Walls & Insulation
The wall assembly is where most of the long-term performance of your sauna is determined. Get this right and the room will perform well for decades. Get it wrong and you'll deal with moisture damage inside your walls.
Framing
Sauna walls are framed like any interior partition, with one important addition: you need blocking at the exact heights where bench ledgers will attach. Studs are typically 16 inches on center and may not land where you need them.
- Stud spacing: 16″ O.C. standard; 24″ acceptable for non-load-bearing walls
- Stud size: 2×4 minimum; 2×6 preferred for exterior walls
- Bottom plate: Treated lumber if the floor is concrete
- Top plate: Double top plate for ceiling rigidity
- Blocking: Horizontal blocking at all bench ledger attachment points
- Door rough opening: Typically 24–30″ wide × 78″ tall; always swings outward
Insulation
Use mineral wool throughout — Rockwool or Roxul. Do not use fiberglass batts. Fiberglass loses its R-value when exposed to humidity and its facing degrades at sauna temperatures. Never use spray foam or foam board inside a sauna.
| Location | Type | Min. R-value | Notes |
|---|---|---|---|
| Exterior walls | Mineral wool | R-15 (2×4) / R-21 (2×6) | Most critical after ceiling |
| Interior partitions | Mineral wool | R-11 | Reduces heat loss to adjacent spaces |
| Ceiling | Mineral wool | R-30 minimum | Heat rises — highest priority surface |
| Floor | Not insulated | — | Drainage required; use duckboards |
Vapor barrier
This is the most commonly misinstalled element in sauna construction. Getting it wrong causes moisture to accumulate inside your wall cavity over time.
The vapor barrier goes on the warm side — between the insulation and the cedar cladding. Use heavy-duty aluminum foil vapor barrier, not plastic poly sheeting. Minimum 0.5 mil; 1 mil preferred.
- Overlap all seams minimum 6″ and tape with foil tape
- Seal every penetration: electrical boxes, vent openings, heater cable
- Run continuously from floor to ceiling with no gaps
- Staple only through the overlap flap, never through the foil face
- Poly sheeting used instead of foil — wrong material, degrades at temperature
- Vapor barrier placed on the cold side of insulation — causes condensation in wall cavity
- Unsealed penetrations — moisture infiltrates insulation over time
Interior cedar cladding
- Species: Western red cedar, clear grade — knots concentrate heat and become burn hazards
- Profile: Tongue-and-groove (T&G), 1×4 or 1×6
- Orientation: Vertical preferred; horizontal acceptable
- Fasteners: Blind-nail through tongue only with stainless steel or hot-dipped galvanized nails
- Gap at floor: Leave ½″ for drainage and air movement
- Finish: None — bare cedar only. No stain, sealant, or paint.
Estimates include 10% waste factor. Labor, fasteners, tape, and door not included.