Part one — before you build

Almost every sauna that disappoints was decided wrong before the first board went up. Bench too low. Heater too small for the glass. No intake vent under the heater. None of it is expensive to get right on paper and all of it is expensive to fix afterwards.

1. Indoor or outdoor, and how much room

An indoor sauna lines a room that already exists — a basement corner, a garage bay, a bathroom that is bigger than it needs to be. That is the cheapest route to a good room, because the shell, the floor and usually the power are already there.

An outdoor sauna means a new structure: foundation, framing, roof, siding and a weatherproof envelope, plus whatever your municipality wants to see first. Check setbacks and whether a small detached structure is exempt from permitting where you live. The interior is built exactly the same way either way.

For interior height, aim for 7 ft (2100 mm) from finished floor to finished ceiling. That is Finnmark’s standard and it sits in the middle of Tylö’s 190–220 cm window. Do not go below about 6 ft 7 in (2010 mm) — that is the minimum room height printed in Harvia’s own heater manuals, and most heaters are simply not listed for a shorter room. Do not go much above 8 ft either; Tylö is blunt about it, over 220 cm and you will struggle to get heat down to the lower bench. Height is never free, because every extra inch is volume the heater has to pay for.

2. Check the power before anything else

This is the step people skip, and it is the one that can kill the project. A typical North American home runs a 120/240V split-phase service, usually 100A or 200A at the main panel. A sauna heater is a large dedicated two-pole load, and the manufacturer’s own table decides the breaker and the wire. These are the published figures at 240V single phase:

Those are Harvia’s and HUUM’s numbers, and they agree with each other. Harvia adds one thing worth knowing before you buy wire: on a 4.5 or 6 kW heater, go up to 8 AWG if the run is longer than 30 ft.

Open your panel and count. Is there room for a two-pole breaker? Is there headroom in the service after the range, dryer, HVAC and any EV charger? If you are already close to capacity, a service upgrade is a real cost and belongs in the budget now, not later.

Wood-burning stoves sidestep all of this entirely — no circuit, no control, no panel capacity question — at the cost of a chimney penetration and a fire that has to be tended. For an outbuilding a long way from the house, that trade often wins. The clearances are stricter, though.

3. Sizing the heater

Start from room volume: width × depth × ceiling height, in cubic feet. The manufacturers state the rule in metric and it is blunter than most retail advice — Harvia and HUUM both publish 1 kW per cubic metre, which is 1 kW per 35 cu ft. Their own model tables are more generous at the top end: Harvia rates a 6 kW heater for 5–8 m³, and 8 m³ is 283 cu ft, or 47 cu ft per kW. So 40 to 45 cu ft per kW is the honest working figure, and when the answer lands between two heaters you take the bigger one.

Every row is checked against the manufacturers’ own published model ranges — Harvia rates 6 kW for 5–8 m³, 8 kW for 7–12 m³ and 9 kW for 8–14 m³, and HUUM’s DROP table agrees within a heater size. Note what the glass column does to a 6 × 6 room: 252 cu ft is comfortably a 6 kW heater until you hang a glass door on it, and then it is not. Add another ~4 cu ft per square foot for windows, a glass wall, or exposed tile or stone, and re-check.

Undersizing is the more common mistake and the more painful one: the heater runs flat out, never quite reaches temperature, and the stones never store enough heat to give you a decent steam. Oversizing makes the heater short-cycle, which dries the air and wastes stone mass. Above about 9 kW on a 240V single-phase home supply it is usually the service, not the room, that runs out first — that is the point to ask us about a three-phase heater. The Sauna Builder does this arithmetic for you, on these figures, as you pick the room and the glass.

4. Setting the heater: clearances, guard and stones

The clearances are on the rating plate and in the manual, and they are the one set of numbers on this page you must not take from a web page. They vary far more than people expect. Side clearance runs from 30 mm (1.2 in) on a Harvia Virta to 127 mm (5 in) on an Amerec 8.0S to 150 mm (5.9 in) on a HUUM DROP 9 — a factor of five between three heaters you can buy on the same afternoon.

