| Air changes per hour | The manufacturer figure. Infrared cabins need less; they run cooler and make no steam. | 6×/hr |
| Intake — mechanical ventilation | Supply vent goes above the heater. | Above |
| Intake — natural ventilation | Supply vent goes under or next to the heater. This is the line the blogs disagree on. | Below |
| Exhaust, either way | Near the floor, as far from the heater as possible. | Low & far |
| Vapour barrier foil | Facing into the room. This is what keeps moisture out of your walls. | Inward |
Why every guide you have read contradicts the last one: intake placement is not a matter of opinion, it is conditional. Harvia’s own installation manual gives two different answers depending on whether your sauna has mechanical or natural ventilation — and most articles quote one of them without the condition attached. Half of them are therefore telling you to put the vent in the wrong place.
From the Harvia Spirit owner’s and installation manual, “Sauna room ventilation”. Harvia is the heater inside a great many of the saunas we track, including Almost Heaven and Redwood barrels:
| Item | What the manual says |
|---|---|
| Air change rate | “The air in the sauna room should change six times per hour.” |
| Supply vent — mechanical | “If mechanical ventilation is used, air supply vent should be placed above the heater.” |
| Supply vent — natural | “If natural ventilation is used, air supply vent should be placed under or next to the heater.” |
| Supply pipe diameter | “The diameter of the supply air pipe must be 50–100 mm.” (2–4 in) |
| Exhaust vent | “Place the exhaust air vent near the floor, as far away from the heater as possible.” Ø 100–200 mm (4–8 in) |
| Exhaust sizing rule | “The diameter of the exhaust air pipe should be twice the diameter of the supply air pipe.” |
| Door gap | If the exhaust vent is in the washroom, the gap under the sauna door must be at least 100 mm (4 in) |
| Sensor warning | “Do not place the supply air vent so that the air flow cools the temperature sensor.” A vent aimed at the sensor makes the heater run hotter than the display claims. |
| Drying vent | An optional separate vent for drying, “closed during heating and bathing”. The manual notes the sauna can also be dried by leaving the door open after bathing. |
Ventilation moves air. The vapour barrier decides whether humid air can get into your wall cavity in the first place, and it is the part that turns a sauna problem into a building problem. The same manual specifies, under room construction:
Two more from the same section, both cheap to get right and expensive to retrofit: ceiling height no higher than 7′6″ (2300 mm) — a taller ceiling wastes heat into a zone nobody sits in — and panelling in kiln-dried softwood with moisture content not exceeding 11%. Wet timber shrinks and gaps after its first few cycles.
An outdoor sauna that vents badly is mostly its own problem. An indoor sauna that vents badly exports the problem to the rest of the house. The usual sequence is: an existing bathroom extractor is assumed to be enough, it isn’t, humidity has nowhere efficient to go, and within months there is warping or discolouration on surfaces outside the sauna. The repair is not a vent — it is opening up finished walls to install one, which is why it routinely costs several times what planning it would have.
So if the sauna is going indoors, design the airflow path before anything else: where air enters, where it leaves, and what carries the moisture out of the building rather than into a cavity. The heater, the cedar and the benches are the easy decisions and they are all secondary.
| Check | Why |
|---|---|
| Is your ventilation mechanical or natural? | It determines intake placement, and it is the one thing the manual makes conditional. Decide before you cut a hole. |
| Is the exhaust twice the supply diameter? | Explicit in the manual, and routinely missed when people reuse existing ducting. |
| Is the exhaust low, and far from the heater? | Air has to actually cross the room. An exhaust near the heater short-circuits the path. |
| Does airflow hit the temperature sensor? | If it does, your heater will run hotter than the controller reports. |
| Is the foil facing into the room? | Backwards is a hidden moisture trap. Check before the panelling goes on — afterwards it is a demolition job. |
| Have you run it for a week before trimming out? | A week of real cycles shows where condensation collects and where heat escapes, while everything is still accessible. |
That last one is the cheapest insurance on this page and almost nobody does it. Every other item is on the printable 27-question buyer’s checklist.
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Source: Harvia Spirit owner’s/operator’s and installation manual, sections “Sauna room ventilation” and “Room construction”, read 2026-08-20 and quoted rather than paraphrased where the wording matters. Air-change guidance elsewhere in the industry ranges from three to eight per hour; six is the figure in this manual, and infrared cabins sit at the lower end because they run cooler and produce no steam. Your own heater’s manual governs — models and brands differ, and local building and electrical codes override any manufacturer figure. We have not bench-tested airflow and do not claim to; this is the published specification, sourced. Spot an error? hello@saunaclear.com — corrections are published, not buried.