Building a sauna is not merely a technical process. It is a comprehensive craft that combines insulation, ventilation, materials, and spatial logic in such a way that the final result provides years of enjoyment.

This article on sauna planning is based on the practical observations and many years of experience of HUUM architect Arden Arroval, who has designed both home saunas and custom-built sauna projects.

According to Arden, a high-quality sauna is born from the details. It is the combined effect of small but critically important decisions that determines the durability, safety, and overall quality of a sauna.

Arden highlights three of the most common construction mistakes: insufficient insulation, faulty ventilation, and choosing a heater with incorrect power.

He then explains why these problems arise and how to prevent them intelligently, ensuring that the sauna provides many 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 operates, and how well the structure withstands high temperatures.

Faulty or insufficient insulation is one of the most common reasons saunas do not function as intended and can lead to moisture-related damage.

Common Sauna Insulation Mistakes

Below are the most typical insulation and installation mistakes that affect the quality and safety of a sauna.

Improper Sealing of the Vapour Barrier and Its Consequences

The hot and humid environment of a sauna requires insulation that is fully airtight and moisture-proof.

Common mistakes include poorly taped joints between insulation boards (such as foil-faced PIR boards), as well as unsealed ventilation openings and cable penetrations.

If foil taping and sealing of penetrations are rushed, the foil layer will not be continuous or airtight.

Risk of Condensation and Mould Due to Inadequate Insulation

Improperly sealed joints and penetrations allow moisture to accumulate within the structures, leading to condensation and mould that damage the wood.
These problems are usually caused by rushing the sealing process.

Lack of a Crucial Ventilation Gap or an Incorrect Solution

There must be an air gap of at least 20 mm between the insulation foil layer and the interior finish (wood panelling).

It is a serious mistake if the insulation foil layer is in direct contact with the wall panelling, because without an air gap there is no air circulation to prevent excess moisture from forming behind the panels.

Consequences of Poor Insulation

Proper insulation is essential for a high-quality and long-lasting sauna. Below are the problems associated with poor or inadequate insulation, affecting both energy consumption and user comfort.

Impact on Energy Consumption:

  • the need for a higher-powered heater;

  • increased energy consumption — uninsulated surfaces increase heater power requirements, leading to higher heating costs.

Impact on the Quality of the Sauna Experience:

  • temperature stratification — warm air accumulates under the ceiling while a cooler layer remains near the floor (at foot level);

  • longer heating times.

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 sauna experience. To achieve this, proper internal insulation is required. The main requirements for sauna heating are as follows:

1) Heat- and Moisture-Resistant Insulation Materials with a Compact Structure:

  • PIR boards — the most common solution, typically 30–50 mm thick with a foil layer;

  • mineral wool — an alternative solution using 25 mm thick mineral wool boards with a foil layer. When mineral wool is used, a separate insulation frame is required.

2) Moisture and Vapour Barrier

Foil-coated insulation boards 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 planned ventilation is one of the most common mistakes in sauna construction. The architect emphasizes that a well-designed ventilation solution is essential to ensure proper air exchange, which is the cornerstone of a high-quality sauna.

Proper ventilation helps prevent oxygen deficiency, uncomfortable temperature stratification, and heat loss at the lower benches. More detailed information on ventilation solutions can be found in a separate guide.

Problems usually begin with incorrect placement of the air inlet or with a failure to consider the ventilation system as a whole. For example, in saunas with wood-burning heaters, the air inlet is often installed too high. As a result, cold air does not come into contact with the heater stones and does not warm up sufficiently, preventing proper air circulation.

Often, the exhaust vent is left open during heating, which allows heat to escape immediately from the room.

Such solutions cause fatigue and lethargy among sauna users due to oxygen deficiency, promote temperature stratification, and in the case of wood-burning heaters, incorrect pressure and airflow direction can even encourage the formation of dangerous carbon monoxide.

Therefore, ventilation must already be considered at the heater selection stage. In saunas with forced ventilation, it is especially important to avoid excessive negative pressure in rooms with wood-burning heaters, as this can cause carbon monoxide to move in the wrong direction.

