Sauna Insulation: Layers, Thickness and the Foil Side

Sauna Dekor Spa Solutions engineering team, Istanbul · project and press enquiries: Alican Aykaç, Managing Partner

Updated: 12 November 2026

In brief (November 2026): sauna insulation is the layer of insulation and vapour barrier behind the timber lining that keeps the heat in the cabin and the moisture out of the wall. It is what lets a Finnish sauna reach 80–100 °C without heating the room around it.

A guide from Sauna Dekor Spa Solutions, an Istanbul manufacturer of saunas, hammams and steam rooms since 1987 — 40 years — with 5,000+ projects in 41+ countries.

Each of those layers does one job, and a sauna that skips one of them shows it within a year: the heater runs longer to hold 80 to 90 °C, the boards cup, or moisture appears where nobody can see it. This guide takes the wall apart layer by layer, compares the three insulation materials that get proposed for saunas, shows how uninsulated surfaces change the heater choice, and explains what changes when the sauna sits inside an air-conditioned building in Dubai, Doha or Riyadh. The arithmetic is written out, and the sources are listed at the end.

Table of contents

• What layers make up sauna insulation?
• How thick should sauna insulation be?
• Which side does the foil go on?
• Why does the 10 mm air gap matter?
• Mineral wool or foil-faced PIR board?
• Why is ordinary foam board a problem?
• How does insulation change the heater size?
• How is the ceiling insulated?
• Where do the cables run in an insulated wall?
• What changes in an air-conditioned Gulf building?
• What should be checked before the panelling goes on?
• Frequently asked questions

Five, counting the structure, from the room outwards: 12 to 16 mm of timber panelling, a 10 mm ventilated gap on battens, aluminium foil as the vapour barrier, 50 to 100 mm of insulation between studs, and the building wall or frame behind it. Harvia’s manual describes exactly this build-up as a wall with efficient thermal insulation.

The table below lists each layer with its thickness, the material that usually fills it and the document the figure comes from. It is the section we draw first on every sauna room, whether the cabin stands inside a villa bathroom or in a hotel wet area, because every later decision, from heater output to the position of the light fitting, depends on it.

100 mm of carefully fitted mineral wool in the walls, with 50 mm as the minimum, and at least 100 mm in the ceiling, according to the Harvia manual. With mineral wool at 0.037 W/m·K, 50 mm gives a wall U-value of about 0.61 W/m²·K and 100 mm about 0.33, so the thicker wall loses roughly half as much heat.

The arithmetic is short. The resistance of a layer is its thickness divided by its conductivity: 0.05 ÷ 0.037 = 1.35 m²·K/W for 50 mm and 2.70 for 100 mm. A 15 mm softwood board at an assumed 0.13 W/m·K adds 0.12, and the two surface resistances add about 0.17. The totals are 1.64 and 2.99 m²·K/W, and the U-value is the inverse of each: 0.61 and 0.33.

What that means in watts depends on the temperature difference. A sauna at 90 °C inside a room at 24 °C has a difference of 66 K. Each square metre of wall then loses about 40 W at 50 mm and about 22 W at 100 mm. A 2.0 × 2.0 m cabin, 2.1 m high, has about 33.6 m² of walls and ceiling, so the difference between the two walls is roughly 600 W, for as long as the room is held at temperature.

That is why 50 mm is written down as a minimum and not a target. On a hotel sauna that runs 12 or 14 hours a day, the extra 50 mm is paid for in electricity every day it is in service.

On the hot side, facing the room, with the glossy surface towards the sauna and every joint taped. Harvia’s manual states it directly: the moisture protection is aluminium paper with tightly taped edges, fitted so that the glossy side is towards the inside of the sauna. In a room at 90 °C and 10 % humidity, the dew point is about 39 °C.

The dew point decides the position. Air at 90 °C and 10 % relative humidity carries water vapour at about 7.0 kPa, using the Buck equation for saturation pressure (70.2 kPa at 90 °C). That vapour condenses on any surface colder than about 39 °C. Inside a 100 mm wall, the temperature falls from near room temperature at the panelling to near the adjoining room’s 24 °C at the back, so the 39 °C plane lies inside the insulation. Foil placed on the room side stops the vapour before it reaches that plane; foil placed behind the insulation would let it condense in the wool.

Tape matters as much as the sheet. A vapour barrier is only as good as its joints, because air leaking through a gap carries far more moisture than diffusion through a solid sheet. Joints are lapped at least one batten width, taped with aluminium tape, and the wall foil runs up behind the ceiling foil, so that any condensate runs down the face of the barrier and not behind it.

The same logic, taken much further, applies to steam rooms, where the room is at 100 % humidity and the barrier is a bonded membrane under tile rather than a foil behind timber. Sauna insulation and steam room waterproofing are different constructions, and a wet-area contractor who treats them as one tends to over-build the sauna or under-build the steam room.

