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Step by step

How to build a sauna

The build in order, using a 6 ft x 7 ft outdoor sauna as the example. The designer writes the same steps with your own sizes.

Updated

This example has 294 cu ft inside, a Harvia 8 kW heater, a top bench at 38 in and 3 seats. Its materials come to $8,490 at the store prices we read in October 2026. Open it in the designer to change any of it.

The section that matters

IN 8 inOUT 24 inDRY 76 in46 inTOP 38 inLOWER 20 in7 ftSECTION, HEATER WALL LEFT, BENCH WALL RIGHT

Read it left to right: heater on the door wall with the fresh-air intake low beside it; open floor; the lower bench; the top bench 46 in under the ceiling; the exhaust low on the bench wall and the drying vent high on it.

The build

  1. Site and footing. Mark a 7 ft 2 in x 8 ft 2 in footprint on level, drained ground. Lay the landscape fabric, dig and compact 6 in of paver base under each deck block, set the blocks level and string-line the skids. Check your setback and permit rules first.
  2. Floor. Set the 4x6 skids in the blocks, then the 2x6 rim joists and floor joists 16 in on center. Square the frame by measuring the diagonals. Glue and screw the 23/32 tongue-and-groove floor.
  3. Walls. Frame four walls in 2x6 at 16 in on center, 81 in studs, with a doubled top plate. Frame the door opening at 27-1/2 in x 75 in in the heater wall. Add 2x6 blocking where the bench ledgers, the heater and the heater guard will screw on, before anything covers the studs.
  4. Ceiling. Set ceiling joists 16 in on center across the 7 ft 2 in width. Then the 4/12 rafters at 16 in, each nailed to a ceiling joist at the heel, ridge board, gable studs, roof deck, underlayment, drip edge and metal panels. Hang the T1-11 siding and the door frame, prime and paint.
  5. Rough-in. Your licensed electrician runs the dedicated 240 V heater circuit (33.3 A heater; conductors sized at 125 percent, 41.7 A or more), the light feed and the control or sensor cable, with heat-rated cable inside the room. Cut the vent openings now. Get the rough-in inspected before you insulate.
  6. Insulation and foil. Fill walls and ceiling with mineral wool. Staple the foil vapor barrier over the studs and joists, shiny side into the room, seams lapped 2 to 3 in and every seam and every hole taped with foil tape. The foil is continuous: no plastic sheeting, ever.
  7. Air gap. Screw 1x2 furring over the foil on every stud and joist. That 3/4 in gap behind the boards lets the wood dry; the heater maker asks for about 10 mm.
  8. Ceiling boards, then walls. Board the ceiling first, front to back, then the walls, horizontally from the top down so the last cut row lands behind the lower bench. Leave about 4 in (100 mm) between the bottom board and the floor and a small gap at the ceiling. Blind-nail through the tongues with stainless fasteners (galvanized can stain cedar). Western red cedar stays unfinished: no varnish or stain inside a sauna.
  9. Benches. Screw the ledgers to the blocking. Build the top bench at 38 in to the slat top, the lower one at 20 in, slats with 1/2 in gaps, all screws driven from below so no metal touches skin. The lower bench stands on its own front and back legs. Ease every edge.
  10. Heater and guard. Mount the heater on the blocking at the height and clearances in its manual, fit the sensor where the manual puts it, and have your electrician make the final connection. Build the wooden guard around it. Stack the stones loosely by hand.
  11. Vents, light and door. Fit the adjustable intake low beside the heater, the exhaust grilles on the back wall under the top bench and the drying vent high on the same wall. Install the light under the top bench. Hang the door to open outward with a catch, never a lock.
  12. First heat. Run the room empty for an hour with the vents open to burn off dust. Check the thermometer at head height on the top bench, then enjoy it.

