In the article How to build a basement sauna – 7 tips, I share few tips when building a sauna in the basement.

Building a sauna in the basement may sound easy, but it needs to be done properly. There are a few things about building a sauna in your basement that you definitely need to keep in mind. 

We have built a Finnish sauna in our basement here in Germany, so I have personal experiences on the subject. Also, I have asked for advice from professionals and used the Finnish Rakennustieto (a great book about building) for this article.

Read more: How I built a Finnish sauna in Germany

Before you start building a sauna in your basement, you should consider a few things: 

1. What is the room height? 
2. What kind of ventilation does the basement have/need? 
3. What kind of floor material is best for a sauna? 
4. Where should the floor drain be placed? 
5. Is it worth installing underfloor heating? 
6. What kind of sauna is suitable for a basement? 
7. What kind of sauna stove/heater is best for my basement sauna? 

How to build a basement sauna – 7 Tips

1. Room height 

Basements are usually lower than rooms on the upper floors. It is worth measuring the height of the basement and considering whether it is suitable for a sauna. If not, is it possible to add more height to the basement? When digging, you should make sure that the groundwater will not get in your way. That’s what happened to us. 

According to my construction book, the ideal height for a sauna is 2000-2200 mm. Often you lose a few centimetres, so take that into account. 

However, the most important measurement is the space between the upper benches and the ceiling. This space should be 1000-1200 mm. 

There is something called the “law of steam”. That means that when you sit on the upper benches, your toes should be higher than the sauna stove, including the sauna stones. Many modern saunas do not follow this rule, but I think it is quite useful.  

This might not always be possible, especially for a basement sauna. I can say from my own experience that my toes get nice and toasty in our sauna even though we were not quite able to keep that rule. 

2. Ventilation 

Ventilation is one of the most important things in building a sauna. Carefully consider the ventilation of the sauna and shower rooms. 

Expert Jarno Vuorenmaa from Vuorenmaa Ilmastointi and Putki Oy clarified the issue as follows: 

Always look at the bigger picture. If your house has passive ventilation, you should use it in the basement as well. An exception can be made if the basement is separate from the rest of the house. 

Moisture is not removed by passive ventilation as effectively as by mechanical ventilation. In both cases, you have to think of air exchange. 

You can also opt for mechanical supply and exhaust air. The shower area or bathroom do not necessarily need supply air, but you still have to make sure you have enough make-up air. 

This is Jarno Vuorenmaa´s professional opinion. 

If you are buying or ordering a bathroom sauna/element sauna, you should check that the sauna has openings for ventilation. Not all models have them (especially the cheaper ones). 

3. The floor 

You can use the usual wet floor covering as a sauna floor covering. The floor must be properly moisture insulated and waterproof. The area around a wood-burning stove must have non-combustible floor material. 

Concrete floors are normally covered with ceramic floor tiles. According to my book, floor tiles should be rough or slightly studded to prevent slipping. 

Remember! The concrete floor should be allowed to dry 1 cm per week. This means that it takes about 3 months for the 30 cm thick floor to dry. Do not rush, let the floor dry in peace. Otherwise, you might have problems later. 

Pay attention to the slope towards the drain, the water should not stay in the corners. 

As a reminder, at floor level, the temperature rarely rises above 30 ° C. The warm air always rises. 

Remember! The shower/bathroom and sauna are wet rooms and need to be waterproofed properly. 

4. Floor drain  

Make sure you have a floor drain in the basement. If not, make sure it is possible to add one. Optimally, you want separate floor drains for the sauna and your bathroom/shower area.  

I am not a construction professional, so I leave the floor drains to the experts. I would advise you to do the same. Unless you are an expert yourself. 

5. Underfloor heating 

The sauna does not necessarily need underfloor heating but it helps the floor to dry faster. I would definitely put underfloor heating to the shower or bathroom. It is so easy, just press a button and turn on the heating. In a basement sauna, you want to be extra careful with getting rid of moisture. Nobody wants mould and other spores in their sauna. 

There are different types of underfloor heating available; electric and water-operated, as well as different thicknesses. You will for sure find the perfect solution for your sauna or bathroom. 

In our basement sauna project, the groundwater rose, and we were only able to add 30 cm to the room height. It was just enough so we could make the floors. If we had been able to add more space, we would have loved to install underfloor heating. 

6. Sauna 

It makes a difference whether you build a sauna or order an element sauna. To build a real Finnish sauna, you need thermal insulation. 

If the walls and ceiling surrounding the sauna are made of stone, additional thermal insulation is applied to the interior surfaces with mineral wool. Mineral wool is the best insulation for walls and ceilings because it does not move around (e.g., it does not leave the upper parts empty). 

“The structures of the sauna must be carefully sealed so that moisture does not penetrate into other rooms. Well-sealed sauna structures require efficient ventilation to dry the rooms after a sauna” Rakennustieto book says. 

The structures are coated with a heat-resistant and vapour-resistant plastic film or paper coated with reflective aluminium. This is called a vapour barrier and the reflective surface is installed towards the sauna. The vapour barrier is installed on the walls and ceiling of the sauna on the warm side of the thermal insulation. 

Remember that thermal insulation is essential, otherwise, the heat will escape. If there is no insulation in your element sauna, you have to add it yourself. 

Rules: 

• If possible, 20 cm of thermal insulation is required for the ceiling. 
• The walls need 10 cm of thermal insulation. 
• The ceiling shows aluminium paper and the walls mineral wool insulation. 

7. Sauna stove 

You should choose the stove when you know the exact room height of the sauna. If you have a very low room height (for example 190 cm), you should check with the stove supplier what the minimum height required for that stove is. 

Some stoves require plenty of space between the stove stones and the ceiling. 

This ensures the safety of your sauna. Make sure to check your sauna´s stove-ceiling ratio. 

This was the “how to build a basement sauna – 7 tips” article. I hope you enjoyed it and that you got new and useful tips for building your own basement sauna. 

Sources: Rakennustieto-handbook, design plans and experiences of building our own sauna and interviews with experts.

Read more: 5 ways to build a sauna