The Perfect Sauna Wall
My name is Ryan Phelps. I’m the owner of Mune Sauna. We build great saunas.
If our sauna is going to help us live a longer life, our sauna better live a long life too.
And that is why I came up with The Perfect Sauna Wall. This is not the definitive text on the subject but it is an informed take speaking from collectively 40 plus years of carpentry experience. If you have an alternative perspective please let us know in the comments! I think this is about as good as you can build a sauna wall. You can get away with walls that are different than the wall I’m proposing but I believe this sauna wall will last longer than the alternatives.
The Perfect Sauna Wall consists of 8-9 layers depending how you count them. Exterior Cladding > Exterior Rainscreen > Housewrap > Sheathing > 2x4 Framing > Insulation > Vapor Barrier > Interior Rainscreen > Interior Cladding.
We will work our way through the layers one by one. I’ll give you an option or two for each layer and explain what I think is the best way of doing it. Let’s Begin!
Exterior Siding:
Cladding is also known as siding. The exterior and interior cladding are the parts of the wall that we interact with most. Every other part of the wall is sandwiched between the cladding layers.
We have many options here and realistically this is mostly personal preference. A few common options are Hardie cement board, LP Smartside, tongue and groove cedar or thermally modified wood, and metal siding. Hardie and LP are classic sidings used on homes. It’s commonly found as lap siding. If theres a siding that says “sauna” it’s wood siding. This is typically found in the tongue and groove variety. Shiplap is also a popular choice for wood cedar siding. Most people will probably use either Hardie Lap Siding, LP Lap Siding, Cedar tongue and groove or thermally modified tongue and groove siding.
A note about wood siding options; Western Red Cedar is known to be a durable wood. It resists rotting better than other wood species. When it comes to sauna, western red cedar is almost synonymous with sauna in the USA, and for good reason. It’s durable, looks beautiful, smells amazing and is reasonably available in the US. You could use pine or fir but they aren’t as durable, don’t look as good, they smell fine and they are available in the US. There’s one more option for wood cladding and that is thermally modified wood. Thermally modified wood, which I will call thermo, goes through a non-toxic process that introduces high heat and steam to the wood….like a sauna but more intense…. This process ends up producing wood that is very stable, very durable and absolutely beautiful. You definitely pay for it. I receive more remarks from strangers about the thermo wood siding than I do any other siding that we use. If you want to go the thermo wood route I would steer you to Arbor Wood Co who’s based out of Duluth, Minnesota. Thermory is another option but their lengths are metric so they do not come in standard building lengths. For example an 8’ board is a standard building length, Thermory’s closest 8’ foot board is around 7.7’. This can complicate the build.
Exterior Rainscreen:
Rainscreens come from the building science world. A rainscreen is an air gap between the siding and the rest of the wall. So, a rainscreen is anything that can create that gap. What’s the point of a rainscreen? The siding is the main shield between the outside environment and the rest of the wall. If and when your siding becomes wet, you want it to dry as thoroughly and as quickly as possible. Why? Water is the enemy of buildings. It is the leading cause of buildings deteriorating under normal conditions. A tornado or tsunami would be abnormal conditions. So, in a buildings day to day life, water is its biggest threat. That means we need to have a plan for water. If and when our sauna becomes wet we want to dry it as quickly as possible so that our sauna can last as long as possible. A rainscreen is one of the ways that we give our sauna the ability to dry. Think of your siding as a wet towel. Use those creative muscles! So, your siding is a wet towel. Siding without a rainscreen is like a wet towel that is wadded up into a pile on the ground. Siding with a rainscreen is like a wet towel hanging from a clothesline. Which towel do you think will dry faster? Which towel do you think will smell better if left there for a few days? The towel that’s hung on a clothesline! Siding installed over a rainscreen is like the clothesline towel. It has air flowing all around it thus leading to a siding that dries quicker and more thoroughly. Rainscreens also let the REST of the wall dry quicker and more thoroughly. Water also travels as vapor. That vapor can get into your walls. If your siding is completely sandwiching your wall then the wall has no where to breathe. Putting air gaps in your wall lets the full wall to dry quicker and more thoroughly if and when your sauna becomes wet.
How do we incorporate an exterior rainscreen into our sauna? We have two options that depend on if our siding is installed vertically or horizontally. If our siding is installed horizontally, like lap siding or tongue and groove installed horizontally, then we will use 1x2 furring strips. The 1x2 furring strips will be installed perpendicular to the siding which in this case means they will be installed vertically. 1x2 furring strips is a type of lumber. You can find these at your lumberyard by asking “do you have any 1x2x8 lumber?”
Now, let’s say you are going to install your siding vertically. We have two options. Option one is based off of how we installed the 1x2 furring strips from our first example. Except now we will have two layers of furring strips. The first layer will be installed vertically and will be installed on the studs. The second layer will be installed directly on top of those furring strips but this time they will be installed horizontally. The reason you install two layers is so that air can still pass under the siding. If the rainscreen is installed horizontally then air cannot freely move behind the siding because the furring strip is creating a dam. The cross batten opens this dam up. The second option is a product called Coravent. Coravent is a composite material that can be installed horizontally. Coravent has a bunch of air channels that allow air to pass freely through it thus not creating the dam effect that happens with 1x2 lumber. Going with coravent will reduce the overall thickness of your wall if you are installing your siding vertically.
