Trumpkins Notes On Building A Sauna

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New and Maintained Version Here: Trumpkin’s Notes On Building A Sauna.

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On the path to building our new sauna we learned that there is a lot of not so good (or downright bad and misleading) information on the web and from U.S. sauna vendors, and that there are a lot of details that are critical to get right and easy to get wrong. We traveled to Finland and Sweden to experience sauna and learn from experts. Like Glenn Auerbach did more recently, we’d noticed a huge difference between saunas in the U.S. and saunas in Europe. Sauna’s in Europe are consistently better. We want to know why.

Disclaimer: I am not an engineer nor doctor. The following is simply a bunch of notes on what we learned building our sauna and that we wish we’d known much sooner. Use at your own risk. I strongly encourage everyone reading this to do further research on all of these topics.

 

Form, Function and Löyly

What follows is about function (and Löyly). It’s about the things that are needed for the best sauna experience- a Finnish (or Swedish or German or …) sauna experience.

Form is important too though and form can enhance function. Sauna is more enjoyable in a sauna that is aesthetically pleasing than one that’s not. Should sauna function ever be compromised for form? Not to me. I’ve had bad sauna and I’ve had good sauna and I’ll take the latter every time no matter how ugly the room. 

Some may be more interested in form and want only what function fits the form they desire. And that’s OK, it just may not be sauna. Saunas Of The World is one of my favorite Instagrams because I love architecture and it has some fabulous looking places, but many or even most of what are on there are either not sauna’s or not good sauna’s.

While I’ve heard a lot of people say that they wished they’d done more for the function of their sauna, particularly ventilation, I’ve rarely heard anyone say (though I have said it myself) that they wish they’d done more for the form.

 

Details Are Important

“90% of saunas in North America are bad. The other 10% are worse.”
– Board Members, Finnish Sauna Society
– Mikkel Aaland

There are reasons for that statement and very good ones. U.S. saunas suffer from a love of don’t-sweat-the-details. U.S. traffic engineers not sweating the details on road design is why we have the most dangerous road system of all developed countries and why a child in the U.S. is 11x as likely to be killed walking or riding a bicycle as a child in Europe. Details are often quite important.

You can tell you’re in a U.S. sauna that hasn’t sweated the details because you get cold feet (they’re actually not cold but quite warm just not nearly as warm as your upper body). Or you have to scrunch up and put your feet on the upper bench (which isn’t good for relaxing, breathing and getting the benefits of löyly). Or you don’t feel a fully enveloping and comforting heat evenly around your entire body front to back and head to toe. Or the sauna simply can’t get hot enough.  

The most critical though is that you have to leave the sauna because you’re feeling out of breath, have brain fog or getting dizzy. These because of too high CO2 from poor ventilation. High blood CO2 also impacts the ability to use sauna for muscle recovery after a workout or from a day of real work as it reduces our body’s ability to deal with lactate. In some cases muscles are even worse after a sauna because of this.

Some things we noticed on our travels in Finland: EVERY sauna required climbing up several steps to the benches from the changing room – feet are nearly always above the stones. EVERY sauna except one had good ventilation. They all had an adjacent shower and with a tiny few exceptions, a window or two. All critical details that make for a much better experience.

The biggest thing we noticed is how much more enjoyable sauna is in Europe. More even enveloping heat front to back and head to toe. No cold feet or chilly backs. And that there’s a huge difference in leaving a sauna having benefited from heat and löyly versus leaving because you feel like you need air to breath. People in U.S. sauna’s who’ve never experienced proper sauna often confuse the latter with the former, we think that feeling of breathing difficulty is normal, but it’s not.

A bit of effort up front sweating the details provides thousands of hours of more enjoyable and beneficial sauna for years to come. It’s worth the effort.

 

Important Details We’ve Learned

So here, some quick notes on what we have learned about sauna design and building (some that we wish we’d learned much sooner). Special thanks to Jarmo Lehtola, Risto Elomaa, Glenn Auerbach and many others for their ideas and patience in answering my endless questions and their perseverance in making sure that I not only built a proper sauna but understood sauna.

Space – Minimum of 2 cubic meters (70 cf) per person plus one for the elf.

Stones Determine Bench and Ceiling Height – Temps are much more consistent, stable and comfortable above the top of the stones than below. Start with setting the foot bench at or above the top of the stones. The sitting bench then is 17-18” above the foot bench and the ceiling 44-48” above the sitting bench.

Bench Widths – Provide 2’ or 60cm of sitting bench per person. If a L shaped bench then a bit more so that the people in the inside corner don’t have their legs on top of each other.

A Bench to Lay On – At least one sitting bench should be at least 76-80” so that someone can comfortably lay down. 

Bench Depth – 24-28” is the recommended minimum as this is both comfortable for sitting and wide enough to lay down on.

Wood – Almost any wood can be used though some are better than others. Cedar is popular but can be too fragrant for some people and the oil in some cedar and other woods can be toxic though I’ve no idea how this plays out in a hot sauna. Avoid woods like pine with too much sap. Be careful of knots, especially for benches, platforms and backrests, as they can get much hotter to touch than surrounding wood.

Groove Down – With T&G walls the tongue should face up and the groove down to avoid moisture, water or sweat collecting in the grooves. 

It Gets Hot – Be careful of any metals, woods or other materials that get hot easily as getting burned on these can be less than pleasant.

