Sauna Building Checklist: Every Step in Build Order

A sauna goes together in one direction, and several of its most important parts — the heater feed, the vent openings, the blocking that carries the benches — disappear inside the wall midway through the job. Miss one and the fix means cutting open a finished vapor barrier. So this checklist runs in strict build order. Work the phases top to bottom, and do not let the walls close until every box in Phase 3 is ticked.

The list assumes the most common first build: an indoor room with an electric heater. Notes flag where wood heat or an outdoor building adds items. The sauna build guide explains the reasoning behind each entry; this page is deliberately the short version — the what, in order, ready to print and pin to a stud.

The heat-lamp box is not a sauna

Free plans that have circulated online since the early 2000s describe a narrow plywood cabinet warmed by four 250-watt heat lamps, hand-wired to a homemade switch and leaned against a wall so it will not tip over. That is not a sauna. Hand-wiring mains cable inside a hot plywood cabinet is a shock and fire risk, not a weekend project. A sauna is heated by a kiuas (the stone-topped sauna stove) sized to the room and installed to its manual, with the wiring done by a licensed electrician — the electric heater guide shows what a proper installation involves.

Phase 1: Decisions — before you buy anything

Every expensive sauna mistake is a decision made too late. Settle these on paper before the first purchase; the planning guide works through each one in depth.

• Bench layout first. Design from the benches outward: two tiers, with the top-bench sitter’s shoulders 100–120 cm (39–47 in) below the ceiling. The room exists to hold the benches, not the other way around.
• Room size and ceiling height. Around 7 ft of ceiling suits most home builds. Extra height is not automatically better — it demands a taller bench stack to match, or the heat sits above everyone’s head.
• Heater type. Electric or wood-burning. Wood heat brings a chimney, a hearth, and a separate inspection with it.
• Heater size. Sized by room volume — and every square foot of glass adds roughly 3–4 phantom cubic feet before sizing. The heater sizing guide has the math.
• Ventilation strategy. Gravity or mechanical, decided now, because the two systems put their openings in opposite places.
• Indoor or outdoor. Outdoor adds a foundation, a weather barrier, and a roof to this list.
• Wood species, walls and benches together — compared in the wood guide.
• Budget and route: kit, precut package, or full DIY.

Phase 2: Permits and paperwork

Call the building department before the lumber run. It is a ten-minute conversation, and it is far cheaper before framing than after.

• Electrical permit. A hardwired 240 V heater circuit needs a permit and an inspection almost everywhere in North America. This is the box people skip and regret.
• Building permit. Rules vary widely: small detached buildings are sometimes exempt, interior remodels sometimes not. Ask rather than assume.
• Outdoor builds: confirm property-line setbacks and the local frost depth for footings.
• Garage conversions: the garage-to-dwelling fire separation (IRC R302.6) must remain intact. Any drywall removed behind the sauna gets rebuilt to the same rating.
• Wood-burning heaters: the chimney is a listed system with its own clearances and its own sign-off — read the wood-burning heater guide before committing.

Phase 3: Framing and rough-in — before the walls close

This is the phase the whole page exists for. Supply cables and vent paths run through the wall cavity and become unreachable once the foil and boards go on. Opening a finished sauna wall later means redoing the vapor barrier, so nothing closes until every box here is ticked.

• Frame the room: 2x4 or 2x6 studs at 16 in on center, with the door rough opening sized to the unit plus shim allowance.
• Heater circuit roughed in. Dedicated circuit, copper conductors rated for the temperature — a typical 6 kW heater wants 30 A on #10 copper. No receptacles in the hot room. Most manuals instruct against a GFCI breaker on the heater feed, and Harvia prohibits one outright; Finland’s electrical-safety authority Tukes draws the same line, requiring 30 mA residual-current protection on every sauna circuit except the heater’s own. Your electrician and inspector have the final word.
• Sensor cable routed exactly where the heater manual says. Many sensor leads must never be spliced.
• Vent openings framed. Gravity systems: supply low near the heater, exhaust high on the far wall. Mechanical downdraft — the pattern argued best in Trumpkin’s Notes, and the standard advice in Finnish ventilation guidance — reverses it: supply high above the heater, exhaust below bench level and far away. Plan for about 6 air changes per hour; the ventilation guide covers both systems.
• Bench blocking installed: 2x lumber between studs at every bench attachment height. Bench weight lands on studs, never on paneling.
• Lighting cable run, rated for the location.
• Rough-in inspection passed, where permits require one.