Housewrap:
Also know as homewrap, wrb, weather rain barrier, and Tyvek.
This is the rain jacket of your sauna. The siding protects the majority of water from getting into your walls. The rainscreen allows the walls to dry if water does get behind the siding. The housewrap is the last stop for water entering your walls. If water gets behind your siding, housewrap is its last stop. the housewrap’s job is to not let the water go any further, just like a rain jacket. Because of our rainscreen, we don’t necessarily have to be too picky about what brand we use for our house wrap. With that said, avoid woven house wrap. Big box stores like Lowes will sell this woven house wrap with their logo on it. If you can make the case for this stuff, please do so in the comments.
Besides being a rain jacket, another thing that housewrap can do is help create an airtight wall. Thus leading to an airtight sauna. What’s an airtight sauna and why would I want that? This is the part where we have to talk about ventilation. Please read Trumpkin’s Notes for all things ventilation. The perfect sauna wall is designed for an electric sauna. In this electric sauna we will have two openings for air. A passive air-intake vent above the heater and an exhaust fan below the benches. We want to confine all of the airflow to those two openings. Buildings are very air-open unless you specifically build to make the building airtight. Typically, buildings get their air tightness from the housewrap. BUT in a sauna, we have a unique situation that we can take advantage of. That unique situation is the Vapor Barrier. A well sealed vapor barrier becomes the airtight layer meaning we do not necessarily need to use a housewrap that is designed to be airtight. Because of that, a more standard housewrap is all that we need. ESPECIALLY since we have a rainscreen. We don’t need the most robust housewrap because the rainscreen is doing the heavy lifting. We also don’t need a housewrap that is airtight because our vapor barrier is doing the heavy lifting. That’s called building smart!
With all of that said, what are our options? We have two- Tyvek as the standard option and Blueskin VP100 as the upgraded option. Blueskin VP100 is the “belt and suspenders” approach. It self heals around nail holes which means it is really blocking water from entering and it is airtight which means if we make a slight mistake with sealing our vapor barrier, the Blueskin will help offset that mistake giving us a little bit of wiggle room.
Some people may be thinking “Well what about Zip?!” The other people may be thinking “What is Zip?” Let’s get into it.
Sheathing:
I have a confession to make. I used to use OSB for the sheathing. But not anymore! Now I use plywood.
Sheathing is OSB or plywood. It’s what gives a building its structure. It’s that 4’ x 8’ flat wood stuff that you may have seen at Lowes or on the side of a new building being built. If we didn’t use sheathing our building would just sway in the wind and eventually come crashing down. We really only have two options here OSB and plywood. So why do I use plywood and not OSB. One reason: Vapor Permeability. For our use case, that’s a fancy way of describing how fast something can dry. Remember, water is the enemy of buildings. If and when water enters a building we want it to dry as thoroughly and as quickly as possible. If vapor has entered our wall, we want to give it the highest potential of leaving our wall. Plywood is more vapor permeable than OSB which means if there is vapor in or behind our sheathing. And the environments are the same. The sauna with plywood will dry quicker and more thoroughly than the sauna with OSB. Knowing that, I choose plywood every time now because I want my sauna walls to have the highest possibility of lasting the longest. One more note here about the thickness of the sheathing. Typical thicknesses are 3/8”, 7/16”, 15/32” and 3/4”. The thicker the material the longer it takes to dry. The thinner the material the less structural it is. The sweet spot for wall sheathing thickness is 7/16” or 15/32”.
Zip sheathing is a fluid applied housewrap over OSB sheathing. Then you tape the seams creating an airtight wall. The problem is they use OSB instead of plywood. I’ve used ZIP before but I no longer do simply because it’s an OSB product.
2x4 Framing:
This is the bones of our sauna. It’s the first part of the wall that you put up. When your 2x4 walls are up it’s the first time that your sauna actually kind of looks like a sauna. You COULD use 2x6 framing here. There is no down side and only up side to using 2x6 framing. The up side, for our purpose, is more insulation. A 2x6 simply allows you to install two more inches thick of insulation into your walls. An airtight, insulated 2x4 framed sauna holds heat very well! Go 2x4x8 KD (kiln dried) unless you have a good reason to go 2x6. Go with kiln dried lumber. Green lumber is going to shrink and shift more than dried lumber is. The less our wood moves the better. Go with dried wood as much as is available to you. You wont regret it. This is also another story about how we really want as little moisture in our walls as possible.