Warm Dry Floor – Stepping on to a cold or cold and wet floor while still in a hot room doesn’t make for a pleasant experience. A wood slat floor (duckboards) makes for a more enjoyable end to each sauna round. They will also help to keep benches cleaner as debris on the bottom of feet are more likely to remain on the boards or fall through them vs a hard floor where debris are more likely to stay on feet to be deposited on to the foot bench. Duckboards actually seemed quite rare in Europe as the entry to almost every sauna hot room was a step, followed by another one or three steps to get up to a platform and the benches.

Stove Sizing – Bigger is not necessarily better. Too high of kW results in shorter heating cycles, faster heating swings and less comfort. A properly sized heater will have longer run times which results in less noticeable temp changes and greater comfort. The EU recommendations from EU manufacturers should be good. Pay attention though to things like very large windows that may result in excess heat loss and require a larger stove. The more stones the better and more stones help to smooth out the temp swings.

Stones – More is better. MINIMUM 8kg / m³ (1/3 lb / cf) of space and 17kg (37 lbs) per person are good starting targets. More is better. 40kg (90lbs) per person is good. More stones result in more even temps, more even softer steam and thus a more comfortable experience. A good sauna is heated by the stones and the stones by the heater. Nothing makes up for proper stones.

Use virgin quarried stones – Landscape stones or river rock may have organic matter (think cow dung) that is unhealthy and can produce unpleasant odors when heated. 

Rough stones are good – Stones should ideally have a rough surface to help hold water to make better steam. Smoother or rounder stones do not do this so well. 

Thermostat – Should be placed at about head height (75cm above the sitting bench) and NOT directly above the stove. Note that this could violate UL guidelines in the U.S. but if you want a sauna rather than an American warm room then this is needed.

Avoid vaulted, coved or similar ceilings – In theory and in experience a flat ceiling is best as it results in the most even temps. Vaulted, coved or sloped ceilings result in heat being up too high and reduce fresh air movement. HOWEVER, I’m not convinced that it’s not possible to have something other than a flat ceiling that works and is comfortable such as a minor cove. A-Frames, Barrel saunas and similar shapes should be avoided though.

A shower is important – It’s important to shower (and dry off) before first entering sauna and a cool shower is often a good way to cool off after each round. The more convenient the shower the more likely it is to be used and the more pleasant an experience. We have two for our sauna; one inside and one outside.

A Window on the world – Being able to see outside while in sauna is quite enjoyable.

Changing Room – Besides changing and showering this space provides a critical air-lock function to prevent or lessen cold chilling air from blowing in to the sauna.

Heated Floors – If you have a concrete floor then adding in-floor radiant heat can make for a more comfortable experience, especially in the changing room and shower but also in the hot room. Extending this to the porch and nearby walks or patios (snow melt system) isn’t a bad idea either.

Privacy – Sauna is best enjoyed nude. Providing for some privacy for both inside the sauna building and for an outdoor patio can make for a much more enjoyable experience for all.

Wood, Gas or Electric Heater?  Wood is more traditional and more romantic. Even the routine of preparing the fire has benefits and for many of us is quite enjoyable, relaxing and a great way to prepare for a good day of sauna. A wood sauna causes you to slow down a bit and be intentional about your sauna which is good. Drawbacks are that it is not as convenient, uses natural resources (trees) and is a direct source of pollution. 

Electric is certainly more convenient, especially with a phone app that allows you to begin preheating before you arrive home and electric maintains more even temps. Electric may be less healthy, this perhaps due to the calrods used for heating (though this, if it is a problem, could be eliminated with better heater design). Electric may have similar environmental impacts to wood though as resources are used to produce electricity and production of electricity often produces pollution (and making solar panels does as well). How the environmental impacts of wood vs electric compare is a much longer discussion.

Gas is not as prevalent but can be a good option. Some local codes will not allow remote app control.

Hybrid. I built a hybrid gas/wood fireplace for our house. Natural gas is used primarily to get the wood going but is sometimes kept on if wood is greener than it should be. Similarly, it should be possible to create a gas/wood hybrid sauna stove that can act as a traditional wood stove (with a convenient gas starter) or have the convenience of a gas stove when desired. 

 

Ventilation

Ventilation in such a small space seems critical. However, the folks building our sauna do not normally do any ventilation and say their saunas have been fine. Tylo-helo, our heater manufacturer, says to put a vent below the heater for air to enter and one 2’ above the floor on the opposite wall for air to exhaust. I’m not sure this will work though. There is not enough height difference to induce much airflow from natural convection and this would seem to make the sauna colder and not do anything for removing CO2.

Online we see recommendations similar to Tylo-helo or some that say to leave a gap under the door (which we did) and to have the exhaust vent in the ceiling.

One question is how much CO2 do we produce and do we need to do something to remove it. My guess is that we will produce enough CO2 to be a problem and that we do need a way to remove it and I’m not sure that what is recommended nor what we currently have will work. I’m beginning to test this now.

We are currently using kind of a reverse flow – bringing fresh air in higher and exhausting it lower. By turning on the ceiling exhaust in the adjacent shower we pull fresh outside air in through the ceiling vent (that would normally be exhaust) that then flows down through the sauna and under the door to be exhausted outside. This makes the sauna feel much better (better heat, fresher air and less lightheaded feeling) so we want to stick with this or something similar. However, this also results in the high limit switch on the heater tripping so that needs to be dealt with somehow.

 

Still To Learn:

We see extremely high particulate matter (1.0, 2.5 and 10.0) when ladling water on to the stones. Part of this and possibly all of it is bits of stone breaking off and disbursing with the steam. I’d guess some of it is also from the calrods which isn’t good.