If you are planning to install an outdoor sauna, you need to do a good bit of research. How much electricity is required? Do you need a drain? Do you need to install additional insulation?
Most outdoor sauna installs require insulation for the best performance, regardless of the “heat-insulating” properties of the wood used in construction. The lack of insulation means that heat will seep out rather than build up in your sauna. Barrel saunas are the exception to this rule as they are specifically designed to better retain heat.
Keep reading to learn why insulation matters, the types of insulation available for saunas, and what to look for when you’re planning and building.
Does an outdoor sauna have to be insulated?
Maybe you have already purchased a build kit for your outdoor sauna and want to know if insulation is really that important, especially if you live in a warmer climate.
Insulation is not absolutely necessary for sauna use, but your sauna will retain and maximize heat best if it is insulated. Barrel saunas are the exception to the rule, but generally, insulation is best for maximal sauna performance.
Here’s a brief science lesson:
According to the second law of thermodynamics, heat naturally flows from a higher-temperature object or area to a lower-temperature object or area. In application to saunas, if the heat in your sauna has the chance to escape to a cooler area, it will.
It’s just physics.
The way to prevent heat from escaping is to insulate your sauna. In addition, if warm air comes up against cooler walls, condensation will develop, which may damage the beautiful wood of your sauna.
Even if you live in a warm climate, the outdoor temperature is unlikely to be as warm as the interior of your sauna. If you set your sauna even in a lower temperature range of 110℉-120℉, it is not typical that your environment will be that warm all the time, and since many sauna users prefer sauna temperatures of 150℉ or higher, heat escape and condensation are inevitable without proper insulation.
Barrel saunas
Unlike other outdoor sauna designs, barrel saunas do not require insulation for ideal performance.
These factors design features of barrel saunas make it possible for them to retain and maximize heat without insulation:
- Shape – The circular design of a barrel sauna causes warm air to constantly rise and cool air to fall, so that the air is in constant motion and being reheated, maintaining the internal temperature more efficiently. And, because of the rounded roof, rain and snow do not collect on top to make the roof cold.
- Area – Even with similar outdoor dimensions, a barrel sauna has about 23% less interior area to heat than cabin-style saunas.
- Wooden staves – The long planks of wood that encircle the cylindrical back and front walls provide some insulation of their own as they are designed to fit together more tightly than in cabin-style saunas.
You can still insulate your barrel sauna if you choose, but it is not as necessary as in other types of saunas. If insulation is a step you would like to skip, it would be wise to choose a barrel sauna.
Pod saunas
Pod saunas have a raindrop shape, rounded on the top and flat on the bottom.
Insulation is your best bet if you have a pod sauna as they are not as efficiently designed as barrel saunas.
While they are similar in some ways to barrel saunas because their rounded roofs allow rain and snow to slide off and reduce the cold exterior temperature and because they have similar-style wooden staves, they do not have the benefits of a fully circular shape for the best convection heating and have interior areas similar to those of cabin-style saunas.
Cabin saunas
Cabin saunas are square or rectangular in shape and require venting or sometimes fans to support the best air circulation patterns for heat convection.
Cabin saunas are not specially designed for heat retention, no matter the material used in construction, and definitely should be insulated for best heat performance.
The staves are not as closely fitted as in a rounded sauna, and the flat roof allows snow to pile up in winter weather.
Luna saunas
Luna saunas are also square or rectangular in shape, but with rounded corners for visual appeal.
Luna saunas are very similar to cabin saunas and will require insulation to retain heat and prevent condensation from forming.
Do pre-built outdoor saunas already have insulation?
If you bought a sauna kit or pre-built sauna, you are probably wondering if insulation is included.
Many manufacturers tout the “heat-insulating properties” of the wood they use, such as cedar, but do not offer other insulation besides the wood itself. While some types of wood do keep heat in more efficiently, it is still best to add insulation for efficiency and to protect the wood from condensation.
Check with your specific manufacturer, because a few do have insulation included, but if not, you will have to purchase insulation for your sauna.
What does the r-value mean for insulation?
So you know you need insulation. How do you know what type of insulation is best, and what is r-value?
