Negative pressure is the state where air pressure inside a grow space is a little lower than outside it. It is created by running the exhaust fan harder than the intake (or using only passive intake vents). Any gaps then let air in rather than out, so all exhaust air leaves through the carbon filter. That controls odour and helps keep dust and pests from blowing out into the home.
In practice
The visual test in a tent is simple: the walls bow slightly inwards. Aim for a gentle concave, not walls sucked tight against the frame; too much negative pressure starves the space of fresh air, strains the fan and can make the tent leak light through stressed seams.
To set it up: size passive intake openings large enough (often 2–4 times the exhaust duct area), or run an intake fan at roughly 70–80% of the exhaust fan's real airflow. Close unused ports and flaps.
Negative pressure is not a substitute for a filter: it ensures the filter handles everything.
Don't confuse with
- Positive pressure: used in clean rooms and some propagation areas to keep contaminants out; air leaks outwards.
- Sealed room: no air exchange at all; climate is controlled by HVAC and dehumidifiers, often with CO2.
