Sizing a Sauna Heater When a Wall Is Glass

Updated: 11 November 2026

Short answer: at Sauna Dekor Spa Solutions we size every heater for the room as built, because a glass, stone or concrete wall absorbs heat that a timber wall would not, and a sauna with a glass front needs a heater sized for a larger room than the one on the drawing. Harvia publishes the most quoted rule: add about 1.2 m³ of room volume for every square metre of glass, stone or concrete. Tylö and HUUM use about 1 m³. EOS does not publish a single figure, so EOS heaters are sized from the maker's own tables.

Why does glass change the heater size?

A traditional sauna is a timber box. The lining warms up quickly and gives very little heat back to the structure behind it. Glass behaves differently: it conducts heat to the outside, it never feels as warm as timber and it keeps drawing heat while the room is in use. Stone and concrete store heat, so they take a long time to come up to temperature.

The result is the same in both cases. If the heater is chosen only from the cabin's cubic metres, the room is slow to heat and struggles to hold its set point. In a Finnish sauna that runs at 80–100 °C, that is the difference between a room that is ready in a reasonable time and one that never quite gets there.

What do the heater makers say?

Each maker writes its own rule, and they are not the same:

Harvia: add about 1.2 m³ to the room volume for every m² of glass, stone or concrete surface.
Tylö and HUUM: about 1 m³ per m² of such surfaces.
EOS: no single published figure; use the selection tables for the specific heater.

A solid masonry wall can need more than these figures. Double glazing is sometimes treated as a reason to reduce the allowance, but that is a designer's judgement, not a manufacturer's rule, so it should not be written into a specification as if it were one.

A worked example, in words

Take a cabin of 2.0 × 2.0 m with a ceiling at 2.1 m. That is 8.4 m³ of air. Now give it a full glass front, 2.0 m wide and 2.0 m high: 4 m² of glass. With Harvia's rule, the design volume becomes 8.4 + (4 × 1.2) = 13.2 m³. The heater is then chosen from the maker's table for 13.2 m³, not 8.4 m³.

With a Tylö or HUUM heater the same cabin would be sized for about 12.4 m³. With an EOS heater, the selection table for the chosen model decides. The point is that the correction belongs to the heater maker, so the correction and the heater must come from the same source.

What else changes with a glass front?

Control unit: it goes outside the cabin, on a reachable wall; only the temperature sensor sits inside. It is chosen with the heater, not afterwards.
Electrical supply: a larger heater may need a larger supply. This has to be known before the electrician runs cables.
Ventilation: fresh air enters low near the heater and leaves high on the opposite side. A glass wall does not change this, but it limits where the outlet can go.
Bio mode: if the cabin will also run as a bio sauna at 50–65 °C, a combination heater with an evaporator is chosen from the start.

What should you send before a heater is chosen?

Inside dimensions of the cabin and the ceiling height.
The area of each glass, stone or concrete surface, marked on the drawing.
Whether the sauna is for a home or for a hotel or club, because use hours change the choice.
The electrical supply available at the cabin.
Your preferred heater maker, if you have one.

We have used EOS heaters and controls for about 20 years and keep spare parts in stock in Istanbul for the equipment we install. The full method, with the rules side by side, is on our page about sauna heater sizing.

Frequently asked questions

Does a small glass door need a correction?

A standard glass door is usually covered by the maker's normal table. The correction matters when a whole wall, or a large part of one, is glass or stone.

Can I use one rule for every heater?

No. The correction is part of each maker's selection method. Mixing Harvia's figure with another maker's table gives a number neither maker stands behind.

What warranty comes with a sauna room?

On the rooms we build: 24 months on workmanship and manufacturing, a 24-month manufacturer's warranty on electrical and mechanical equipment, and 5 years on structure and waterproofing.