What is consistent across the manuals is the vertical geometry, and it is what decides whether your room is tall enough at all:

One catch worth naming: Harvia’s very small European clearances are stated for non-combustible surfaces. The number you build to is the combustible-material figure on the rating plate, which is larger. And Tylö adds a layout rule no one else states — put the heater on the same wall as the door, as close to it as you can, so incoming air sweeps the stones. The ventilation geometry pushes you the same way.

Stones

The stones are not decoration. They are the thermal mass that turns a heating element into a sauna, and they are the single most common cause of a heater that will not hold temperature.

• Use the right rock. Peridotite, olivine, olivine diabase or vulcanite — Harvia names those four and HUUM specifies the same family. Buy them. Harvia is explicit that stones picked up off the ground are not suitable, and that ceramic “stones” and soft soapstone do not store enough heat. Decorative stones void Harvia’s warranty beyond a single top layer.
• Size. 2–4 in (50–100 mm) for an electric heater, 4–6 in (100–150 mm) for a wood-burning stove with the smaller ones on top. Discard anything under 2 in.
• Rinse the dust off under running water before they go in. Every manufacturer says this and most people skip it — it is why a new heater smells.
• Lay them one at a time. Do not pour them in. Big stones at the bottom and around the sides, smaller ones locking them in, elongated stones standing on end.
• Loose enough for air, dense enough that no element shows. Never wedge a stone between two elements and never rest one on the frame.
• Packed tight is the classic fault. Harvia says it cuts heating capacity; Helo says it trips the heater’s high-limit switch. If your heater keeps cutting out, this is the first thing to check, before you call anyone.
• Restack once a year — HUUM puts it at every 200 hours of use — and replace the ones that have started to crumble. Under heavy use that is annual; a lightly used home heater can run several years, and Harvia’s US manual says five.
• Order enough. 25 kg for a Harvia KIP45B, around 55 kg for a HUUM DROP, 70 kg for an 11 kW Virta. It always takes more than people think.

5. Electrical, and what a sauna counts as

A sauna is a hot, humid, conductive room, and the rules that matter come from three places at once: the heater’s own manual, the electrical code, and — this catches people out — the residential building code.

All of this goes to a licensed electrician who pulls the permit and signs it off. That is not a formality — it is what makes the room insurable.

6. Ventilation, planned now not later

A sauna needs air moving through it or it turns into a stale hot box, and the wood stays damp instead of drying between sessions. The target is roughly six air changes an hour. Most home saunas run gravity ventilation — no fan, just the heater pulling air through — and the placement below is what makes that work.

• Supply, low and beside the heater. A 50–100 mm (2–4 in) duct below or next to the heater, so incoming cool air is drawn straight up through the stones instead of across the room at ankle height. Tylö puts a number on the free area: at least 125 cm² (about 19 sq in) for heaters up to 8 kW, rising to 300 cm² for 16–20 kW.
• The gap under the door is the secondary supply. Leave ½ to ⅝ in between the door bottom and the finished floor and never seal it — 2 in (50 mm) if you are burning wood.
• Exhaust, low and as far from the heater as possible — under the benches on the far wall. This is the vent that actually does the work, and it should be about twice the diameter of the supply. Low, not high: pulling the air out at floor level drags the hot air down across the bathers first.
• A drying vent, high near the ceiling. Keep it closed while the room is heating and while you are in it, then open it afterwards so the room dries out.
• Keep every vent well away from the temperature sensor. Harvia wants 1000 mm from an omnidirectional supply valve and 500 mm from a directional one; HUUM wants 500 mm from any opening. A draught across the sensor makes the control think the room is cold, and it will run it hot.

Cut and rough the duct penetrations while the walls are open. Retrofitting a vent through a finished, foil-lined, clad wall means undoing all three layers.

7. Flooring and drainage

Tile is the right answer for almost every sauna. It is impervious, it survives being splashed, and it can be sloped to a drain. A wet tile floor at 180°F with bare feet is exactly where people fall, so ask for a slip number — and ask for the right one.