Because the foil does two jobs and both need air in front of it. Harvia recommends a 10 mm vent gap between the moisture protection and the panel boards, plus a gap of a few millimetres at the top of the wall covering where it meets the ceiling. The gap lets the boards dry from behind and lets the foil reflect heat.

A reflective foil only reduces radiant heat transfer when it faces an air space; pressed directly against timber, its low emissivity does almost nothing. With 10 mm of air in front of it, the glossy side works as a radiant barrier as well as a vapour barrier, which is why the manufacturers specify it glossy side in.

The drying path is the second reason. Panelling takes up moisture every time water is thrown on the stones and releases it as the room cools. A 10 mm cavity behind 12 to 16 mm boards, open at the ceiling edge, lets that moisture leave from the back of the boards instead of sitting against the foil. Boards fixed hard against foil tend to cup and stain from behind within a few seasons. Our guide to sauna wood covers which species hold their shape best on that cycle.

Both work; they trade thickness against fire class. 30 mm of foil-faced PIR sauna board at 0.022 W/m·K has a thermal resistance of about 1.36 m²·K/W, the same as 50 mm of mineral wool at 0.037. The board saves 20 mm per wall, but its reaction-to-fire class is E, while stone wool is usually non-combustible.

Finnfoam’s FF-PIR SAUNA board, one example of the type, is 30 × 600 × 1,200 mm with a diffusion-proof aluminium laminate on both faces, a declared conductivity of 0.022 W/m·K and a service temperature of −50 to +120 °C, with short peaks up to 200 °C. Because the foil is already on the board, the vapour barrier becomes a matter of taping the board joints rather than hanging a separate sheet.

In practice, the fire class usually decides. In a villa, a PIR board behind timber inside a masonry room is common. In a hotel, a residential tower or a club, the fire consultant and the civil defence authority often want the non-combustible option in the wall around a heater, so we specify mineral wool there and accept the extra 20 to 70 mm. Where floor area is tight, the difference is real: on a 2.0 × 2.0 m cabin, 70 mm less wall build-up on three sides gives back about 0.14 m in each direction.

Because standard polystyrene softens close to sauna temperature. Expanded and extruded polystyrene has a glass transition temperature of about 90 to 100 °C, and the air under a sauna ceiling can reach that. A board that holds its shape at 24 °C can slump or shrink behind the panelling after a season at 90 °C.

The problem is not the insulation value. XPS board insulates well and is a normal material under a steam room bench or behind tile. The problem is the combination of heat and the fact that nobody will see the board again once the panelling is on. If a foam board is proposed for a sauna, the question to ask is the manufacturer’s stated service temperature, and the answer needs to be comfortably above the room’s design temperature.

Harvia’s manual adds a warning that sounds like the opposite case: heat protection such as mineral board fixed directly on a wall or ceiling can make the temperature of the materials behind it rise dangerously. The point is the same. Insulation belongs inside the wall, behind the foil and the gap, not as a hot face fixed on top of the timber.

Every uninsulated surface makes the room behave like a bigger one. Harvia’s rule is to add 1.2 m³ to the sauna volume for each square metre of non-insulated wall, such as glass, tile, concrete or stone. A 10 m³ sauna with a glass door therefore needs about the heater output of a 12 m³ room.

The correction adds up quickly on a modern design. Take a 2.4 × 2.0 m sauna, 2.1 m high, which is about 10.1 m³. Replace one 2.4 m wall with full-height glass and the uninsulated area is about 5.0 m². At 1.2 m³ per m², that adds 6.0 m³, so the heater must be chosen for about 16 m³, not 10. A tiled or stone feature wall behind the heater adds its own share in the same way.

Log walls are corrected differently: Harvia multiplies the volume of a log sauna by 1.5, because solid timber heats slowly. Our guide to types of sauna heaters compares heater families and output ranges, and the calculation above is the step that comes before choosing one.

With at least 100 mm of insulation between the joists, and no thinner fallback, according to the Harvia manual. The manual also recommends no more than 1,200 mm between the top bench and the ceiling, and allows the ceiling to be lowered to a minimum height of 2,100 mm so that the heater serves a smaller volume.

The ceiling carries the highest temperature in the room and the largest pressure of hot, moist air, so it is where gaps in the foil do the most damage. The ceiling foil is laid over the wall foil, not under it, and taped along the whole perimeter. Light fittings, sensor holes and the exhaust sleeve are all ceiling or upper-wall penetrations, and each one is sealed to the foil before the boards go up.

