[DRAFT]

Sauna design and the resulting sauna environment, particularly stratification of hot air and steam, may have a significant effect on physiological response and thus benefits.

As well, it’s quite difficult to communicate effectively when we all have different understandings of what words mean. A sauna for instance …is not necessarily a sauna. This especially in the U.S. where all manner of hot boxes are mislabeled as a sauna. A common understanding of what things are is critical to the value of these studies.

Some standardization of thermal interventions would be a good thing.

 

Sauna Design Likely Affects Physiological Response

There is likely a significant difference in the physiological response from a small box with low benches and poor ventilation that’s typical in the U.S. compared to a proper sauna with feet above the stones and good ventilation that’s typical in Finland.

A proper sauna should likely provide an increase in Tcore of ≈1.5°c after 15 minutes of exposure while a Finnleo novelty sauna like that used in Atencio 2025 resulted in less than 0.2°c in 10 minutes.

The current environment is akin to a study where participants are given ‘drugs’ with no specification of what drugs were given. Just saying participants were ‘in a sauna’ is kind of meaningless. In some cases the ‘sauna’ in a study may not even be what most in the world consider a sauna and may not deliver the same benefits.

This is also misleading for consumers who may think that they are getting X benefits from their novelty kit sauna when in reality they are not. This empowers bad actors who are then able to take advantage of consumer ignorance and make all sorts of claims that ‘sauna’ provides x benefits when that is likely not the case for the product they are selling.

Following then is a proposal for a Standard Sauna.

 

A Standard Sauna:

The intent here is not so much that every study should use a sauna built to these specifications, though numerous studies using this exact sauna and support facilities would be beneficial, the critical thing is that people fully document the saunas used in a study and how they differ from this standard. That should provide a firm basis for comparison and increase our knowledge of what specific design elements provide what physiological outcomes.

The sauna specified here should provide a good, comfortable and enjoyable sauna experience for bathers, healthy air quality and appropriate increases in core body temperatures and other measures. The bather experience and health benefits should be consistent with good saunas in Finland and other areas of Europe. And considerably better experience and benefits than typical saunas in the U.S. 

1. Interior 
1. Dimensions: 250 cm x 250 cm x 260 cm high
2. Wood: Preferably spruce or pine.
3. Ceiling Shape: Flat. Rounded or flat coves in the corners would be beneficial but not necessary.
2. Heater: 12 kW Helo Laava or 12 kW Narvi Ultra located adjacent to the wall opposite the bench wall.
1. Approximately middle of the wall opposite the benches for a side entry door. Approximately centered between the door and side wall for a front entry door. 
3. Benches:
1. Foot Bench: 250 cm x 40 cm @ 90 cm high
2. Sitting Bench: 250 cm x 70 cm @ 130 cm high.
1. Include a 5-10 cm open air gap between the bench and bench wall.
3. Air Gaps: Benches should be air permeable with 
4. No Bench Skirts.
4. Ventilation: Mechanical Downdraft. 
1. Passive Supply: 200 cm2 face area vent
1. located in the ceiling and centered 10 cm closer to the bench wall than the center of the heater.
2. Or located on the heater wall, centered on the heater and 20 cm below the ceiling.
3. Passive supply includes back flow prevention of either an updraft duct, passive flapper gate or similar.
2. Mechanical Exhaust: Located 30 cm below the foot bench (60 cm above the floor).
1. Blower should be speed adjustable and able to produce at least 50 l/s of airflow for the completed sauna.
2. The blower should be mounted for as quiet of operation as possible including mechanical isolation from the structure and the use of a silencer between the exhaust vent and blower
3. No other air entry points.
5. Door: 60cm x 200cm
6. Measurement:
1. Sauna Temperature: On the bench wall, 100cm above the sitting bench and 30 cm from the corner furthest from the heater.
1. Probe sensor must be at least 10 cm from wall to avoid cold layer.
2. [Is this still best location after Lassi experiments?]
2. Temp/Humidity Sensors: Located on the bench wall in a line 60-90 cm from the corner furthest from the heater.
3. Heights (in order of priority): 
1. Head: 100cm above the sitting bench.
2. Feet: 10 cm above the foot bench (on a bench support). 
3. Sitting: 10 cm above the sitting bench.
4. Calf: 25 cm above the foot bench (on a bench support).
5. Low Back: 40 cm above the sitting bench.
6. High Back: 70 cm above the sitting bench.
4. CO2 Sensor:
1. Std Temp Sensor (50°c): 30 cm below the foot bench in line with exhaust vent.
2. High Temp Sensor (>80°c): 20 cm above the sitting bench or near bather faces if possible.
7. Commons Area / Thermal Suite:
1. Support Hot/Cold/Rest/Rinse/Repeat protocol.
2. Cooldown: Temperate / cool room, cold plunge, etc.
3. Showers: 
1. Immediately adjacent to the sauna for cooling down and rinsing off sweat. 
2. Sufficient count that bather participants will not need to wait.
4. Rest:
1. A comfortable area for bather participants to rest between rounds in the hot space.
5. Provide a space for bather participants to warm to room temperature before entering sauna (or other thermal experience) and without going outside in to cold weather.

