Sauna Rocks

Aug 2, 2022

If the heart of the sauna is the kiuas, or sauna heater,
then the soul is the rocks.

—Rob Licht

Every brand of heater I’ve installed has a different approach to the rocks. Some, like a wall hanging 6 or 8 kilowatt unit, use less- maybe 40 pounds, and some, like the Harvia Cilindro or Club heaters use more— up to 200 pounds. Wood burners vary too, Kuuma’s heater takes 150 pounds or so, but you have to provide your own. My own custom built stoves ( Lämpimämpi ) take a similar amount and like the Kuuma stove, the rocks are piled on top, mounded as high as you can. The Harvia Legend, an elegant wood burner from Finland, basically has a cage that surrounds the stove. By the time you are finished loading it, you won’t see the stove. There is even an optional cage that surrounds the stove-pipe to hold even more rocks.

sauna rocks in electric heater

The Finnish and Swedish heaters use Olivine Diabase or Peridotite, igneous rock found in Scandinavia. All of the rocks I have used that come with the electric heaters look the same: grayish chunks, some flatter, some chunky, all in the size range you’d find in a bag of potatoes. These are amazingly cheap considering that they come all the way from some quarry in Finland or Sweden. The UPS driver has stopped asking what is in the boxes (“yup, rocks”). Once I found a note in one from a young Swede hoping that I was happy with my rocks.

My favorite rocks are hand selected glacial erratics: various igneous granitic and metamorphic stones that I find in places where glacial melt-waters sluiced out potato sized rocks. (shown below) These are heavier and denser than our native shale, which is worthless for the sauna since it tends to explode when heated. They have a crystalline structure that you can usually see and a solidity and heft that is evident when you pick them and bang them together (they are not easy to chip.)

glacial erratics
Lämpimämpi with Glacial Erratic Rocks
Lämpimämpi with Jagged Olivine Diabase Rocks

A big consideration when selecting a heater is the rock capacity. The more rocks, the longer you should let your sauna heat up. Clients always ask:” how long should we let it heat up?” While I usually say “about 45 minutes” I really want to answer with the same sort of retort I give when someone asks how long will take for paint to dry (as long as it takes). The correct answer should always be: the sauna is ready when the rocks are hot enough to produce good löyly or steam—löyly is the soul of the sauna’s mysterious expression. And how hot is that? 450° Fahrenheit, more or less. If you want more steam, then you want more rocks.

Löyly is the soul of the sauna’s mysterious expression.

—Rob licht

If you expect several rounds or people coming and going, then more rocks are good. No one wants to pour water on the rocks expecting a stimulating burst of steam (but not too sharp) only to hear the fizzle of water barely boiling. That’s like trying to make an omelet in a pan that is barely hot. Disappointing, to say the least, but also bad for the heater. There’s Finnish saying that goes something like this: when you leave the sauna put another log on for the sauna elf or he will pee on your stove. Which means, don’t keep throwing water on the stove, especially at the end, if the rocks are not hot. The warm water will just sit on the steel stove and rust it. When you throw water on the rocks, it should all turn to steam and when you are done, the stove should dry quickly. When I finish eating my omelet, I rinse my cast iron skillet and let it dry over the flame, I don’t let it languish all wet in the dish rack; same principle applies to the sauna stove.

Jagged rocks will catch and momentarily hold more water, allowing less to get past the rocks and into the heater. The dense roundish rocks I use produce a nice soft steam but they have to be hot. I’m not so worried about water getting down to my stove, which is always very hot (and mine have 3/8” steel plate on top so they will never rust out). By the way, always use clean water and be wary of city water with chlorine or even country water with minerals or methane. We use only use filtered water for löyly at home. Spring water or distilled water might be prudent in some cases. Never use pool water or worse, let some disrespecting novice wring out their bathing suit over the rocks, as I have seen happen in a gym. Pleasant smells should emanate from the rocks. Special sauna oils or even Sauna Brew can be added to the löyly water for aromatic effect. Heck, we used to add beer to the water (cheap lager was always best).

Custom Build Electric Sauna
Custom Sauna with Harvia Cilindro with carefully placed olivine rocks.

If your electric heater comes with the grey jagged olivine rocks, I suggest using them. They are selected to fit between and around the elements. When setting the stones first I rinse them, then I take care to arrange the rocks one by one. It can take me 45 minutes to fill a large capacity heater. For the Cilindro, I place large rocks at the bottom to hold up the others and then work a flattish layer against the cage wall. I then backfill the rocks against the elements with smaller and irregular rocks. On the smaller wall-mounted units I try to fit the flat stones between the elements without forcing or bending them. The goal is loosely fill the cage so there is good airflow and to completely conceal the elements. Lastly, I top off the heater with larger stones and try to arrange them so they will catch water and force it inward, not splashing it out. In Finland there might a “spirit stone” placed here: a special stone or stones from a favorite place (make sure it is igneous or metamorphic—test it in a campfire first if not sure). The top might even have a layer of more decorative stones, which in Finland you can buy from sauna dealers, but here in the US, I have only seem the same dull grey stones. The Harvia web page has a good section on stones and how to place them into the pillar.