A floor drain is optional in a home sauna and expected in a commercial one — SaunaFin calls it non-essential at home, Finnmark recommends it, Tylö requires it in every public room and adds one hard placement rule: never directly under the heater. If you fit one, waterproof first, then slope the substrate at ¼ in per foot (2%), the same figure the plumbing code uses for a shower liner, and carry the membrane up the wall above the finished floor. On a wood subfloor, code-minimum L/360 deflection is not enough for tile — TCNA and APA both want a stiffer floor and an uncoupling membrane over it. Without a drain, keep the floor sealed and let the drying vent do the work.

Cedar duckboard over tile is comfortable underfoot and lifts out for cleaning, which is the point — it must lift out, or it becomes a mould trap. Never lay wood flooring in a sauna as the finished surface.

8. What goes into the wall, in order

From the framing inward, a sauna wall is five layers, and every one of them earns its place:

1. Framing — 2×4 studs at 16 in centres is the North American standard, and it is what Finnleo, SaunaFin and Cedarbrook all specify. 2×6 buys the depth for thicker insulation, which is what Harvia’s 100 mm is asking for. Block solidly wherever a bench frame, heater guard, backrest, wall light or the sensor will fasten. Frame in kiln-dried spruce rather than cedar — SaunaFin warns that cedar’s acids attack fasteners.
2. Insulation — unfaced mineral wool or fibreglass between the studs, full depth, no gaps.
3. Vapor barrier — foil-faced, foil side facing into the room, lapped and taped at every seam with aluminium foil tape. This is the single most important layer in the wall.
4. Battens — over the barrier, creating the air gap the cladding needs.
5. Cladding — tongue-and-groove boards, fixed to the battens.

Batten and cladding together add about 1¼ to 1½ in (32–38 mm) to each wall face — so roughly 3 in off the width and 3 in off the depth of the opening you framed. If you are lining an existing finished wall and adding fire board and a sauna insulation board on top of it, that figure climbs to 2¾–3¼ in (72–84 mm) per face. Either way: measure the framing, then subtract, then order.

9. Insulation and the vapor barrier

How much. Harvia specifies 100 mm (4 in) in the walls and the ceiling and calls 50 mm the minimum. Tylö asks for at least 45 mm of mineral wool or fibreglass at home and 95 mm in a public room. The North American kit makers speak in R-values instead — Finnleo wants R-11 or better in the walls with a 6 in batt in the ceiling, SaunaFin R-12 throughout. In a 2×4 wall an unfaced R-13 batt packed full is the practical answer; in a 2×6 wall, or a ceiling where you have the depth, take more. Only Finnleo asks for more in the ceiling than the walls, so treat that as an upgrade rather than a rule.

Use unfaced batts and add the foil separately. Do not use kraft-faced insulation: the facing is combustible and, in the manufacturer’s own words, must not be left exposed.

The foil vapor barrier goes on with the reflective face toward the room — Harvia’s wording is “the glossy side is towards the inside”. Lap the joints (published figures run from 2 in to 5 in, so lap 3 in and stop worrying), tape every lap with aluminium foil tape, carry it up onto the ceiling and tape the wall-to-ceiling junction, then seal around every penetration: cables, vent ducts, light housings. Not duct tape and not house-wrap tape — both let go at sauna temperature.

One honest note, because you will find contradictory advice: Tylö is the outlier and does not recommend foil or plastic in the wall at all, and a couple of North American kit suppliers ship foil but tell you to staple it and skip the tape. Harvia, Superior Saunas and Finnmark all specify taped aluminium foil, and so do we. What nobody disputes is that polyethylene sheet and house wrap have no place in a sauna — they melt, or they smell, or both.

