Everything else in a sauna has a settled answer. Where the intake vent goes does not.

Let the heater manual decide the intake height

This is the honest answer to the most argued question in sauna building, and it is not a dodge. Finnmark states that fresh air should not enter above the top of the heater because it can interfere with the thermostat reading and disrupt convection, and then instruct readers to consult the heater’s installation manual for intake positioning relative to the unit. Other capable builders advocate the opposite arrangement. The reason both positions survive is that the answer depends on where your heater takes its temperature reading, which is a property of that heater. Published figures across the field range from four to six inches below the heater, to four to twelve inches off the floor, to six inches below the ceiling directly above it. Read your manual, follow it, and treat any general number, including any in this paragraph, as background.

Opposite sides, or you have not ventilated anything

This is the part with no disagreement at all and it is the one most often gotten wrong. The intake and exhaust have to be positioned so air crosses the room, which in practice means opposite walls or diagonally opposite corners. Put them on the same wall and air short-circuits between them without moving through the space. There is a documented retrofit where the exhaust sat six inches off the floor on the same wall as the intake, and the outcome was no measurable air change and visible mold by the second year. Before cutting anything, draw the path the air has to take and confirm it passes through the zone where people actually sit.

Cold feet and a hot head is a ventilation diagnosis

The most useful symptom to know, because it is felt from the bench rather than measured. If the room is fierce at head height and the floor stays cold, air is not being drawn through the bench zone. Typically the intake is low, the cool air runs across the floor, and it exits without ever rising through where the bathers are. The room stratifies, the air where people are breathing goes stale, and their feet never warm up. The instinct is to buy a bigger heater, and a bigger heater will not fix it, because the problem is the path the air takes rather than how much of it is being heated.

Give both vents adjustable covers, and add a drying vent

Fixed holes are not ventilation, they are a permanent heat leak that you cannot tune. Both the intake and the exhaust should have a damper or a sliding cover on the inside so airflow can be adjusted during a session, and the exterior side of each wants a weather-resistant hood so rain, wind and debris stay out. Then add a third opening near the ceiling that stays shut while bathing and gets opened fully afterward. That drying vent is what pulls the humid air out at the end of a session, and it is the single cheapest thing you can do to protect the wood inside the room from the moisture staining that eventually turns into mold.

Outdoors, both vents need real outside air

A specific trap on outdoor saunas with a changing room or a covered porch. If the intake and the exhaust both open into that same enclosed space, the room is breathing its own exhaust and the air change rate is effectively zero no matter how correctly the vents are positioned inside the hot room. Each vent needs its own path to genuine outside air. It is easy to miss because the layout looks correct from inside the sauna, and the failure presents as a room that is stuffy for no visible reason with both vents open and working.

Wood-fired helps, but it is not the ventilation system

A wood-burning heater consumes combustion air and the flue creates draft, so a wood-fired sauna does have air moving through it as a side effect of the fire. That assists, and it is why passive ventilation behaves more reliably in a wood-fired room than an electric one. It is not a substitute. The room still needs its own intake and exhaust so that air exchange happens across the bench zone and continues to work when the fire is dying down. Gas heaters are a separate matter again: their combustion air and exhaust are dedicated, sealed and ducted to the outdoors, installed to code by a licensed professional, and entirely independent of the room ventilation described here.

Every vent is a hole through the vapor barrier

Worth flagging because ventilation and the wall envelope get planned by different people or at different times. Each vent penetrates the insulation and the foil vapor barrier, and an unsealed penetration undoes the barrier locally no matter how carefully the rest of it was installed. Cut the openings and seal around them with foil tape as part of the barrier work, not as a later addition. This is also the strongest argument for locating vents at framing stage: cutting them afterward means cutting through finished paneling, insulation and barrier, and then trying to reseal a penetration you can only reach from one side.

The heater needs a guard, and codes say so

Ventilation decides comfort. This one decides whether somebody gets burned. A sauna heater reaches temperatures that will injure on contact instantly, and it sits in a small room with people moving around it, sometimes in the dark, sometimes unsteady after a long session. Massachusetts building code has a section specifically on sauna heaters requiring that they be protected from accidental contact with a guard made of material with low thermal conductivity. Other jurisdictions handle it differently, so check yours, but treat a guard as part of the installation rather than an accessory. The same thinking applies to the door: sauna doors are built to swing outward and without a latch that can hold them shut, and neither of those should ever be modified.