The Vault

My brother and his wife heard about this sauna concept a few years ago: a vault-shaped underground smoke sauna made of cylinder blocks.

So they called the manufacturer and asked for a demo as they happened to be driving in that part of Finland. The owner could not make the demo at their showroom but offered to heat up the sauna at his home instead. He had even bought sauna beers and sausages!

Turns out that the owner Jari had heard stories from his dad about the underground dug-out saunas during WW 2 so he decided to develop something similar: first a cellar sauna and then these vault saunas. To be fair, the dug-out saunas during the war were quite different than this one but let’s leave reviewing that sauna type for another time.

The concept

The idea of building saunas underground is pretty unique but not unheard of. In fact, all kinds of cellar saunas were and still are typical in Finnish apartment complexes and houses. One benefit of the underground saunas is higher humidity.

This manufacturer sells underground saunas with continuously heated stoves and with smoke sauna stoves (or ovens as they call them). This article reviews the smoke sauna configuration.

Smoke saunas do not have chimney so they are sometimes also called chimney-less saunas. While the idea in most saunas is to keep the stove and the stones warm by keeping the stove on while using the sauna, smoke saunas are very different. The idea is to heat up a bunch of stones (300 kilos or 650 pounds in this case). After heating up the stones, the fire is killed and carbon monoxide in the sauna room is ventilated out before using the sauna. This is quite important if you want to use this sauna type for more than one time in your life..

What then makes this particular smoke sauna different is the shape and the material: it is underground, made of cylinder blocks, and the ceiling is semi-circle-shaped vs. flat. The underground structure keeps temperature and humidity more constant in winter and summer. Cylinder blocks are fireproof, which is a plus considering how easily Finnish smoke saunas burn down – some sources say that these saunas last on average six years. So this one won’t burn down. According to the manufacturer, the benefit of the semi-circle ceiling is better rotation of the steam.

Construction

A few years after consideration, my brother and his wife decided to order this vault sauna. It comes as a kit but the construction process is a bit more laborious than putting together an IKEA shelf or a barrel sauna, but with the help of a local bulldozer guy and a bricklayer, and after getting the needed permits, the phase 1 was completed in weeks. I had the privilege to help to build a deck.

After the initial construction, the surfaces were rendered and the oven, the hot water boiler and the benches were added. The seating deck is above the the top of the oven for heat stratification between your head and your feat and to comply with the law of löyly.

There is still some landscaping to be done to properly hide the dungeon.

Heating up

Heating up this sauna takes about two hours of active heating. Before heating, you carry the benches and armrests out because the would otherwise soot. You open the oven latch above the stones and make sure that the upper air vent is open to let the smoke out. There are two other latches in the oven that you should open too.

This sauna is heated from the outside of the sauna in the bathroom area, which is very comfortable. We used gray alder for heating up these saunas. Aspen is also an option but it generates less heat. Birch would soot too much and pine and spruce would crack and would not heat the stones enough.

It is important to keep adding wood constantly every 20 minutes or so. If you let the fire almost die and then add more wood, the flames will hit high and that generates more soot and possible more gasses.

After 1.5-2 hours of heating, you should see the stones below the top layer turning glowing red. At this point you should the fire die: make sure that no wood is burning and turn the ashes around in fireplace. Then you close the latches on the stove and open the lower air vent on the back wall and let the sauna work its magic to route fatal gasses (carbon monoxide) out of the room. There is actually a word for all this sauna magic in Finnish language: siintyä. That word is only used to describe this phase of preparing smoke sauna.

You can start saunaing right away or later. The stones should stay hot for about 20 hours. The more you wait, the less you burden the sauna stones. Typically you will have to change the stones after every 100-150 heat-up times.

When you are ready to start the sauna session, you clear the oven of all the ashes. When you open up the latch of the oven, you will notice how a layer of ash and soot has accumulated on the stones.

The next phase is ”häkälöylyt”, which translates to “carbon monoxide steam”. In reality, the idea is to get rid off the soot and ash on stones before saunaing – there should no longer be carbon monoxide in the room if the ventilation works. You pour water on different parts of the oven and quickly close the door. The ash and soot will land on benches and will be swept out.