Tools you will use

  • Circular saw and a miter saw for the boards and bench slats
  • Drill-driver, a hammer-tack or staple gun for the foil, a finish nailer for the boards
  • Level, square, chalk line, tape measure
  • A table saw helps rip the door jamb and the last board row, but a circular saw with a guide will do

Questions people ask

Is it cheaper to buy or build a sauna?

Usually cheaper to build. A pre-cut 6 x 7 ft indoor kit in clear cedar, interior parts only and no heater, listed at $9,197 on Oct 9, 2026; the whole 6 x 7 ft indoor room in the planner, framing, insulation and a heater included, prices at $6,329 before the electrician. You trade money for your own time and a lower board grade.

How much does it cost to build a sauna?

For the 6 ft x 7 ft outdoor sauna on this page, $8,490 in materials and heater at the store prices we read in October 2026, before the electrician, freight and tax. An indoor room of the same size is cheaper because it needs no footing, roof or siding.

What type of wood is best for a sauna?

A soft, low-resin wood that stays cool to the touch: western red cedar, or thermally modified aspen or spruce. Leave it unfinished inside. Never use pressure-treated lumber or plywood for the inside surfaces.

Is building your own sauna hard?

It is basic carpentry: framing, insulation, nailing boards and building benches. The parts that need a licensed professional are the 240 V heater circuit and, for a wood stove, a chimney installed exactly to its manual.

What is the cheapest way to build a sauna?

Keep it small with the ceiling near 84 in so a small heater covers it, buy tongue-and-groove cedar from a lumber yard, build your own door, and frame inside an existing room so you skip the footing and roof. Do not save on the heater circuit, the foil vapor barrier or the heater guard.

How do I calculate the size of a sauna heater needed?

Multiply inside width, depth and height in feet to get cubic feet. Add glass and bare masonry: Harvia counts 1.2 m3 per m2, about 4 cu ft per sq ft of glass. Then pick a heater whose maker-rated range covers that number. The designer does this for you.

How big should my sauna heater be?

For the 6 ft x 7 ft x 7 ft room on this page, 294 cu ft: a 8 kW heater rated 250 to 425 cu ft. A 4.5 kW heater covers roughly 100 to 210 cu ft and an 8 kW about 250 to 425, by the makers' ranges.

What goes behind sauna wood?

From the room outward: the boards, a 3/4 in air gap on furring strips, a continuous aluminum foil vapor barrier with taped seams, mineral wool insulation between the studs, then the outside wall.

Sources

Every rule the planner applies comes from the heater maker's own manual or product data where one exists. Where a number is our own default, the designer says so next to it.

  1. 01HUUMDROP 2.0 electric heater user manual (EU, v6)Room 2000 to 2300 mm high, never under 1900 mm; top bench 1100 to 1300 mm below the ceiling; 50 to 100 mm insulation, foil with taped joints, 10 mm gap behind the boards, 12 to 16 mm boards, 100 mm from the floor; supply air 50 to 100 mm at or below heater mid-height (400 mm), exhaust twice that on the opposite wall at least 200 mm higher (600 mm), drying vent under the ceiling; six air changes an hour; only a certified electrician connects the heater.Read 2026-10-09
  2. 02HarviaHow do I select the correct heater power?Each square meter of stone, glass or similar uninsulated surface adds as much load as 1.2 cubic meters of room volume.Read 2026-10-09 (search listing; the page answered 403)
  3. 03HarviaKIP45 product dataRoom 106 to 212 cu ft, minimum room height 6.56 ft, 3.61 ft from the heater to the ceiling.Read 2026-10-09
  4. 04HarviaM3 wood-burning heater product dataRoom 212 to 459 cu ft; to combustible material 11.81 in front and rear, 9.84 in each side; minimum room height 6.56 ft.Read 2026-10-09
  5. 05NFPA 70, National Electrical CodeArticle 424.4(B), branch-circuit sizing for fixed electric space-heating equipment (2020 text)Branch-circuit conductors are sized at no less than 125 percent of the load. Your jurisdiction's adopted edition and the heater's nameplate govern.Read 2026-10-09 (secondary summary)