There are two defensible schools. The mechanical-downdraft camp argues electric saunas lack a chimney's draft, so a fan should pull fresh air from above the heater down past bathers and out below the foot bench. The passive camp argues heat-expansion positive pressure suffices in smaller or outdoor builds, and that exhaust fans pull make-up air from bad places. Neither is code.
The two schools, stated fairly
Every incumbent guide in this category picks a side and writes as if the other side doesn't exist. It does — sauna ventilation is a genuine, unresolved disagreement between two named camps of practitioners, and this article isn't going to adjudicate it for you. What it will do is name both positions, attribute each to who actually argues it, and give you the placement and sizing detail either approach needs, so the decision is yours to make with the full picture in front of you.
Mechanical downdraft (Trumpkin's Notes; builders including Midwest Sauna & Steam and Cedarbrook) argues an electric heater has no chimney to create draft, so a fan should actively pull fresh air down from above the heater, across the bather zone, and out low below the foot bench. Passive (the SaunaTimes/Glenn circle) argues heat-expansion positive pressure — warm air rising and pulling cooler make-up air in low — does the job on its own in smaller and outdoor builds, and that adding a mechanical exhaust fan risks pulling make-up air from places you don't want it pulled from. What both agree on: a sauna needs a real intake and a real outlet, CO2 buildup is a genuine concern worth designing around, and neither position is a code requirement.
What both camps agree on
Before the disagreement, the agreement — because it's easy to lose track of how much common ground there actually is underneath two camps that argue past each other online. Both sides take it as given that a sauna needs a dedicated intake and a dedicated outlet, not just whatever gaps happen to exist around the door; neither camp argues for sealing the room and letting bathers breathe whatever's already inside it. Both sides treat carbon dioxide buildup during a session as a real design constraint, not a hypothetical one — the actual target figures both camps converge around are covered in full below, in “What the numbers say.” And both sides keep heater-to-bench clearance in the conversation for the same underlying reason: whatever airflow plan you run, bathers still need to sit a safe distance from a hot heater, independent of which ventilation school you're following.
Where the camps split is not whether ventilation matters — it's what actually drives the airflow, and where the vents themselves belong. That's the real disagreement, and it's covered camp by camp below.
The mechanical-downdraft case (Trumpkin camp)
The mechanical-downdraft position is most fully articulated by Trumpkin's Notes (LocalMile/Medium), and echoed by builders working at the installation end of the category — Midwest Sauna & Steam advocates for it directly, and Cedarbrook's own FAQ content leans the same direction Tier-2 — Trumpkin's Notes; Midwest Sauna & Steam; Cedarbrook FAQ. The case starts from a single physical observation and follows it to a specific layout.
Why electric ≠ wood-burning (no combustion draft)
A wood-burning sauna stove has a chimney, and a chimney does real ventilation work on its own: hot combustion gases rising up the flue create a continuous draft that pulls replacement air into the room, whether anyone designed for it or not. An electric heater has no flue and no combustion — it has no draft engine built in. Trumpkin's own framing is direct about what that means for the traditional passive layout most guides still describe by default, vent low near the heater, exhaust high on the opposite wall: without a real combustion draft doing the pulling, that arrangement is, in Trumpkin's words, “a recipe for bad air” Tier-2 — Trumpkin's Notes. The room can still warm up fine on an electric heater — heat and airflow aren't the same problem, and an electric sauna can run hot while still handling air poorly.
The downdraft layout (supply above heater; mechanical exhaust below foot bench)
The fix Trumpkin's Notes proposes flips the traditional layout rather than tuning it: fresh-air supply enters high, directly above the heater, and a mechanical exhaust fan pulls air out low, below the foot bench, on the opposite side of the room. Air is drawn down across the bather zone instead of rising past it — the fan supplies the draft the missing chimney can't Tier-2 — Trumpkin's Notes. One practical detail worth carrying over from the same source: place the heater's temperature sensor at head height on the bench wall, not directly over the heater itself, since a sensor mounted right above the heat source reads hot air that hasn't actually reached the bathers yet Tier-2 — Trumpkin's Notes.
