronze or grey, which softens the view in and reduces glare.
• Partial glazing: a glass door in a timber wall, or a glazed strip rather than a full wall.
• Orientation: placing the sauna so the glass faces a private view, not a corridor or the street.

Glass and heater sizing

Why glass costs heat

An insulated sauna wall holds heat in. Glass does not. It has no insulation, so it keeps drawing heat out of the room for as long as the sauna runs. Brick, concrete, tile and stone behave similarly. Heater manufacturers therefore add notional volume for every square metre of these surfaces before choosing a heater.

Harvia’s method (as published in current heater manuals such as the Cilindro):

• add 1.2 m³ to the room volume for each square metre of non-insulated wall (brick, glass block, glass, concrete, tile);
• multiply the volume by 1.5 for log walls;
• then choose the heater whose published volume range covers the result.

Harvia’s own example: a 10 m³ room with a glass door needs about the same output as a 12 m³ room.

Other manufacturers use different allowances. Several use the same 1.2 m³ per square metre; one adds 1.5 m³; another adds 1 m³ per square metre of glass, tile or stone, and more for solid masonry. The manual for the selected heater governs.

A worked example

This example uses Harvia’s published method. It is illustrative only: the selected heater’s manual and the actual glass areas govern.

A sauna measures 2.0 m wide × 1.8 m deep × 2.1 m high inside, a timber volume of about 7.6 m³.

The same small room needs a heater for about 7.6 m³ with a timber door, but about 12.6 m³ with a full glass front and over 17 m³ with two glazed walls. That more than doubles the design volume. Each heater is rated for a minimum and maximum volume, so glass can move the selection up one or more sizes, with knock-on effects for the electrical supply and the control. See sauna heaters and stones.

Why this matters: a heater sized for the timber room alone works flat out, heats slowly and leaves the stones short of heat for löyly. Glass areas need to be known before the heater is chosen, not after.

Heat-up and comfort

Even with a correctly sized heater, a heavily glazed room takes longer to come up to temperature. The glass itself stays cooler than timber on the inside, so bathers near it feel less radiant warmth.

The outside of the glass gets hot

Sauna glass is in contact with very hot air on one side, and its outer face warms up too. One European heater maker states that touchable glass surfaces on the outside of a sauna should not exceed 76 °C, with protection fitted if needed.

Why this matters: a glass front facing a living area, hallway or pool deck is a surface people, and especially children, may touch. Position glass with this in mind and ask the heater and glass suppliers how hot it will run.

Safety glass and AS 1288

Purpose-made sauna doors use toughened (tempered) safety glass. Harvia’s sauna glass doors, for example, use 8 mm tempered safety glass. Toughened glass is much stronger than ordinary glass, and if it does break it shatters into small granules rather than sharp shards. Harvia’s installation instructions warn that a sharp knock, for example against the floor during installation, can shatter a tempered door, so it must be handled with care.

In Australia, glass in buildings is governed by AS 1288:2021 Glass in buildings – Selection and installation. Its scope covers selecting and installing glass subject to wind loading, human impact and special applications. Which glass type, thickness and fixing a particular sauna needs, including the effect of heat, is for the glazier, the glass or door supplier and the certifier to confirm.

Doors

Open outward, never lock

Every heater manufacturer we reviewed says the same thing. Harvia’s instructions say the sauna door must open outward and must not have a locking or latching system. Acceptable hardware is:

• magnetic catches;
• friction catches;
• spring- or gravity-loaded closers.

Harvia’s installation instructions also ask the installer to loosen the roller latch so a new door cannot lock itself the first time it is closed.

Why this matters: someone who feels faint or unwell in the heat must be able to get out by pushing the door, without finding a handle or turning a key. A door that swings outward also cannot be blocked by a person who has collapsed against it.

Some sauna controls offer a door switch, which supports safe remote and timer starts and, in commercial rooms, monitoring. It detects whether the door is open; it does not lock it. See sauna controls explained.

Size and fitting

Sauna doors are usually narrower and lower than house doors, which keeps heat in. Harvia’s standard glass doors are named by nominal width × height in decimetres (8×19 is roughly 0.8 × 1.9 m). Its current data sheets give these sizes:

All five have 8 mm tempered glass, come left- or right-handed, and have frames 92 mm deep in timber or 60 mm deep in aluminium. Harvia’s installation instructions also list a minimum 1000 mm opening width for its 10× sizes. Other door makers and custom doors have their own figures, so set out from the selected door’s data sheet.

Opening height. Harvia does not publish one rough-opening height, because the frame stands on supports at whatever under-door gap the design needs. Its installation instructions ask for:

• about 10 mm between the top of the frame and the opening, with room for wedges at both sides;
• the frame fixed on the hinge side first (top screw first), then wedged and fixed on the handle side, with an even gap between glass and frame all round;
• the end grain of timber frames and trims treated with a sauna protective agent;
• care with fixing and sealing in porous walls such as lightweight block, checked with the supplier.

The opening height is therefore the frame height, plus about 10 mm at the head, plus the gap under the door. As illustrative arithmetic only, an 8×19 door with a 25 mm gap needs an opening of about 1925 mm, and with a 100 mm gap about 2000 mm.