Lowering the ceiling is often the cheapest energy decision in a sauna. In a villa with a 3.0 m slab-to-slab height, a suspended sauna ceiling at 2.2 m removes about 0.8 m³ for every square metre of floor. On a 2.4 × 2.0 m room that is almost 3.8 m³ of volume the heater never has to warm, and the void above becomes the place where cables and ducts run on the cold side of the insulation.

Preferably on the cold side of the insulation. IEC 60364-7-703, the international standard for rooms containing sauna heaters, says wiring systems should be installed outside its three heat zones, which are bounded by the cold side of the thermal insulation. Cables that must run on the warm side need insulation rated for at least 170 °C.

The standard divides the room into three zones. Zone 1 contains the heater and extends 0.5 m around it; only the heater’s own equipment may be there. Zone 2 is the lower layer up to 1.0 m above the floor and has no special heat requirement. Zone 3 is everything above 1.0 m, where equipment must withstand 125 °C and cable insulation 170 °C. The EOS Cubo 2+ installation manual repeats the 170 °C requirement for every electrical installation laid inside the cabin.

For the insulation section, this means the cable route is designed before the wool goes in, together with the breaker and cable sizes in our guide to sauna electrical requirements. Supply cables to the heater, the light and the sensor come through the structure behind the insulation and cross the foil at a sealed point as close to the device as possible. A cable laid loose in the 10 mm gap, on the hot side of the foil, is in zone 3 whether or not anyone planned it.

Two things: the room around the sauna is kept at about 24 °C all year, and outdoor air in summer can hold more water vapour than that room. At 45 °C and 40 % humidity, outdoor air has a dew point near 28 °C, so a sauna placed against an exterior wall can see condensation behind its foil when it is switched off.

When the sauna is running, nothing changes from the European case. The room holds vapour at about 7.0 kPa, far above the 1.5 kPa of air-conditioned air at 24 °C and 50 % humidity, so the vapour drive is outwards and the foil on the room side stops it. The Gulf-specific case is the idle sauna. Once it cools to the building’s 24 °C, an exterior wall with outdoor air at 3.8 kPa behind it pushes vapour inwards, towards a foil that is now colder than the outdoor dew point of about 28 °C.

The practical rules follow from those numbers. We place saunas against interior walls wherever the plan allows. When an exterior wall is unavoidable, the building envelope carries its own air and vapour control on the outside of that wall, and the sauna wall is built as a separate insulated lining with its own foil, rather than relying on the building’s insulation. Air-conditioning diffusers are kept away from the sauna door, because a constant cold draught at the threshold is where the door frame and the lowest boards stay damp. Our Dubai office checks these three points on the drawings before a cabin is manufactured.

Sauna insulation in a bathroom has one more neighbour: the shower. Where a sauna shares a wall with a wet area, the wet side is waterproofed as a wet room and the sauna side is built as a sauna, with the two constructions meeting at the structure. Our guide to building a sauna in a bathroom shows how that shared wall is detailed.

Eight items, because none of the sauna insulation is visible afterwards: thickness, material, foil orientation, lap and tape continuity, ceiling lap direction, penetration seals, the 10 mm gap and the top vent gap. On our projects each wall and the ceiling is photographed at this stage for the handover file.

1. Insulation thickness: 100 mm in walls where the manual’s recommendation is followed, never under 50 mm; 100 mm minimum in the ceiling; measured, not read from the delivery note.
2. Insulation material: mineral wool or a sauna-rated board with a stated service temperature above the design temperature; no standard polystyrene.
3. Foil orientation: glossy side to the room.
4. Laps and tape: every joint lapped and taped with aluminium tape, no open seams at corners.
5. Ceiling over wall: ceiling foil lapped over the wall foil around the whole perimeter.
6. Penetrations: heater supply, light, sensor and ventilation sleeves each sealed to the foil.
7. Air gap: 10 mm battens between foil and panelling, continuous.
8. Top vent gap: a few millimetres left open where the wall boards meet the ceiling.

Ventilation openings are the only holes meant to stay open to air. How much air they carry and where it enters is a separate calculation, set out in our note on sauna ventilation.

Sauna Dekor has designed and built saunas, steam rooms, hammams and pools since 1987: 40 years of manufacturing in Istanbul, in our own facility in İkitelli OSB, with a team of 19 employees, working under TS EN ISO 9001:2015, and with 5,000+ projects delivered in 41+ countries. We design the sauna room, manufacture the cabin, benches and panelling, and install it with our own teams travelling from Istanbul. Recent sauna work includes the residential sauna and steam room at Istinye Park Residence in Istanbul and a villa sauna and hammam in the Sedra community in Riyadh, delivered in 2024.

What we do not make is equally clear. Heaters, control units and fans are not our products; EOS and Harvia make them, and any CE marking or EN 60335 conformity on that equipment belongs to its manufacturer. Insulation boards and foils are also bought in from their manufacturers. The TS EN ISO 9001:2015 certificate is ours.