The passive case (SaunaTimes circle)
The passive position comes from SaunaTimes and its founder Glenn, whose venting philosophy has circulated widely enough in practitioner circles to be treated as the passive camp's baseline Tier-2 — SaunaTimes (Glenn). It doesn't dispute that an electric heater lacks a chimney — it disputes that a chimney's draft is the only thing that can move air through a sauna well.
Heat-expansion positive pressure in smaller/outdoor builds
Hot air expands and rises on its own, with no fan required — that's the entire mechanism behind heat-expansion positive pressure, and SaunaTimes's position is that it's genuinely sufficient in smaller rooms, outdoor builds, and leakier barrel-style construction, where the room volume is small enough and the envelope loose enough that passive convection has an easy job to do Tier-2 — SaunaTimes (Glenn). A related, deliberately-embraced detail from the same source: some thermal stratification — a genuinely hotter layer up near the upper bench than down at floor level — isn't a ventilation failure to engineer away. Glenn describes this as the “Heavy Heat Hug,” a desirable feature of a well-run passive sauna, not a defect Tier-2 — SaunaTimes (Glenn). A mechanical downdraft system, by actively pulling air down across the whole bather zone, tends to flatten that stratification out — which the mechanical camp would call more even air quality, and the passive camp would call losing something bathers actually want.
The negative-pressure objection (make-up air from bad places)
SaunaTimes's core objection to mechanical exhaust isn't that fans don't work — it's what a fan does to the rest of the room's pressure balance. A mechanical exhaust fan pulls air out of the room faster than it can enter through the intended intake, which puts the room under slight negative pressure; that pressure difference doesn't just pull air through the intake you designed for, it pulls make-up air in through whatever gap is available, including gaps you didn't design as intake at all Tier-2 — SaunaTimes (Glenn). A passive, heat-expansion-driven system stays closer to neutral pressure by comparison, because nothing is actively forcing air out faster than the intake can replace it. SaunaTimes's own framing extends this caution to a specific passive configuration too, not just to mechanical exhaust: a lower exhaust vent, on its own, “only works if integrated with mechanical ventilation” Tier-2 — SaunaTimes (Glenn) — a low exhaust without a fan behind it doesn't reliably pull air the direction you want.
Inlet & outlet placement, both configs
Placement and sizing for both configurations is owned here, in this hub's ventilation guide — not repeated or re-derived in the build guide, the insulation guide, or anywhere else in this hub. A general sizing convention that applies before you split into either camp's specific layout: total vent area, intake plus exhaust combined, should run at least roughly 1 square foot per 100 cubic feet of room volume Convention. From there, the two configs place their openings differently.
| Config | Intake | Exhaust | Typical sizing |
|---|---|---|---|
| Passive (heat-expansion) | Low, near or behind the heater | High, on the opposite wall — the more defensible default if you're going fully passive, with no fan anywhere in the system. Some builds substitute roughly a 3-in. gap under the door instead, but SaunaTimes's own reasoning is skeptical of that swap: a low exhaust vent like this, on its own, “only works if integrated with mechanical ventilation” Tier-2 — SaunaTimes (Glenn) — so treat the door-gap option as the weaker of the two, not an equivalent alternative | Intake roughly 8–16 sq in.; exhaust roughly 12–23 sq in. Convention, DIRECTIONAL — practitioner guideline |
| Mechanical downdraft | High, directly above the heater | Low, below the foot bench on the opposite side of the room, with an inline exhaust fan | Sized to the fan's own rated CFM against the room volume — no separate passive-style opening-area figure applies once a fan is doing the pulling |
The passive sizing figures above are a practitioner starting point, not a manufacturer specification — no Harvia or HUUM manual reviewed for this article specifies vent placement or opening area directly, so these numbers stay labeled as convention rather than upgraded to a manufacturer figure Convention; no manufacturer vent-placement spec found this pass. Whichever config you build toward, get the low-and-high (or high-and-low) relationship right before worrying about the exact inch count — the direction of the intended airflow path matters more than shaving a few square inches off either opening.