Phase 4: Insulation and foil

The wall gets exactly one vapor barrier — the aluminum foil — and everything outside it stays vapor-open so the assembly can dry. The foil does two jobs at once: it reflects radiant heat back into the room, and it stops steam from reaching the framing. It does both only if it is continuous.

• Mineral wool in every stud bay: R-15 in 2x4 walls, R-23 in 2x6. It handles sauna temperatures, does not burn, and does not hold water. Avoid foam unless it is specifically approved for sauna assemblies.
• Foil stapled to the interior stud faces, reflective side facing the room, every seam and staple line sealed with aluminum tape.
• Foil carried unbroken across the ceiling. Hot air pushes moisture hardest at the highest point of the room.
• No second vapor barrier anywhere in the assembly. Two low-perm layers with framing trapped between them is the classic hidden-rot detail.
• Basement builds: keep an air gap between sauna framing and foundation concrete. Below-grade concrete is permanently damp.

Phase 5: Cladding, floor, door, and glass

The paneling needs two things the finished room will hide: acclimated boards and an air gap behind them.

• Acclimate the boards one to two weeks inside the sauna room, packages sealed until install day — Thermory’s installation guides are explicit about this. Wood movement should happen before nailing, not afterward as gaps and cupped boards.
• Furring strips over the foil: 3/4 in of ventilated air gap behind every board, strips running perpendicular to the cladding. Skip them and the paneling stays damp behind its back face.
• Horizontal boards go in from the top down, tongue up, so a splash of löyly (the steam from water thrown on the stones) drains out of the joint instead of pooling in it.
• Vertical boards get about 0.4 in of expansion gap at each corner, hidden by moulding.
• Floor decided on purpose: duckboard, tile, or a deliberately bare slab. Concrete pulls heat from bare feet fast, which is why the slab-only floor should be a choice, not a leftover.
• The door swings outward and has no latch — roller or magnetic catch only. A bather who faints against an inswing door turns it into a barricade.
• All glass is tempered, confirmed by the etched label on every panel. Tempered glass cannot be cut or drilled after manufacture: frame the opening, measure it, then order. Spans and thicknesses are in the doors, windows, and glass guide.

Phase 6: Benches

A loaded bench is a live load of several hundred pounds in a hot, humid room. Build it like furniture; fasten it like framing.

• Top bench at least 38 in high, with the second tier about 18 in below it — a natural sitting step that doubles as the top sitter’s footrest.
• Feet at or above stone-top level where the layout allows. Feet parked on the floor sit in the coldest air in the room.
• Fasten through the cladding into the Phase 3 blocking, never into paneling alone.
• Center legs under long spans: lower-bench runs past about 63 in want mid-span support.
• First climb about 12 in, never much past 14 — overheated bathers have reduced balance.

Depths, layouts, and the reasoning behind the heights are in the bench guide.

Phase 7: Heater and stones

The heater arrives last among the trades, and its manual outranks this page on every dimension it prints.

• Mount to the manual’s clearances exactly — wall, front, bench, and ceiling. Wood inside the listed distance slowly chars toward its ignition point over years of heating, even if it never looks burnt.
• Electrician makes the final connections: the high-temperature heater whip, the disconnect, and the control panel mounted outside the hot room where the manual requires it.
• Wood stoves get a rated hearth pad beneath and around the stove, and a chimney installed as a listed system.
• Stack the stones loosely: angular stones roughly 5–10 cm across, with air paths between them, and heating elements fully surrounded where the manual demands it — HUUM makes that a hard rule.

Phase 8: Commissioning — before the first fire

Every route through this list ends at the same gate: an electrician checks the heating and the lighting before anyone bathes.

• Electrical inspection passed, sign-off in hand.
• One heating cycle run empty. Most heater manuals call for a first full heat-up with the room ventilated and nobody inside — new elements and protective coatings give off fumes once.
• Warm-up timed. A well-sized room reaches bathing temperature in roughly 60–90 minutes. Much longer suggests an undersized heater or glass nobody accounted for.
• Vents verified: supply open, exhaust open, air actually moving through the room.
• Door verified: swings out freely, nothing latching it shut.
• Sensor placement matches the manual — the thermostat’s whole behavior depends on it.
• Stones re-checked after the first heat. They settle.

Then throw the first löyly and find out what all the careful sequencing was for. Hyvät löylyt!