Sauna construction is not merely a technical process. It is a comprehensive skill that combines insulation, ventilation, materials, and spatial logic in such a way that the end result provides years of enjoyment.
This article on sauna planning is based on the practical observations and years of experience of HUUM’s architect Arden Arroval in planning both home and custom saunas.
According to Arden , a high-quality sauna is born from the details. It is the combined effect of small but critical decisions that determine the durability, safety, and quality of the sauna.
Arden highlights three of the most common construction mistakes: insufficient insulation, faulty ventilation, and choosing a heater with the wrong power.
He goes on to explain why these problems arise and how to prevent them in a smart way so that the sauna will provide years of safe and enjoyable use.
Sauna construction mistake #1: inadequate insulation
According to the architect, sauna insulation is “the be-all and end-all of sauna construction.” It determines how well the sauna retains heat, how efficiently the heater works, and how well the sauna’s structure withstands heat.
Faulty or inadequate insulation is one of the most common reasons why saunas do not work as they should, and it can lead to moisture damage.
Common mistakes in sauna insulation
Below are the most common mistakes made in insulation and related installation work that affect the quality and safety of saunas.
Improper sealing of the vapour barrier and its consequences
The hot and humid environment of a sauna requires insulation that is airtight and moisture-proof.
Common mistakes include poorly taped joints between insulation boards (e.g., foil-faced PIR boards) and unsealed ventilation openings and cable penetrations.
If foil taping and sealing of penetrations are rushed, the foil layer will not be complete and airtight.
Risk of condensation and mould due to inadequate insulation
Improperly sealed joints and penetrations allow moisture to accumulate in the structures, leading to condensation and mould, which damages the wood. These problems are usually caused by rushing the sealing process.
Lack of a crucial ventilation gap or incorrect solution
There must be at least a 20 mm air gap between the insulation foil layer and the interior finish (panelling).
It is a big mistake if the insulation foil layer touches the wall panelling, because without an air gap, there is no air circulation to prevent excess moisture from forming behind the panelling.
Consequences of poor insulation
Proper insulation is essential for a high-quality and long-lasting sauna experience. Below, we list the problems associated with poor or inadequate insulation, which affect both energy consumption and the comfort of using the sauna.
Impact on energy consumption:
- Higher heater power requirements.
- Higher energy consumption. Since uninsulated surfaces increase the power required by the heater, the energy consumed for heating and the associated costs also increase.
Impact on the quality of the sauna experience:
- Temperature stratification. Warm air accumulates under the ceiling, but a cooler layer of air remains on the floor (at foot level).
- Longer heating time.
Principles of sauna heating
The purpose of a sauna is to maintain a stable desired temperature in the room and provide a long and pleasant experience. To achieve this, proper internal insulation is necessary. The main requirements for heating a sauna are as follows:
1) Insulation materials should be heat- and moisture-resistant and compact:
- PIR board – the most common material is 30–50 mm thick PIR board with a foil layer.
- Mineral wool – an alternative is 25 mm thick mineral wool board with a foil layer. When using mineral wool, a separate insulation frame is required.
2) Moisture and vapour barrier
Insulation boards with a foil coating should be used, as they act as a vapour barrier and reflect infrared radiation.
3) Air gap
There must be an air gap of at least 20 mm between the insulation (foil layer) and the interior finish.
Sauna construction mistake #2: inadequate or incorrect ventilation
Incorrect or poorly thought-out ventilation planning is one of the most common mistakes in sauna construction. The architect emphasizes that a well-thought-out ventilation solution is necessary to ensure good air exchange, which is the cornerstone of a high-quality sauna.
Proper ventilation helps prevent oxygen deficiency, uncomfortable air stratification, and temperature drops on the lower steps of the sauna. You can read more about ventilation solutions here.
Problems usually start with the incorrect location of the air inlet or the fact that the ventilation system is not considered as a whole. For example, in a sauna with a wood-burning stove, the air inlet is often installed too high – the cold air does not come into contact with the stove stones and does not heat up sufficiently, which means that the necessary air circulation does not occur.
Often, the exhaust vent is left open during heating, which immediately lets all the heat out of the room.
Such solutions cause fatigue and lethargy in sauna users due to oxygen deficiency, promote temperature stratification, and in the case of wood-fired stoves, incorrect pressures and air flow directions can even promote the formation of dangerous carbon monoxide.
Therefore, ventilation must be considered already at the heater selection stage. In saunas with forced ventilation, it is important to avoid a situation where excessive negative pressure occurs in the room with a wood-fired stove and carbon monoxide can move in the wrong direction.
The fresh air inlet should always be located near the heater. It can be behind, below, or at a suitable height above the heater, depending on the type of heater, so that the incoming air can warm up before entering the circulation.
The right ventilation solution for the sauna and washroom
The ventilation system is selected depending on whether forced (mechanical) or natural (free-flow) ventilation is used.
Ventilation in the sauna
The inlet must be located near the heater and its height depends on the type of heater (for a shell heater, near the floor or above the heater; for a mesh heater, at the height of the centre of the heater) so that the fresh air can warm up and create sufficient air flow.