Insulation:
Without insulation your sauna would be an empty, echo-y feeling building which would feel weird. We pretty much have one option here for our Perfect Sauna Wall. Mineral wool is the gold standard for good reason. It won’t mold and degrade when exposed to moisture and it has fantastic sound deafening properties. If you live in a suburban or city environment there can be a lot of noise. From Honda Civics to lawnmowers and leaf blowers. A sauna can be exceptional when experienced in a quiet, slow way. Mineral wool has the ability to greatly reduce the noise of the outside environment. Theres a theme happening here and that theme is moisture management. We want our wall to handle moisture to the highest level at every layer. Mineral wool does that better than any other insulation and as another plus it has better sound deafening properties as well. And on top of that mineral wool has a higher r-value than fiberglass.
This is the part where I address the elephant in the room: Insulating Foam. XPS, EPS, POLYISO, Closed cell, Open cell, ETC, ETC. Trumpkin Notes recommends rigid foam as the vapor barrier. Two birds one stone. You get your insulation and vapor barrier all in one easy package. And on top of that you reduce thermal bridging! This sounds like a miracle product! So why do i not use POLYISO or PIR insulation in my saunas? Because foam, like polyiso is a form of plastic. I don’t trust that over time the polyiso wont expand and contract so much that it loosens the tape seal and thus exposing the interior of the saunas air to hot plastic. That’s a hot soup that I would like to avoid! Am I being too cautious here? I don’t know. You tell me.
Vapor Barrier:
The vapor barrier plays a critical role in a sauna by containing the heat and steam. Hotter air wants to move into cooler air. If we did not have a vapor barrier the air in the sauna would continuously be rushing into the rest of our wall. Taking all of the heat and steam with it. With an impervious layer we are able to contain the sauna environment allowing us the experience we all love. A hot sauna with steam rich air. We want to avoid plastic vapor barriers and aluminum vapor barriers coated in plastic. A few examples that we want to avoid are bubble foil and perforated foil. The best foil to use here is a kraft backed aluminum vapor barrier. 4’ tall rolls are better than 3’ tall rolls because you can cover more wall in less time. If you only have access to 3’ tall rolls; don’t think twice about it and grab that.
Now we need to seal the vapor barrier. We will be doing this mostly with aluminum tape which is also known as HVAC tape. The only real consideration here is we want to avoid tape that is coated in plastic. Tape that is about 2” wide is the sweet spot. You don’t need high temp HVAC tape. Many HVAC tapes can handle the heat of a sauna which 230 Fahrenheit at its absolute peak.
We have one more option for sealing our vapor barrier and that is silicone. Standard 100% Silicone is inert up to about 350 Fahrenheit. Well above any temperature our sauna will be seeing. If you want to get very specific, grab a tube that says something like “neutral cure” rather than something that says “acetoxy”. If you are trying to seal around a penetration and the aluminum tape isn’t working as well as you need it to, don’t hesitate to grab for a tube of 100% silicone.
Our vapor barrier is our best defense against moisture entering our walls from the interior. It’s also our best defense when it comes to directing air where we want it to enter and where we want it to exhaust, thus helping with the luxurious feeling of our sauna. Take your time to seal the full sauna. Double check your vapor barrier after you have installed it. An airtight sauna is a long lasting luxurious sauna.
Interior Rainscreen:
We are almost to the end of our Perfect Sauna Wall layers. This layer and the next layer are redundant layers meaning we have these layers on the exterior and therefore we have discussed the whys and the different options. This rain screen is here to create a gap between the interior siding and the aluminum vapor barrier allowing our interior siding to dry quicker and more thoroughly than it would if it was stuck right next to the impervious vapor barrier. Therefore giving you a sauna that feels fresher and lasts longer. The only difference here that I want to mention is that we will not be using a composite version of a rainscreen inside of the sauna like Coravent. A sauna is too hot for a material like that. What we will be using is 1x2 furring strips. Just like on the exterior. These do not need to be made of cedar. They can be Fir, Pine or whatever is readily available where you live. If your siding is installed horizontally, then all you need to do is attach the furring strips to the studs (2x4 framing) vertically. If you are installing your interior siding vertically then just add one more layer of furring strips horizontally. This is the cross batten approach that I mentioned earlier. Now air can pass behind your siding effortlessly.
Interior Cladding:
The final layer! We did it! This is the layer that you will become intimate with as you use your sauna through the years. We basically have one option here which is tongue & groove wood cladding. The wood that we use needs to be a soft wood. As long as you are using a soft wood you can really use whatever you would like. Just research it a bit before actually pursuing it. The most normal wood options here in the US is Western Red Cedar or a thermally modified wood from Arbor Wood Co or Thermory. You can use pine or fir I suppose but as I mentioned earlier in the exterior siding section, it is not as durable or as beautiful as the other options. Always go with dried wood here. Also known as kiln dried. You will find siding labeled something like this: 1x4 T&G KD Western Red Cedar. The 1x4 refers to how wide and thick it is. In this example 1x4 is .75” thick and 3” wide. T&G refers to tongue and groove. KD refers to kiln dried. A 1x4 or 1x6 is going to be the most standard options for our purpose.
If you incorporate all of these layers into your wall and take your time to detail them correctly, you will be well on your way to having a luxurious feeling sauna that lasts a very long time.
Ryan Phelps
Mune Sauna
Salem, OR ✌🏻