 

Deviation is good.

While many or most studies should likely use the standard sauna and baseline protocol, studies that deviate from this are also valuable. This particularly once the standard sauna and baseline protocol have been well studied and understood.

It is critical that the differences between the standard sauna / baseline protocol and those used for a study be clearly documented for each element above as each can have an effect on outcome. This will make the resulting data more valuable.

FWIW, I think a well designed larger sauna will likely have little effect on physiological response while a smaller sauna is likely to have a measurable negative effect. There appear to be two inflection points with saunas for both enjoyment and health benefits with one being this size and another at about 180x180x230cm sauna.

 

Some Terminology

A Round is one continuous period in a sauna hot room (or steam bath, vitality pool, etc.) followed by a rest/rinse/cooldown period.

A Session is a collection of repeating rounds.

A typical sauna session might be 3 rounds, each round comprised of 15 minutes @ 90°c + tepid rinse + 4 minutes cold plunge @ 16°c + 20 minutes rest @ 22°c.

Total Session Heat Exposure is the total amount of time spent in a hot experience during one session.

Total Session Cold Exposure is the total amount of time spent in a cold experience during one session.

Degreetime is the sum of degrees celsius for each minute of exposure. This can be a measure of ambient air temperature, latent heat of steam, or core body temperature.

For example, a good sauna should result in an increase in Tcore of ≈1.5°c in 15 minutes. Ambient degreetime for this sauna would be about 1350 and Tcore degreetime about 573.

An IR device is slower to effect Tcore, taking about 2 hours, about 8x as long, to achieve the same 1.5°c increase. Bathers are likely to tolerate more time with this than a sauna however. Ambient degreetime here would be 5256 while Tcore degreetime would be 4592.

This might help us to better understand how time, temp, Tcore and peak Tcore affect physiological response. Can a Tcore increase of 1.5°c in a standard sauna provide the same benefits as taking 2 hrs to reach that same 1.5°c in the IR dome used for Mason 2024?  How about 3 sauna rounds over 2 hrs resulting in a Tcore degreetime of 2310?  And similar for effects of Tskin, Tmuscle, etc.

It’s important as well that everyone use common and accurate terminology for different modalities. For instance, in a sauna, by definition, bathers are heated by convective heat produced from heated stones and humidity is increased by steam produced from water on the stones. An IR device, be it a cabin, dome or individual panels, is not a sauna or any sort.

Some additional terminology and thermal taxonomy: https://localmile.org/thermal/

 

 

Known Environment and Attributes

One other advantage to a defined standard sauna and baseline protocol is that we can fully analyze many aspects of this sauna from both a sauna dynamics and physiological response standpoint – we can get to know this sauna well. We can know in detail what ambient air temperatures and density of steam various parts of the body are exposed to.

This will help us to better understand variations; larger, smaller, higher elevation, etc.  

Corentin Macqueron is producing some valuable fluid and thermo dynamics models of a few saunas and having a standard like this will help him to produce better models.

 

 

 

But perhaps also other elements like CO2 or ??
And these certainly affect short term and long term use patterns due to comfort. Atencio noted not well tolerated likely due to CO2.

 

This is by the way not a case of fault. 

 

 

Deeper Dive

Physics and Physiology

From a design perspective we want to deliver a product that is comfortable, enjoyable and compelling for bathers and one that results in desired physiological responses. The medical world is looking for benefits (and to avoid problems.

At the heart of the design/physiology world are the environmental variables. 

Environmental Variables – These include things like overall temperature (head temperature), air / heat stratification, humidity stratification, steam stratification, air movement, air quality (CO2, etc.), radiant heat and so forth.

Design Elements – There are a number of tools available to create a desired sauna environment. Some of the more critical are ventilation scheme, bench heights, size and shape of the sauna, heater and heater location. 

For each environmental variable then, there are a number of design elements that affect it and in turn each environmental variable affects bather comfort along with numerous physiological measures.

We can create a similar chart for each environmental variable.

 

 

 

 

Notes on Sauna Design – Why This Design:

The specifications listed are those most likely to affect physiological response.

 

The overall size will result in appropriately even temperatures and steam distribution.

The placement of the heater will result in a good convective loop that will provide more even temperatures and comforting evaporative cooling of bathers skin.

A larger sauna would likely be more comfortable and enjoyable and possibly provide greater health benefits

A smaller sauna will be less comfortable and enjoyable and likely provide lessor health benefits. We can be fairly certain for instance that the increased stratification on bathers bodies of a lower ceiling and benches will result in significantly less increase in core body temperature.

 

Similar standards should be established for other thermal modalities including bio-sauna, steam bath, IR treatment, Laconium, Tepidarium, Caldarium, Ultrasound, etc.

 

Note: this is not a one person’s design but an amalgam of best design practices from numerous sources, primarily the book ‘Secrets of Finnish Sauna Design’ and associated Saunologia.fi website.

 

Some of this same rigor should be applied to other modalities such as steam baths, laconiums, tepidariums, etc.

 

As well, a Baseline Usage Protocol.

Revisions:

2025.07.31 – First Draft