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 about 7 ft (2.1 m) from finished floor to finished ceiling. Higher than that and you are heating air nobody sits in; the heat that matters gathers in the top 18 in of the room. If you have an 8 ft ceiling and want to keep it, size the heater for the extra volume.

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 6 kW sauna heater draws roughly 25A at 240V and wants a dedicated 30A two-pole breaker; an 8 kW heater is closer to 33A and wants a 40A or 50A breaker depending on the manufacturer's table.

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.

3. Sizing the heater

Start from room volume: width × depth × ceiling height, in cubic feet. For a well-insulated cedar room the working figure is roughly 1 kW for every 45 cubic feet. A 6 × 6 room with a 7 ft ceiling is 252 cu ft, which lands at about 5.6 kW — a 6 kW heater.

Then add for glass and for any uninsulated surface. Glass sheds heat far faster than a clad and insulated wall, so it has to be paid for in kW. A workable allowance is to treat every square foot of dual-pane glass as if it added about 2.5 cubic feet of room, and to double that for single-pane. Exposed stone or tile on a wall behaves the same way.

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. The Sauna Builder does this arithmetic for you as you pick the room and the glass.

4. Electrical, and what a sauna counts as

A sauna is a wet, hot, conductive environment and your local electrical code treats it as a special location. The parts that matter in practice:

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.

5. 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 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.
• The gap under the door is the secondary supply. Leave roughly 5/8 in between the door bottom and the finished floor and never seal it.
• 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.

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.

6. 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. Use a floor tile with a real slip rating — R11 or better for a home sauna, R12 or R13 for anything commercial or heavily used — because a wet tile floor at 180°F with bare feet is exactly where people fall.

A floor drain is a luxury indoors and near-essential in a commercial room; if you have one, slope the substrate to it before tiling. If you do not, keep the floor sealed and rely on the drying vent to dry the room out afterwards.

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.

7. 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 or 2×6 studs at 16 in centres, blocked wherever a bench bracket, heater guard or wall light will land. Add the blocking now.
2. Insulation — mineral wool or a sauna-rated board 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.

Together these add roughly 3 to 3.5 in (72–84 mm) to each wall, which is why you measure the framing and then subtract before ordering a kit.

8. Insulation and the vapor barrier

Use mineral wool or a purpose-made sauna insulation board — the sauna-rated boards are safe at sustained +100°C, which ordinary foam is not. Fill the stud bays completely; a gap is a cold spot and a cold spot is where condensation forms inside the wall.

The foil vapor barrier goes on with the reflective face toward the room. Lap every joint by at least 2 in and tape every lap with aluminium foil tape — not duct tape, not house wrap tape. Carry it up onto the ceiling and tape the wall-to-ceiling junction. Seal around every penetration: cables, vent ducts, light housings.

9. Battens and cladding

Battens run perpendicular to the direction the cladding will lay, fixed through the vapor barrier into the studs. Use battens at least 3/4 in thick and 1-3/4 in wide (20 × 45 mm), spaced about 16 in (400 mm) apart. The air gap they create lets the back of the cladding dry, and it is what keeps the boards from cupping.

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 a small expansion gap at the floor and at the ceiling — the boards will move. Start from a level line, not from the floor, which is never level.

10. Bench heights and the geometry of sitting

The upper bench is the room. Everything else is circulation.

• Upper bench: about 41 in (1050 mm) off the finished floor. This puts your head in the hottest layer of air.
• Lower bench: about 24 in (600 mm), which also serves as the step up.
• Bench depth: 24 in for the upper tier so you can lie down, 18 in for the lower.
• Head clearance: leave at least 42 in between the upper bench and the ceiling.
• Board spacing: leave 1/4 to 3/8 in between bench boards so water drains and air moves through.

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.

11. 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 magnetic catch only. If someone faints against the door, it still has to open.

Glazing is tempered, and dual-pane if you want the room to hold heat. Handles inside the room are wood, never metal. Hinges are stainless or brass; plain steel rusts within a season.

Leave the 5/8 in gap under the door for the secondary air intake, and hang the door plumb to the finished cladding, not to the rough opening.

12. Which timber to use where

Never use pine, plywood, MDF, treated lumber or anything with a glue line in a sauna. Resin bleeds, glue outgasses, treated timber releases what it was treated with. Order 5 to 15% over your measured quantity for cutting waste — more if the room has angles.

13. 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 and the humidity.

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.