After all that, we bring the seats in the sauna and start saunaing. The oven latch remains closed during the breaks and open when people are in the sauna. The idea is to preserve as much heat on the stones to allow longer sauna sessions.

You are not supposed to throw water on the stones. Instead, you gently pour water on the hot stones. You should work your way from one corner to another during different sessions to make sure that you get a perfect steam each time. In other words, you should not pour water on the same spot twice. When you hit it right, the steam concerto is just amazing.

The upper air vent latch on the wall is closed and the lower one under the bench is opened when saunaing. This directs the steam up to the ceiling and then down under the bench, which keeps the steam in the room longer and also warms the feet and the entire body more comprehensively.

The Vault Sauna Experience

I tried the sauna for the first time in May 2022. The outside temperature was around 15c. We did about five rounds each day in the sauna and poured about five-six ladles water on each round. I would estimate that in each round we stayed in the sauna on average about 10-15 minutes and cooled down in the lake in between so with breaks, our sessions ended up lasting probably about three hours.

The outside temperature was about 10c and it was raining a bit. My gut feeling was that the sauna was about 60c warm on the bench level. This sauna type will not heat up as hot as a traditional smoke saunas that are typically 80c or more. I personally liked the experience. You end up pouring more steam on the oven, which also keeps the air extra humid. Then if you just want to chat a bit longer in the sauna room, you feel comfortable and sweating but not exhausted.

The steam was very gentle and moved relatively slow in the room. It was difficult to say, how the steam circulated in the room, which was a cool experience in itself. In many smaller saunas, the steam hits your upper body hard and then disappears quickly so you know exactly the direction from which the steam hits you and where it goes. In this sauna, the steam just comes from somewhere and kind of drops on you. Very unique experience!

Overall, the experience was fantastic with relatively intense smoke aroma in the room too.

Rounds 2 and 3 with Temperature and Humidity Sensors

I tried the sauna next time in October, 2022. This time, the outside temperature was around 10 c. I brought in a wireless Ruuvi sensor to capture temperature and humidity in the air. I placed the sensor on the backrest of the bench so it was roughly at the chest level. It looked like that the temperature was hovering at around 55 c and the humidity reached to 90 g/m3. This means about 80% relative humidity, which is very humid compared to a regular sauna.

In December, I paid another visit to this sauna. This time, we enjoyed the darkest time of the year in Finland. The temperature was at around -10C and we had to break the ice to be able to cool off in a lake between the sauna rounds.

This time I placed two Ruuvi sensors in the sauna. The other one on the foot level and the other one on the chest level. This time it turned out that the temperature dropped a few degrees during the breaks as we closed the lid of the stove. Had we kept it open, I think the temperature would have risen a bit above 60 C. The temperature on the feet level was at about 37C after the sensor heated up.

The humidity readings were interesting: on the chest level, the humidity peaked at 66 g/m3 at 54.8C and 998 hPA, which means about 60% relative humidity. On the feet level, we measured 49 g/m3 at 39 C and at 998 hPa pressure. That means 98% relative humidity! You definitely feel like you are sweating a lot (although the sweat is mostly just condensed steam, read About Sauna Temperature and Humidity).

This sauna is definitely different and probably not ideal to those who are obsessed of getting the temperature to 220 F. The steam heats the whole body up, but it is gentle and super humid. The steam and how it hits you resembles Rajaportin sauna, which is hands down the best public sauna in Finland.

For The Next Visit

Now that we have enjoyed and the analyzed and enjoyed the steam, the bucket list item in this sauna would be to cook food there. Traditionally, Finns have cooked ham in smoke saunas. A whole ham was hung from the ceiling during the heat up and it absorbed all the good smoke, carbon monoxide, and carcinogens. They actually still sell smoke sauna cured ham as cold cuts in grocery stores (called “saunapalvi”). I doubt that those products are actually smoked in a sauna, but that’s where the name comes from.

This sauna does not necessarily heat up enough for curing a ham in it, but salmon might be an idea.

More reading in Finnish: manufacturer’s site and Lassi Liikanen’s comprehensive review at Saunologia.fi.