By Vadim, licensed general contractor · Last reviewed October 2026
Why a sauna has to breathe
Ventilation does three jobs: it brings in oxygen for the people on the benches, it carries moisture out so the wood dries between sessions, and it moves the heat around so the feet are not cold while the head is hot. Get it wrong and the symptoms are familiar: a stuffy room, uneven heat, weak löyly, a musty smell that cleaning does not shift. Those are the complaints we see on service and repair visits.
The reference most builders cite is the Trumpkin series on ventilating an electrically heated sauna. It gives the air change as a minimum of 10 liters per second per person, which is 30 cubic meters an hour or about 20 CFM, and sets the target by carbon dioxide: below 700 ppm, ideally below 550. It also warns the other way: "too much ventilation can lessen löyly by removing too much heat and moisture".
Where we put the intake
On our drawings the fresh-air intake belongs in the low zone by the heater: beside it or under it, no lower than 4 inches off the floor and no higher than 20. Our default is 4 inches off the floor with a 4 inch opening. It can be round or square, and on the plan it is labeled with two distances, off the floor and from the nearest wall, so the framer cuts it where it was drawn.
It stays low and close to the heater so that cold air entering there is pulled across the stones and warmed before it reaches anyone's feet.
Where we put the exhaust
The exhaust goes on the wall away from the heater, under the lower bench. Our default is 6 inches off the floor with a 6 inch opening, round or square, dimensioned on the plan like the intake. Our rule allows it anywhere from 4 inches off the floor up to 2 inches below the ceiling, so when a room asks for a high exhaust we can draw one; but the default on our builds is low, under the bench, and it is low on purpose.
Pulling air out low on the far wall draws the warm air down through the room before it leaves, which keeps the lower bench usable.
The Finnish reference, and where it differs from our practice
Trumpkin's guidance for an electric heater is that "the fresh supply air should enter ABOVE the heater", with the exhaust "on the wall opposite the heater, under the foot bench". On the exhaust we agree completely: far wall, under the lower bench, low. On the intake we differ: his is above the heater, ours stays in the low zone beside or under it.
His argument against low intakes is that "any updraft ventilation that pulls cooler air upwards results in lower temps for the foot bench". Our practice comes from the rooms we have built and sat in, with the intake kept tight to the heater so the incoming air is warmed by the stones first. We will not average the two into a height neither of us recommends. If you want the Trumpkin layout on your sauna, say so at the design stage and we will draw it that way; the intake can sit anywhere in its range.
Ventilation and insulation work together
Ventilation and the wall are designed together, because moisture that does not leave through the exhaust goes into the framing. On our estimates the wall is described as it is built: "Rockwool R-15" insulation behind a "vapor and air-proof reflective foil to create a 'thermos' effect", with "continuous foil wrapping" so there is no seam for steam to find. The foil keeps the moisture in the room, where the exhaust can carry it out; without it, no vent layout saves the framing. Our room conversion guide walks through that shell, and the custom indoor sauna page describes how ventilation and moisture control are planned at design time rather than patched on at the end.
After the session
Trumpkin leaves the ventilation running "while we're using the sauna and for about 60-90 minutes afterwards to clear moisture". Our maintenance guide says the same in plainer terms: prop the door and leave the vent open until the room is dry. Ten seconds of habit that heads off most odor and mildew.
What ventilation looks like on our estimate
It is its own line, and it reads the way the work is done: "Installation of fresh air intake and exhaust ventilation systems in the sauna to ensure proper airflow and humidity control, including cutting openings in the wall, installing ventilation ducts, and covering both interior and exterior openings." On a custom build the vent covers are listed with the finish materials. Both openings are on the plan with their sizes and heights before anyone cuts a wall, and the planner draws them the same way, so what you approve on paper is what the room gets.
Sauna ventilation FAQ
Ventilation questions, straight answers.
Where does the fresh-air intake go in a sauna?
On our builds, beside or under the heater, between 4 and 20 inches off the floor; our default is 4 inches up with a 4 inch opening. It stays low by the heater so the incoming air is warmed by the stones before it reaches the benches. The Finnish reference places it above the heater instead; we say so plainly rather than blending the two.
Where does the exhaust go?
On the wall away from the heater, under the lower bench, about 6 inches off the floor, with a 6 inch opening. Our rule allows it anywhere from 4 inches off the floor to 2 inches below the ceiling, but low under the bench is the default on our drawings, and that is where the reference puts it too.
How much air does a sauna need?
The Trumpkin reference gives a minimum of 10 liters per second per person, about 20 CFM, and sets the target by CO2: below 700 ppm, ideally below 550. The flow depends on the room, so we size the openings on the plan rather than quote one number here.
Can a sauna have too much ventilation?
Yes. In Trumpkin's words, "too much ventilation can lessen löyly by removing too much heat and moisture". The aim is a steady, modest flow in the right places, not the biggest fan you can fit.
How long should the sauna vent after use?
The reference practice is about 60-90 minutes after the session to clear the moisture. In practice, prop the door and leave the vent open until the benches are dry; our maintenance guide has the rest of the routine.
Planning a sauna?