The gap under the door: Harvia’s documents differ. Harvia’s 2015 planning and installation guide allows no more than 25 mm. Its 2022 glass door installation instructions recommend 100–200 mm under a sauna door for ventilation (about 10 mm in other rooms). Separately, Harvia’s heater manuals require at least 100 mm, with mechanical exhaust, where the sauna exhausts into a washroom under the door. Set the gap from the ventilation design for the selected heater and the instructions supplied with the door.

Why this matters: the under-door gap changes both the opening the builder frames and how air moves through the room, so it needs to be settled before framing, not on site.

Swing. The door opens outward, so plan clear floor space outside for the leaf to swing (about 650–850 mm for these sizes) and choose the handing so it does not open into a walkway or against a fixed object.

Harvia notes that the door seal on its glass doors keeps heat in the sauna rather than leaking to the bathroom.

The door gap and ventilation

The gap under a sauna door is part of how air moves through the room, so its size follows the ventilation design, not habit.

• In a standard installation, Harvia’s 2015 planning guide allows up to about 25 mm under its glass doors, while its 2022 glass door instructions recommend 100–200 mm for sauna ventilation. The selected heater’s ventilation design decides which applies.
• Where the sauna is designed to exhaust into a washroom under the door, Harvia requires a gap of at least 100 mm and mechanical exhaust in the washroom. Another maker gives a smaller gap for the same route; with Harvia equipment, Harvia’s figure applies.
• A door gap is a poor fresh-air inlet unless the heater stands beside the door, because air entering under the door does not reach a heater on the far side of the room.

Our guide to sauna ventilation explains how the gap fits the overall scheme.

Handles

Handles on sauna doors are usually timber. Timber stays comfortable to hold in a hot room, while metal conducts heat. Harvia, for example, offers a long vertical timber handle in aspen or alder as an alternative to a chrome knob on its sauna glass doors. Provide a handle on both sides so the door can be pulled from outside and pushed or pulled from inside.

Hinges and frames

Hinges and fixings must cope with heat and, on doors shared with a bathroom or steam room, moisture. Harvia uses stainless or acid-proof hinges on its glass doors, with frames in pine, aspen, alder or aluminium. Harvia’s North American manuals also advise sealing timber around door glass on both sides to limit warping.

Large glazed walls

Floor-to-ceiling glass fronts and glass corners are among the most striking saunas, and the most demanding to build. Beyond heater sizing, they raise several design questions:

• Structure. Large glass panels are heavy and need suitable framing, fixings and floor support. Harvia notes that additional structural support may be needed. Issue the loads to the structural engineer early.
• Airtightness. One manufacturer states that glass sections must be airtight, because leaks around glazing upset the ventilation design.
• Heater in front of glass. Where the heater stands in front of a glass wall or in the middle of the room, the supply air cannot come from the wall behind it. One European heater maker feeds fresh air from below through a floor duct in this layout, which has to be designed into the floor early.
• Clearances. The heater’s safety distances still apply, whatever the glass is.
• Lighting and reflections. At night a glass wall becomes a mirror. Concealed, low lighting reduces reflections inside and makes the sauna a soft glow from outside. See sauna lighting.

Outdoor saunas and the Queensland sun

Glass on an outdoor sauna, or one facing a sunny window, adds questions of its own:

• Solar heat and glare. Sun through glass heats the room and can make it uncomfortable to look out of during the day. Orientation, eaves and tinted glass help.
• Timber colour. Harvia notes that sunlight is one of the things that darken sauna timber over time, so sun-struck benches and walls will change colour faster.
• Weather. Exposed glass and frames need to shed rain, and the junction between glass, frame and wall needs to be weathertight.
• Glass selection. The glazier considers sun exposure along with the heat inside the room when selecting the glass.
• Privacy. A glass front that looks onto the garden by day can be on show at night.

For freestanding outdoor saunas, see our guide to outdoor sauna kits in Queensland.

Steam room glass is different

Steam rooms face constant moisture rather than dry heat, and their doors and glass follow different rules. Harvia specifies 8 mm tempered glass, 8 mm double-layer laminated and tempered glass, or polycarbonate for steam room doors, with a gap of about 15 mm under the door for supply air. Some other manufacturers use gasketed, sealed doors instead, and some specify double-glazed windows. Which approach applies depends on the steam generator system. See steam room design and construction.

Cleaning glass

Sauna glass shows water marks, fingerprints and the residue of löyly splashes. Manufacturers’ care guidance is simple: clean it like other glass. One heater maker suggests ordinary window or dish cleaner on the glass as part of routine cleaning. Clean when the room is cool, avoid abrasive pads that can scratch, and keep cleaners off the timber frame and benches. Our guide to caring for your sauna covers the rest of the room.

Timber door, glass door or full glass front

None of these is the right answer for every room. The decision is a trade between openness, privacy and heater size, made early so the heater, structure and ventilation are designed around it.

Questions to settle at design stage

1. How much glass, and where? Measure every square metre for heater sizing.
2. Clear, tinted or satin, for the privacy each side of the glass needs?
3. Who touches the outside of the glass, and how hot will it get?
4. Does the heater stand in front of the glass, and how will fresh air reach it?
5. What role does the door gap play in the ventilation?
6. Which door hardware: catch type, timber handles, hinges, frame?
7. Is the glass exposed to sun, and how will glare and heat gain be managed?
8. Who confirms the glass under AS 1288:2021: glazier, supplier and certifier?

If you are planning a glass-fronted sauna, talk to us before the heater and glazing are chosen. Our guide to planning a home sauna covers the rest of the room.