Once you have the sauna going and have been using it for a year or so, expect the stones to settle and flake off. You should remove them, vacuum out the heater and replace the rocks loosely, discarding the broken ones and adding more if needed. Failure to do so will cause the heater to eventually overheat and lead to the elements failing.

Care in selecting and placing the rocks will ensure that your sauna…. Rocks!

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The Last Word on Foil.

Dec 21, 2021

Lately I have been thinking about the application of the foil I use in my saunas as a radiant vapor barrier. Perhaps this is because it almost Christmas and I was thinking of how we decorated the tree each year. The final touch would be to drape foil tinsel over everything; our mother would have to constantly damp down our enthusiasm by reminding us to place it carefully on each branch, not to throw it. 

NOT sauna foil.
This suspicious “sauna” foil is Aluminum-coated Plastic—upper working temperature of only 55-120° C.

There are tricks to using the foil but the first and most important step is to buy the right stuff. Like the tinsel we put on the tree, the foil may actually be aluminum-coated plastic— which you don’t want to use. That plastic is likely polyethylene which, if you look it up on the material specification sheet that every product has, it has an upper working temperature of 55-120° C, meaning it will likely melt at typical sauna temps. Sauna Foil, available from any of the familiar sauna suppliers, is aluminum foil on a kraft paper backing. I used to find it with fiberglass reinforcing thread, which is helpful because the stuff tears easily. Also helpful is 4 ft. rolls, rather than 3 ft so you can do a wall in 2 passes, but I have trouble finding this too. I recently tried a new supplier selling 4 ft rolls of  “sauna” foil, but upon opening it had a suspicious plastic look to it. That night I put it in the sauna and within seconds it began to distort and curl up like the polyethylene I suspected it was made of. (See illustration above)

The second thing is to design the wall correctly. I read and see a lot of misinformation that touts using no air gap with foil.  The air gap is essential.  The foil works by reflecting radiant heat. All “black bodies” give off and absorb radiant heat that travels in a straight line from one hotter object to another cooler one, the hotter the body, the more heat it emits. The sauna rocks radiate a “soft” heat to you, the walls, and the benches, and that is why you want the sauna to be laid out so that everyone has a view of the rocks. The fire, if seen through a clear glass door, also radiates heat— but at a higher intensity. Too high for a comfortable sauna (but great for ambiance.) When that heat hits foil, it is reflected back into the room or the backside of the cedar—if there is an air gap of at least 1/2″. If it touches the backside of the cedar the foil— also a perfect conductor—pulls the heat away from the cedar and transfers to the wall space behind.

Proper sauna insulating with an air gap on backside of cedar.
Air Gap. A Sauna Building Best Practice.

I’ve understood this for along time. The first semester of college I took a class: Solar Design and the Energy Efficient Home. We learned all about insulation, heat transfer and basic building skills. The first day of lab, where we were building a timber frame house, I was handed a Makita 12″ circular saw. My building career started right then and there.

With the web of misinformation out there I had to think of a way to illustrate this basic principle of thermodynamics that I learned my freshman year. So, one slow day in the shop I rigged up an experiment and photographed it. (see illustration below) I set up a section of cedar wall about 18″ from my infrared shop heater and fastened 2 pieces of foil to the back, one with a 3/4″ air gap, and one with no gap. After an hour the cedar was 250°F on the front—like it is often is in my sauna. The back of the cedar was 121° F, which is impressive by itself. The back of the foil with no gap was 115°F, meaning it was acting as a perfect conductor, and the back of the foil with an air gap was 71°F: room temp. The air gap was clearly making a difference, 45° in this case. 

Sauna thermodynamics by sauna builder Rob Licht Custom Saunas
The thermodynamic experiment begins.
  • After an hour on cedar
  • back of cedar
  • back of foil, no gap
  • air gap makes big difference!

The foil is a perfect vapor barrier rated at zero perms— meaning no vapor moves through it. But unless you layer it properly, with insulation behind it, the moisture will condense on it, or the first cold surface it hits. Even in a perfect build, there might be cold spots in the insulation (typically about the size of a mouse hole), so there likely be some condensation, but not a problem if there is air movement. The air gap behind the cedar allows air to circulate around the cedar, removing any moisture and ensuring that the wood heats and dries evenly and remains stable. Heating one side of a board and wetting or cooling the other is how you make curved boat staves.

There are other tricks to using the foil- like unrolling it and re-rolling it foil-in, or using temporary magnets when working a commercial job with metal studs, but the key is to use care. Use plenty of hi-temp foil tape and patch tears as you go and work with a partner if possible.

I suppose you could build a sauna by putting a heater in a refrigerator box- but that would last about a day and be incredibly wasteful. Cedar touching foil won’t ruin your sauna and neither will plastic melting in the walls where you don’t see it. But if you are going to take the time and bear the expense of building a sauna, you might as well do it right and so it will last generations. I guess my mother was right: applying foil carefully and not just throwing it up is the way to work.