How to decide for your build
This is a decision frame, not a verdict — the honest answer is that the right config depends on what you're actually building, and both camps' own reasoning points at the same conditions from opposite directions.
Strengthens the mechanical case: electric heat (no combustion draft to lean on), an indoor build, and a tight, well-sealed envelope — exactly the conditions where passive convection has the least room to work with. Strengthens the passive case: a smaller or outdoor build, a leakier barrel-style envelope, and — notably — a wood-burning heater, since a wood stove's own flue already supplies the combustion draft the mechanical camp's entire argument is built around replacing. Neither condition set is exclusive; most real builds land somewhere on this spectrum rather than at either extreme.
One sequencing approach worth naming, without crowning it as the answer: run passive first, and add mechanical assistance only if it proves insufficient once the sauna is actually in use. It's a reasonable, low-commitment sequence for a build that sits in genuinely contested territory — say, a small indoor electric sauna, where either camp's reasoning has a real point — but it's this article's own synthesis of both camps' reasoning, not a settled recommendation being put ahead of building mechanical in from the start if your build leans clearly toward the conditions that strengthen that case above.
What the numbers say
The figures below are the actual numbers both camps' reasoning gets built on. None of them are code minimums — they're practitioner convention, sourced and labeled individually, and this article isn't going to blur that distinction to make any number sound more authoritative than it is.
Alongside that CO2 target, Trumpkin's Notes puts fresh-air supply at roughly 20–25 CFM (9–12 L/s) per person, and heater-to-bench clearance at roughly 3 ft or more Tier-2 — Trumpkin's Notes. SaunaTimes's own CO2 position lands in a closely related range, stated in Glenn's own words:
“below 500 ppm but up to perhaps 700 is likely acceptable.”Glenn, SaunaTimes
The two camps disagree sharply on how to get there, but the actual CO2 numbers each side cites sit close enough together — both roughly in the sub-700 ppm range, both treating sub-550 as the more comfortable target — that this isn't really a numbers dispute at all. It's a mechanism dispute dressed up as one.
Separately, one practitioner estimate puts typical CO2 levels in an electric sauna running passive ventilation during active use at roughly 800–1,200 ppm, against an outdoor ambient baseline of roughly 400 ppm Convention/Tier-2 — practitioner estimate, one secondary source; label as estimate, not measured canon. That figure is a single practitioner's estimate, not a measured study result, and this article treats it accordingly — it's included because it's the only figure on record for what passive ventilation actually achieves in practice, not because it carries the same weight as the sourced targets above.
You'll see a claim, repeated across vendor pages in this category, that a VTT (Finland) study found mechanical downdraft ventilation outperformed passive ventilation for CO2 and humidity control. This article did not locate the primary VTT source behind that claim — it traces back to a single vendor page, secondhand, with no direct citation to the underlying research. Until a primary source is located, treat this as a claim repeated in vendor literature, not as an established finding, and don't let it tip a build decision on its own Literature — UNVERIFIED.
| Mechanical downdraft | Passive | |
|---|---|---|
| Draft source | Inline exhaust fan, actively pulling | Heat-expansion positive pressure — no fan required |
| Vent positions | Supply high above heater; exhaust low below foot bench | Intake low near heater; exhaust high on opposite wall |
| CO2 control | Actively driven — sized to fan CFM against target ppm | Passive convection — typically estimated 800–1,200 ppm in an electric sauna running passive ventilation (practitioner estimate) |
| Cost | Fan, wiring/power, and install — a real added cost over passive | No fan or added power draw — lower cost by design |
| Best-fit build | Electric heat, indoor, tight envelope | Smaller/outdoor build, leakier envelope, or wood-burning heat |
| Who advocates | Trumpkin's Notes; Midwest Sauna & Steam; Cedarbrook FAQ | SaunaTimes (Glenn) |
Neither approach is code — nothing in this table is a regulatory minimum, and no jurisdiction this article reviewed mandates one configuration over the other for a residential sauna. Both are practitioner convention, argued in good faith by people who build and use these rooms.
