How often should I ventilate a room?

There’s no single correct schedule here — it depends on room size, how many people are in it, and the season — but there is a genuinely useful way to know when it’s needed.

Let a CO2 reading be the cue

A monitor like the TFA Dostmann AIRCO2NTROL Mini gives a practical, at-a-glance signal — its traffic-light indicator changes as CO2 accumulates — rather than relying on a fixed schedule that doesn’t account for how many people are actually in the room.

Occupancy changes the picture significantly

A room with several people generates CO2 and humidity considerably faster than an empty one — a fixed “every two hours” rule that works for one person working alone won’t hold for a room full of guests.

Short, full ventilation beats a window left slightly open

A few minutes of a window fully open, ideally with cross-ventilation to another opening, exchanges air more effectively than a window cracked open for a longer period — the latter also loses more heat in winter for less actual air exchange.

Balance ventilation against heating cost in winter

Ventilating effectively doesn’t mean leaving windows open constantly — short, deliberate bursts of full ventilation, guided by a CO2 reading rather than a fixed timer, balance air quality against heating cost more sensibly.

What we would not do

Give a single numeric CO2 threshold as a health guideline. A rising reading is a practical cue to ventilate, not a diagnosis — what specific threshold matters for health is outside what this comparison site addresses.