# Negative pressure

> Negative pressure means the grow space is held slightly below surrounding air pressure by exhausting more air than comes in, so air and odour leak inwards, not out.

- Canonical: https://genetixbay.com/knowledge/glossary/negative-pressure.html
- Also written: negative air pressure, neg pressure, sucked-in tent, slight vacuum, negative pressure grow room
- Glossary category: Environment and light · Evidence: Grower practice
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) 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](https://genetixbay.com/knowledge/glossary/carbon-filter.md). 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](https://genetixbay.com/knowledge/glossary/dehumidifier.md), often with [CO2](https://genetixbay.com/knowledge/glossary/co2-enrichment.md).

## Related terms

[Exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md), [Carbon filter](https://genetixbay.com/knowledge/glossary/carbon-filter.md), [CFM](https://genetixbay.com/knowledge/glossary/cfm.md), [Tent](https://genetixbay.com/knowledge/glossary/tent.md)

## Read more

- [Airflow: air exchange, canopy air speed and fan placement](https://genetixbay.com/knowledge/environment/airflow.md)