Diagram comparing two sauna ventilation configurations, drawn as the same room at identical scale so they can be read side by side. Both share a heater in the floor corner, a two-tier bench on the opposite wall, and a door. Left panel, passive: an intake vent sits low, next to the heater; an exhaust vent sits high, on the opposite wall; cool air is drawn in low, warms at the heater, and rises in a convection loop out the high exhaust — this approach relies on heat-expansion positive pressure and is strongest in smaller, outdoor, or wood-fired builds, per the SaunaTimes camp. Right panel, mechanical downdraft: a fresh-air supply sits high, directly above the heater; a fan-driven exhaust sits low, below the foot bench on the opposite side; air is pulled down from above the heater, across the bather zone, and out low by the fan — this approach is meant to make up for the lack of chimney draft in an electric heater, per the Trumpkin camp. Both configurations target carbon dioxide under 700 parts per million, and both place any thermostat sensor at head height on the bench wall, never directly over the heater. Neither configuration is a code requirement; both are practitioner convention.
Passive
CO2 target < 700 ppm- Intake — low, by heater
- Exhaust — high, opposite corner
- Thermostat — head height, not over heater
Mechanical downdraft
CO2 target < 700 ppm- Supply — above heater
- Fan exhaust — below foot bench
- Thermostat — head height, not over heater
FAQ
Should sauna ventilation be mechanical or passive?
There's no single correct answer — practitioners genuinely disagree. The mechanical-downdraft camp (Trumpkin's Notes and builders including Midwest Sauna & Steam) argues electric saunas lack a chimney's draft and need a fan; the passive camp (SaunaTimes/Glenn) argues heat-expansion positive pressure is sufficient in smaller or outdoor builds. Neither is a code requirement Tier-2 — Trumpkin's Notes; SaunaTimes (Glenn).
Where should sauna vents be placed?
It depends on which configuration you're building toward. A passive layout puts intake low, near or behind the heater, and exhaust high on the opposite wall. A mechanical downdraft layout reverses it: supply high above the heater, mechanical exhaust low below the foot bench on the opposite side Tier-2 — Trumpkin's Notes; SaunaTimes (Glenn).
What CO2 level is safe in a sauna?
Practitioner targets from both camps land close together: under 700 ppm, ideally under 550 ppm Tier-2 — Trumpkin's Notes, with SaunaTimes citing a closely related range of below 500 ppm, up to perhaps 700 Tier-2 — SaunaTimes (Glenn). These are practitioner figures, not a regulatory limit. A separate practitioner estimate puts typical CO2 in an electric sauna running passive ventilation during use at roughly 800–1,200 ppm — an estimate, not a measured study result Convention/Tier-2 — practitioner estimate.
Does an electric sauna need a fan for ventilation?
Not by any code requirement — but the mechanical-downdraft camp's core argument is specifically that electric heaters lack the chimney draft a wood-burning stove provides, so a fan does work that combustion draft would otherwise handle Tier-2 — Trumpkin's Notes. The passive camp disputes that a fan is necessary, particularly in smaller or outdoor builds, and raises a real tradeoff of its own: a mechanical exhaust fan can pull make-up air from unintended gaps Tier-2 — SaunaTimes (Glenn). Both are genuinely defensible positions.
Is there a code requirement for sauna ventilation?
No. Neither the mechanical-downdraft approach nor the passive approach is a code minimum — both are practitioner convention, and no jurisdiction reviewed for this article mandates one configuration over the other for a residential sauna.