The four things that make it a sauna
- A tight, insulated box. Mineral wool in the walls and ceiling, then a foil vapor barrier with every seam taped, then a 3/4 in air gap, then wooden boards. The foil keeps steam out of the insulation; the gap lets the boards dry.
- A heater sized to the room. Heater makers rate each model for a range of room volumes. Glass and masonry count extra: Harvia adds 1.2 m3 of volume for every m2 of glass.
- Benches up in the heat. The top bench sits 43 to 51 in below the ceiling. That puts your head in the hottest layer and your feet off the cooler floor.
- Air that moves. A fresh-air intake low by the heater, an exhaust low on the far wall, a drying vent high on that wall.
Indoor or outdoor
An indoor sauna goes into a basement corner, a garage or a large bathroom: four stud walls on the slab, a ceiling, and the envelope above. An outdoor sauna is a small cabin: footing, floor, 2x6 walls, a roof and siding, then the same inside. The outdoor build costs more because it is a building, not a room.
| Priced in October 2026 | 5 x 7 ft indoor | 6 x 7 ft outdoor |
|---|---|---|
| Site, floor, shell and roof | $195 | $1,996 |
| Framing | $334 | $581 |
| Insulation, foil, furring | $758 | $988 |
| Cedar boards | $2,392 | $2,272 |
| Benches and guard | $380 | $380 |
| Heater, vents, light, door hardware | $2,210 | $2,273 |
| Total priced | $6,268 | $8,490 |
Open either in the designer: 5 x 7 ft indoor, 6 x 7 ft outdoor. The electrician, freight and tax are on top.
How big
Count about 24 in of top bench per person. A 5 ft wide room seats two side by side, 6 ft seats three, 8 ft seats four. Depth has to hold the benches and still leave you 30 in to stand: two tiers take about 43 in, so 6 to 7 ft deep is comfortable. The door and the heater with its guard need about 6 ft of wall between them; in a narrower room the planner moves the door to the side wall. Keep the ceiling at 84 to 90 in. Every extra inch is volume the heater has to warm.
The 5 ft x 7 ft room above has 245 cu ft and takes a Harvia 6 kW heater; the 6 ft x 7 ft cabin has 294 cu ft and takes 8 kW.
Where DIY saunas go wrong
- A plastic vapor barrier. Use foil rated for sauna heat, taped at every seam.
- Boards straight on the foil. Without the furring gap the back of the wood never dries.
- Benches too low. If the top bench is 30 in from the floor in a 7 ft room, your head sits in the cool half.
- A heater sized by guesswork. Too small never reaches temperature; too big cycles hard. Use the maker's range.
- Finish on the inside boards. Varnish gets sticky and smells. Leave the inside bare.
- A door that locks. Sauna doors open outward with a catch, never a lock.
Questions people ask
Can I put a sauna in my back yard?
Usually yes, as an accessory building, but most towns set a size above which a building permit is needed, setbacks from property lines, and an electrical permit for a new 240 V circuit. Call your building department with the footprint before you buy lumber.
How much does a DIY sauna cost?
With the prices we read in October 2026, a 5 x 7 ft indoor room comes to about $6,268 in materials and heater, and a 6 x 7 ft outdoor cabin to about $8,490, before the electrician, freight and tax.
How do I build a sauna cheap?
Keep the room small and the ceiling near 84 in so a small heater covers it, build your own door, and pick the board wisely: tongue-and-groove cedar from a lumber yard was the cheapest lining we priced, and the same 5 x 7 ft room in thermo-aspen sauna boards comes to $13,460 against $6,268. Do not save on the heater circuit, the vapor barrier or the heater guard.
Can I use a regular electric heater?
No. Use a heater built and listed for saunas, sized to the room, installed as its manual says and connected by a licensed electrician. Space heaters are not made for 190 F and steam.
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.
- 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
- 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)
- 03HarviaHow to design a saunaThe seat for one person is usually 60 cm wide.Read 2026-10-09
- 04NFPA 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)
- 05Haven of HeatHeater listings with the maker's room rangesHarvia KIP 4.5 kW 100 to 210 cu ft, 6 kW 170 to 300, 8 kW 250 to 425; Cilindro 9 kW 141 to 494; Harvia M3 212 to 459; Pro 20 283 to 706.Read 2026-10-09