Air extraction and ventilation
The exhaust vent should be located on the opposite wall and either lower in saunas with forced ventilation (up to 60 cm from the floor) or higher in saunas with natural ventilation, with the final ventilation opening closed during sauna use but opened after to remove moisture.
Washroom solution
The washroom requires a separate ventilation system as part of the building’s general ventilation system to remove moisture, and the piping and air ducts must be insulated with diffusion-tight material to prevent condensation and dripping.
Sauna construction mistake #3: incorrectly selected heater power
According to the Arden, choosing a heater with the wrong power is a very common problem. The most common mistake is choosing a heater that is too small.
The size and construction of the sauna affect the choice of heater power. The power requirement of the heater increases if the room has glass windows or doors and heat-retaining brick, concrete, or log walls.
A common mistake is to choose a heater based solely on its appearance, assuming that power is not that important. The reality is quite the opposite: the design of the heater may be pleasing to the eye, but it is the right power that ensures a functional, comfortable and energy-efficient sauna.
If you want to know how to choose the correctly powered heater for the dimensions and construction of your steam room, see our comprehensive guide here.
Factors influencing the choice of a heater
1) Actual size/real capacity of the sauna room (VR): The suitable power of the heater depends on the width, depth and height of the room (VR=a×b×h). A steam room that is too large requires a powerful heater, which can be expensive or may not have sufficient electrical capacity.
2) Non-insulated surfaces: These surfaces absorb heat and require more power from the heater to compensate for the heat losses. To take them into account, an additional capacity (VS) is added to the real capacity.
- Glass surfaces (e.g. glass doors)
- Log walls
- Tiled surfaces
- Monolithic wood surfaces
- Windows
3) Insulation quality: The sauna must be properly insulated, even if it is used in summer or in warm climates, because the temperature in the room is higher than natural.
Consequences of choosing the wrong heater
An incorrectly selected heater has several fatal consequences, which affect both the efficiency of sauna heating and the quality of the sauna experience. The main consequences are:
Excessive heating time or inefficient steam
- Insufficient power: If the heater power is too small for the room (i.e. the calculated cubic capacity is underestimated), the heating time will be unreasonably long or the desired temperature will not even be reached.
- High heat loss: The heater needs more power to compensate for heat losses on non-insulated surfaces (such as glass doors, tiled walls or log walls). For example, one square meter of non-insulated wood wall is equal to 1 cubic meter of sauna room volume. If these surfaces are not included in the computational cubic capacity (VA), the heater is too weak and heats the room inefficiently.
- Uneven warming: If the heater is too powerful, the room heats up quickly, but the heater stones are not yet hot enough to throw water on the stones for steam.
Poor steam quality and an uncomfortable sauna experience
- Crisp and short steam: If there are not enough stones on the heater (less than 50 kg), their temperature will rise very high (sometimes up to 400 °C). The water thrown on the hot rocks evaporates abruptly, creating a sharp and intense steam in the face, not a long and mild one.
- Overheated sauna: A heater that is too powerful (especially if there are not many stones) can overheat the room. If the heater stones are hot, the sauna itself is already too hot.
Heater life shortened
- Overheating: Continuous intensive heating of the heater at full capacity significantly shortens the service life of the heater.
- Damage to the heater: In the case of a combined heater with both wood-burning stoves and electric heaters, excessive heat generated by heating with wood can harm the electric heater.
Sauna calculator will help you with your calculations. You can find HUUM’s sauna calculator here:
The general rule of thumb when choosing the power of a heater is that one calculated cubic meter (1 m³) of a sauna requires one kilowatt (1 kW) of heater power. The appropriate power of the heater is found through the computational cubic capacity (VA) of the steam room, which is obtained by adding the real capacity (VR) of the room and the additional capacity (VS) of non-insulated surfaces (e.g. glass or non-insulated wooden wall).
Finally, the architect gives three specific recommendations for everyone who is thinking to build a sauna:
Ensuring proper ventilation
The secret of a good sauna is that fresh air must warm up near the heater, either in the heater’s air intake zone or above the heater (from a height of 80 cm), to avoid temperature stratification and lack of oxygen.
It should also be borne in mind that the final ventilation opening must be closed during the sauna.
Selecting the heater based on the correct cubic and large stone mass
Suitable power is determined by a computational cubic capacity (VA) that takes into account the actual size of the room and the extra capacity of heat absorbing surfaces (e.g. glass doors, tiled walls).
If the electrical capacity is limited, the heater with the maximum amount of stones (more than 50 kg) should be preferred to ensure a long and mild sauna experience.
Accurate bench height ratios planning and safety distances
The ceiling of the sauna should be at a height of 2.1–2.3 meters, and between the upper bench and the ceiling should be 110–130 cm.
In addition, it is vital to observe the safety distances of the heater from wooden structures and to ensure that the bench is stable and installed at sufficient intervals so that the hot air circulating in the room can also get under the bench. This reduces air stratification and maintains an even air circulation.
