#15 The Sauna

If there is one thing I regret, it's not building the sauna first before starting the house. Every Finnish person will tell you this. The reason I didn't was due to the pandemic and all the logistics, skyrocketing prices for building materials, and availability problems it brought. I often thought about this while I was working on the house during the long, cold weekends and winter vacation days, spending the nights camping on the site before I had any heating systems in place (or even a roof). 

But that's all in the past, and I fondly remember those days now that the house is ready. Time to finally build the sauna! Unlike the other posts on this blog where I reflect on the build after it is completed, this entry was written as I was building, and the entire building might not be 100% complete before I publish this.

Just a Sauna?

Unlike in many (most) other countries, a sauna in Finland is not a luxury item as such. Pretty much every house has one. Apartment buildings typically have a common sauna one can reserve, or even one as part of the bathroom in each apartment. According to the statistics, there are 3.2 million saunas in Finland for a population of about 5.8 million people. The word 'sauna' is likely the only Finnish loanword in the English language - read the Wikipedia article for more fun facts. 

Because I'm off-grid, the sauna is of course wood fired. For many Finns it's the only 'real' one (leaving the smoke sauna, known as Savusauna, out of it), not that electric powered stuff those city dwellers have to use :-) That said, I didn't want to build just a sauna. I also wanted the place to serve as a guest room, with a fully equipped bathroom and small kitchen area - for a total of  about 25m^2, not including the deck. Like with the house, I opted to go with Kuusamo Hirsitalo and went with a slightly modified version of their Halme 25 model:

This floor plan pictured above is the original one; I made changes so the 'middle' section is now an actual bathroom with toilet, sink and shower. True, it's not the largest sauna, but it's perfectly fine for our family. There is no need to oversize, since it will only take longer to get hot. The guest room also serves as a back-up space: imagine there are issues with the heating system in the main building, or we come back form a long trip and the house is cold. This smaller space can be heated in no time and provides everything needed while we wait for the main building to warm up. I designed it so the sauna room can be a sleeping place as well.

The Build

As with the house, we start with the foundation. While it is 'just' a sauna and guestroom, I went with the same type of foundation as the main house: concrete with insulated floating floor. The picture below shows the foundation before the floor - which is poured only after at least the first course of logs is placed since it extends to half the first log (but insulated from it of course, same as in the house). The inner walls are also log, so they have their own concrete support.

There is 150mm insulation on top, the rest below is filled with two types of gravel: a course one at the bottom to facilitate drainage, and a finer one on top to provide a even, compressed base on which the insulation is placed. The pipe work is embedded in the gravel, and provides tubing for wastewater from shower and basin/kitchen, incoming fresh water, hot/cold PEX water distribution pipes and incoming electricity. 

I was less strict regarding the insulation. It's still good, but not to the same standards as the main building: the logs are less thick (135mm), the windows slightly less performant, less insulation in the ceiling and floor, etc. It's not needed for a guest room and sauna.

Hot water is provided by an on-demand propane heater. This provides a way to get hot water quick and easy, without the need for electricity or any dependencies on the 3000L tank/gasification boiler/heatpump and serves as a back-up solution in this way as well. They're not very common here, so my options were fairly limited. I also didn't feel like going thought the trouble of ordering it from abroad. In the end, I went with a Beretta Fonte Lx 14 (also sold under the Sunwind brand here).  

There is a small kitchen area in the guest room. This houses the hot water heater, a propane cook top and the sink. No other appliances are installed (i.e., no fridge). The ones in the main house will do fine, and I didn't want to occupy more space for something that would likely sit idle/unused most of the time anyway. 

The guest room has its own fireplace (a Contura 820T). There is no floor heating like in the main house, but I did embed an electric floor heating cable in the bathroom section. This is both for additional comfort but also to make sure the floor can be dried out properly. While I could have gotten a slightly more efficient fireplace than the Contura (it's 81%, the other one I looked at had 86% efficiency), the advantage was that the safety distances are very small due to it being surrounded with soapstone which means it takes up minimal space. The soapstone also adds thermal mass, so it adds a level of comfort as well. 

The kiuas (sauna stove) is a Harvia Linear 16. It's rated power output sits around 18kW, and holds about 36kg of stones. Stones serve as thermal mass: they store and radiate heat evenly and thus serve as regulation, and keep the sauna warm even if the fire dies out. Of course, they are also key to creating excellent löyly (the wave of heat and steam when water is thrown on the stones, key to a good sauna experience).  It's probably a bit overkill, but it heats up the sauna in no time.

Ventilation

One of the most important aspects of a proper sauna is the ventilation, and this is very often misunderstood and designed wrong - especially in saunas outside Finland. Without correct air movement, even a well-built sauna with a good heater can feel uncomfortable and unhealthy, and will be ineffective. A sauna is typically a small-ish space, and (especially with a heater relying on combustion) can quickly become poor in oxygen if there is no supply of fresh air. This can lead to dizziness, headaches, nausea, etc. You can not rely on gaps or the space under the door to supply that air. In a properly functioning sauna the air always feels fresh, not stale or suffocating. 

Proper ventilation also plays a major role in heat distribution in the sauna. Fresh air entering near the heater mixes with hot air, helping create even temperature distribution from floor to ceiling. Without airflow, hot air stagnates near the ceiling while cooler air pools near the floor, leading to cold feet and overheated heads. Good ventilation enhances the löyly, allowing it to circulate smoothly instead of feeling harsh or suffocating.

Finally, ventilation is critical for moisture control. Saunas produce large amounts of moisture with all that water poured on the stones. Ventilation allows humid air to exit, preventing condensation inside walls and ceilings, which reduces the risk of mold and other nastiness, significantly extending the life of the sauna building as well as wood surfaces used, as they dry between sessions, preserving their appearance and scent.

I went with a traditional, non mechanical (electrically powered intake/extraction) ventilation set-up. This relies on natural convection, with an air intake low and close to the kiuas (which heats up the incoming air immediately), and an exhaust on the opposite side, high up.

The Bench

The bench in the sauna needs to be placed correctly. You typically have two levels: a lower level often reserved for children, and a higher level that is optimally situated to enjoy the heat. The top bench is typically placed about a meter from the floor, while the lower bench sits in the middle between the floor and the top bench. I placed my top bench at 1.2 meter from the ground, leaving 1.1 meter from seat to ceiling, which is pretty much optimal. I maximized the total length of the benches the length of the room, 2.3 meter with a width of 0.6 meter. 

The "Law of Löyly" states that the top bench should be positioned so that the user's feet are ideally above the top of the heater's stones. Since the kiuas I use has its stones at around 0.8 meter from the ground, this is accomplished using a footrest railing for the top bench. 

The material for the benches here in Finland is typically Aspen or Alder. These are used because they are knot-free, resin-free, stay cool to the touch, resist moisture, and don't splinter. In my case, I was able to buy a huge old sauna years ago (specifically for the material) for almost no money and repurposed the Aspen wood from that - for some reason this material has become very expensive over the past few years.

Conclusion

A sauna is more than just a random room with a heater. Placement, kiuas size, material choices: they will all impact the experience. As I mentioned in the beginning, the sauna might not be fully ready by the time I publish this. And it isn't. Winter meant that some aspects such as the kitchen, plumbing, and details such as window/door/ceiling trim are left for Spring. The sauna itself however has been wonderful to use this winter, especially as we hit those very cold days...