10. Battens and cladding

Battens run perpendicular to the direction the cladding will lay, fixed through the vapor barrier into the studs. Horizontal cladding needs vertical battens; vertical cladding needs horizontal ones. Finnmark specifies 20 × 45 mm at 400 mm centres, Harvia 22 × 50 mm, SaunaFin 1 × 2 spruce at 16 in on centre — the same wall described three ways. What matters is the cavity they create: HUUM puts a number on it and says at least 20 mm (¾ in) between the foil and the back of the boards. That gap lets the back of the cladding dry, and it is what keeps the boards from cupping.

Some North American kits skip it and nail the cladding straight to nailers over the foil. It works, and plenty of prefab rooms are built that way. The vented cavity is still the better wall.

Every time you drive a fastener through the foil, you have made a hole. Keep them to the battens, and tape any tear you make.

Cladding is nailed or stapled through the tongue so the fixing is hidden by the next board. Leave about ¼ in (10 mm) at the floor and the same at the ceiling — the boards will move. Start from a level line, not from the floor, which is never level.

11. Bench heights and the geometry of sitting

The upper bench is the room. Everything else is circulation. And the height that matters is not measured up from the floor — it is measured down from the ceiling, because what you are really setting is how much of the hot layer you are sitting in.

• Upper bench: set it 1100–1200 mm (43–47 in) below the ceiling. That is Harvia’s figure, and Tylö states the same thing as a limit — no more than 120 cm. In a room with a 7 ft (2100 mm) ceiling that puts the bench at about 41 in (1050 mm) off the floor, which is the number European build guides quote. North American kits often sit the top bench at 34–35 in because they are working to lower ceilings. Derive it from your ceiling; do not copy someone else’s number.
• Lower bench: about 24 in (600 mm) in the European layout, 16–18 in in the American one. Same rule — it is the tier below the top one, not a fixed height.
• Put in a step. The 17–18 in rise between tiers is more than twice the 7¾ in maximum riser the residential code allows on a staircase. That is not a step, it is a climb, and it is the detail that makes a sauna unusable for older parents. Harvia calls 350 mm (14 in) the comfortable step height; Finnmark uses 300 mm (12 in). Add a footrest or step at that height.
• Depth: 24 in (600 mm) on the upper tier so you can lie down — Harvia’s minimum is 500 mm and 600 is preferred. 16–20 in (400–500 mm) on the lower.
• Length and width: a clear 6 ft (1800 mm) run if anyone is going to lie down, and 24 in (600 mm) of width per seated person.
• Clear of the heater: at least 20 in (500 mm) horizontally, and check your heater’s own figure for anything above it. Helo does not permit a bench directly over the heater at all.
• Board spacing: ¼ to ⅜ in between boards so water drains and air moves through. (Superior Saunas run ¾ in on their wide 5½ in boards; on narrow boards that is too much.)

Never fasten a bench board from the top. Fix from below, into the frame, so there is no metal where skin lands. A bench screw at 190°F will burn.

12. Doors, glass and hardware

Three rules that are not negotiable: the door opens outward, it has no lock, and it has no latch that can jam — a roller catch or a magnetic catch only. If someone faints against the door, it still has to open.

That is not us being cautious. Tylö and Helo print it in the same words in their manuals — “DO NOT install a lock or mechanical locking device onto the door of the sauna room” — and Finlandia says the door must open out and should have no lock. To be accurate about where it comes from, though: it is a manufacturer instruction, not a residential code requirement. The building code only forces an outward swing at 50 occupants or more. Some commercial jurisdictions do write it into law — New York City’s health code requires a free-swinging or outward-swinging door, a viewing window, a thermostat capped at 194°F, and either a one-hour timer or an attendant checking every 15 minutes. Build the home room to that standard anyway.

What is code everywhere is the glazing: any glass in a swinging door has to be safety glazing (IRC R308.4.1). Sauna doors are typically 8 mm toughened, with a 6-12-6 dual-pane option if you want the room to hold heat.