R-value is a measure of thermal resistance, or how well a material prevents heat from traveling through. Higher R-values mean superior insulation. For an outdoor sauna, recommended R-values are R19 for exterior walls and R26 for ceiling insulation.
These recommended R-values will prevent warm air from escaping and keep condensation from forming on your sauna walls and ruining the wood. There are multiple types of insulation available, but only some may work for your outdoor sauna.
Sauna insulation types
There are several types of insulation available at any hardware store, but only a few can work in an outdoor sauna.
In a hot, moist sauna environment, Polyiso insulation boards or recycled cotton insulation protected by a vapor barrier are your best bets for safe and efficient insulation.
You might have some thoughts about what type of insulation you might want to use in your sauna, so read on for some pros and cons to help you make the best choice.
Paper-backed fiberglass rolls
Most people are familiar with puffy pink fiberglass insulation. With an R-value of 2.9-3.8 per inch and a low price point, it’s a popular choice.
While fiberglass is great for retaining heat, it has a couple of disadvantages when it comes to a sauna:
- It often has a paper backing, which encourages moisture retention, counterproductive to insulating your sauna.
- It sheds glass splinters that can damage your skin and lungs, so you have to work with it carefully and wear protective equipment.
If you do choose fiberglass insulation, be sure to buy it “naked,” without the paper backing, and wear protective gear.
Recycled cotton
Recycled cotton insulation is made from recycled textiles, mostly jeans, which gives it a blue coloring.
Recycled cotton insulation is a little bit more expensive than fiberglass, but there is no paper to deal with and no risk of handling the material.
The R-value of recycled cotton insulation is 3.0-3.7 per inch, slightly better than fiberglass, which may make up for the additional cost.
Insulation boards
Insulation boards, made of expanded polystyrene foam, extruded polystyrene foam, or polyisocyanurate, are easy to install and have high R-values compared with fiberglass or cotton insulation.
It is important not to use expanded polystyrene foam as these boards have air bubbles that can retain moisture and cause issues.
Polyiso boards, on the other hand, have R-values ranging from 7.0-8.0 per inch of thickness. Polyiso boards also have reflective foil on both sides, which provides water resistance and reflection of heat.
Vapor barriers
Vapor barriers used in homes are often made of plastic, but this would not work in a sauna given the high temperatures and moisture.
An aluminum foil vapor barrier is essential to insulating your sauna. A foil vapor barrier prevents moisture from reaching your insulation and wall studs, and it has the added advantage of reflecting heat back into the sauna.
The vapor barrier should be applied over insulation and is an important step that should never be skipped.
Adding insulation to your outdoor sauna
Now that you know you need to insulate your outdoor sauna, how do you go about it?
The floor, ceiling, and walls of your sauna may require different types of insulation and installation methods. If you are unsure, it is wise to consult a sauna service professional or your manufacturer for assistance.
Read on for some specific tips if you choose to DIY the insulation in your sauna. If not, reach out to a professional for insulation to make your sauna as efficient and enjoyable as possible.
Flooring
When insulating your sauna, it’s important to be thorough, but how thorough? Do you need to insulate the floor?
It is not necessary to insulate the floor of your sauna. Because warm air rises and cool air falls lower to the ground, heat will not escape from the floor, but condensation may form there. Since the floor is not made of wood, condensation on the floor is not a problem in terms of damaging the sauna.
In most cases, the subfloor is made of concrete, and the best options for flooring include non-slip tiles and floor mats.
Ceiling
Much of the heat in your sauna will be lost through the ceiling as heat rises. This is doubly true if there is a layer of cold rain or snow resting on the top of your sauna building.
The ceiling of your outdoor sauna should be insulated to an R-value of 26R and then protected by a vapor barrier. You should consider using Polyiso boards with a high r-value to meet the 26R recommendation.
Although Polyiso has reflective foil, it is not a substitute for a vapor barrier, so it should be covered with a foil vapor barrier intended for sauna use.
Walls
Even though most of the heat in a sauna is lost through the ceiling, you’re likely to lose quite a bit through uninsulated walls as well.
The walls of your outdoor sauna should be insulated to an R-value of 19R. Polyiso boards are again a good option, or if you need something more affordable, recycled cotton batt insulation will work.
Either way, be sure to cover it with a foil sauna vapor barrier!