The fresh air inlet should always be located near the heater. Depending on the heater type, it may be positioned behind, below, or at an appropriate height above the 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 based on whether forced (mechanical) or natural (free-flow) ventilation is used.

Ventilation in the Sauna

The air inlet must be located near the heater, and its height depends on the heater type:

  • for shell-type heaters — near the floor or above the heater;

  • for mesh-type heaters — at the height of the centre of the heater.

This allows fresh air to warm up and create sufficient airflow.

Air Extraction and Ventilation

The exhaust vent should be located on the opposite wall:

  • lower (up to 60 cm from the floor) in saunas with forced ventilation;

  • higher in saunas with natural ventilation.

During sauna use, the final ventilation opening should be closed, and after use it should be opened to remove moisture.

Washroom Solution

The washroom requires a separate ventilation system as part of the building’s overall ventilation to effectively remove moisture. Pipes and air ducts must be insulated with diffusion-tight materials to prevent condensation and dripping.

Sauna Construction Mistake #3: Incorrectly Selected Heater Power

According to Arden, choosing a heater with the wrong power is a very common problem. Most often, a heater with insufficient power is selected.

The size and construction of the sauna directly affect the required heater power. Power demand increases if the room includes glass windows or doors, as well as brick, concrete, or log walls that retain heat.

A common mistake is selecting a heater based solely on appearance, assuming that power is not particularly important. In reality, the opposite is true: while the design may be visually appealing, it is the correct power that ensures a functional, comfortable, and energy-efficient sauna.

If you want to learn how to choose the correctly powered heater for the dimensions and construction of your steam room, refer to our comprehensive guide.

Factors Influencing Heater Selection

1) Actual Sauna Room Volume (VR)

The suitable heater power depends on the width, depth, and height of the room (VR = a × b × h). A sauna that is too large requires a powerful heater, which can be expensive or exceed available electrical capacity.

2) Non-Insulated Surfaces

These surfaces absorb heat and require additional heater power to compensate for heat losses. To account for them, extra volume (VS) is added to the actual room volume.

Non-insulated surfaces include:

  • glass surfaces (e.g., glass doors);

  • log walls;

  • tiled surfaces;

  • monolithic wooden surfaces;

  • windows.

3) Insulation Quality

The sauna must be properly insulated even when used in summer or warm climates, as the temperature inside the sauna is significantly higher than ambient conditions.

Consequences of Choosing the Wrong Heater

An incorrectly selected heater has several critical consequences that affect both heating efficiency and the overall sauna experience.

Excessive Heating Time or Inefficient Steam

  • Insufficient power. If the heater power is too low for the room (i.e., the calculated volume is underestimated), heating time will be excessively long or the desired temperature may not be reached at all.

  • High heat loss. The heater requires additional power to compensate for heat loss through non-insulated surfaces such as glass doors, tiled walls, or log walls. For example, one square metre of non-insulated wooden wall is equivalent to one cubic metre of sauna room volume. If these surfaces are not included in the calculated volume (VA), the heater will be too weak and operate inefficiently.

  • Uneven heating. If the heater is too powerful, the room heats up quickly, but the heater stones are not yet hot enough to throw water on them and produce steam.

Poor Steam Quality and Discomfort

  • Sharp and short steam. If there are not enough stones in the heater (less than 50 kg), their temperature can rise very high (sometimes up to 400 °C). Water thrown onto such hot stones evaporates abruptly, creating sharp, intense steam rather than long-lasting, mild steam.

  • Overheated sauna. A heater that is too powerful (especially with a small stone load) can overheat the room. By the time the stones reach operating temperature, the sauna itself may already be excessively hot.

Shortened Heater Lifespan

  • Overheating. Continuous intensive operation of the heater at maximum power significantly shortens its service life.

  • Heater damage. In combined heaters that include both wood-burning and electric components, excessive heat from wood firing can damage the electric heating element.