• Size. 24 × 80 in is the standard leaf. Finlandia keeps it at 24 in wide on purpose — less heat out, and less chance of the leaf warping. Accessible doors run up to 36 in.
• Rough opening. To your door maker’s number, not a rule of thumb: Superior Saunas frames 3 in larger than the jamb in both directions, others say 2 in. Getting this wrong is a framing job you do twice.
• Gap underneath. ½ to ⅝ in for the secondary air intake, never sealed. 2 in (50 mm) if you are burning wood.
• Hardware. Wooden handles inside the room. Stainless or brass hinges — plain steel rusts within a season.

Hang the door plumb to the finished cladding, not to the rough opening.

13. Which timber to use where

“Never use pine” is one of those rules that gets repeated past the point of being true. Finnish saunas have been clad in spruce and pine for as long as there have been Finnish saunas. The real rule is narrower and worth getting right: no knots and no resin where skin lands. A knot is denser than the wood around it, so it runs hotter than the board it sits in, and the sap inside it can melt and weep at sauna temperature. So — clear, low-density, knot-free boards on benches and backrests (cedar, aspen, thermo-aspen, alder, abachi, basswood), and kiln-dried, well-graded spruce or pine is perfectly good on walls and framing.

Keep plywood, MDF, chipboard and anything else with a glue line out of the room, along with pressure-treated lumber. No manufacturer manual bothers to spell this one out, so take it as reasoning rather than a citation: the adhesives in sheet goods are not made to cycle at 90°C and high humidity, and preservative-treated timber releases what it was treated with.

One honest caveat about cedar. Western red cedar contains plicatic acid, a documented respiratory sensitiser — the evidence comes from people who work with the dust all day, not from sauna bathing, but if scent or allergy is a concern in your household, aspen and thermo-alder are the low-odour choices. Either way, wear a mask when you are cutting it.

Order 5 to 15% over your measured quantity for cutting waste, which is Finnmark’s own allowance. Take the high end if the room has angles.

14. Lighting and the last 5%

Light in a sauna should be indirect and low. Behind a backrest, under the upper bench lip, or in a shielded corner — anywhere but overhead where it shines in the eyes of someone lying down. Fixtures must be rated for the heat as well as the humidity — an IP number covers water and dust, not temperature, so a good bathroom fitting can still be the wrong fitting here. Buy something the maker lists for sauna use, and keep it out of the heater’s clearance zone.

Dimmable is worth paying for: the same room at full brightness and at 10% is two completely different experiences. Beyond light, the pieces that get used every session are a thermometer and hygrometer, a bucket and ladle, a sand timer, backrests, and a stone cup for essential oil — oil goes in the cup, never straight onto the stones.

15. Heat, löyly and looking after the room

Building it right is most of the job. The rest is knowing what the room is supposed to do once it is finished, because a surprising number of “my sauna is not right” problems are settings and habits rather than construction.

• Temperature. Harvia calls 160–185°F (70–85°C) normal; Tylö’s traditional range is 75–90°C. The control’s legal ceiling is 194°F (90°C). Start around 175°F and adjust from there — there is no correct number, and Harvia refuses to give one.
• Humidity, and the myth about it. A traditional Finnish sauna is dry between throws — Tylö’s published figure is 5–15% relative humidity. The “40–60% humidity” you will read online describes a different bath entirely; at 85°C it would not be survivable. Humidity in a sauna arrives in bursts and leaves again. That is the whole point of löyly.
• Water on the stones. One ladle at a time. Harvia and Saunum both cap the ladle at 0.2 litres (about 7 oz), HUUM says roughly 80 ml, Helo says half a cup. Let the stones dry between throws. Clean tap water only.
• Heat-up takes 30–60 minutes for an electric room at its rated kW. If yours takes much longer, look at the stones before you look at the heater.
• After every session, leave the door open and open the drying vent. The room has to dry out or the wood never does.
• Once a year, restack the stones, replace the crumbled ones, wash the benches and re-oil them.
• Finishes. Paraffin oil on the benches. Wax on the walls and ceiling if you want the protection — but not on top of oil, they are mutually exclusive on the same surface. Harvia is blunt about everything else: using lacquer on the interior surfaces of a sauna is strictly prohibited. No varnish, no paint, no sealer, ever.