# Airflow: air exchange, canopy air speed and fan placement

> Aim for gentle, constant movement across and under the canopy (roughly 0.3–1.0 m/s at leaf level) plus enough exhaust to remove heat and humidity, typically one full tent air exchange every 1–3 minutes for a lit tent. Moving air thins the leaf boundary layer, which raised canopy photosynthesis in non-cannabis research, and dries out the microclimates where mould starts.

- Canonical: https://genetixbay.com/knowledge/environment/airflow.html
- Category: Environment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How many [fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md) do I need in a grow tent?** A typical cannabis grow tent needs at least two fan jobs covered: one exhaust fan mounted high to swap out hot, humid air, and one oscillating or clip fan above the canopy blowing across the tops. In flower, add a second circulation fan below the canopy blowing across the pots to dry the lower canopy. This layout is grower practice rather than a researched cannabis standard.

**How fast should air move over cannabis leaves?** Air over cannabis leaves should move at roughly 0.3–1.0 m/s (60–200 ft/min) at leaf level, enough that leaves flutter gently but never whip. Research on other crops found that raising air speed above the canopy from 0.1 to 1.0 m/s roughly doubled canopy net [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md), but no cannabis-specific optimum has been published, so the range is grower practice.

**What size exhaust fan for my tent?** An exhaust fan for a lit grow tent without added CO₂ should replace the full tent volume every 1–3 minutes. For a 1.2 × 1.2 × 2 m tent (2.9 m³) that is about 60–175 m³/h of raw airflow, plus 25–50 % extra to cover carbon filter and ducting losses. Run it on a speed controller tied to temperature and humidity rather than at full blast.

**Can too much wind hurt plants?** Yes, too much wind can hurt cannabis plants: a strong fan pointed straight at them causes windburn, seen as constantly bending top leaves and clawed or tacoed edges on the fan side, while the far side stays stagnant. The fix is gentler, spread-out circulation that keeps leaf-level air speed around 0.3–1.0 m/s, aimed across the canopy rather than down onto it.

**Should fans run at night?** Circulation fans in a cannabis grow should run 24 hours a day, including lights-off. Turning them off at night to 'let the plants rest' is a common mistake, because humidity spikes when the lights go off and still air lets moisture build up in dense [colas](https://genetixbay.com/knowledge/glossary/cola.md), where [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md) and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) start. Moving air dries those microclimates and keeps the leaf boundary layer thin.

**Short answer.** You need two separate things: **exhaust** (to swap room air and remove heat/humidity) and **circulation** (to move air across and through the canopy). Target leaf-level air speed of about **0.3–1.0 m/s (60–200 ft/min)**: leaves should flutter gently, never whip. In research on other crops, raising air speed above the canopy from 0.1 to 1.0 m/s roughly doubled canopy net photosynthesis [1]. Circulation also dries the pockets where *Botrytis* and powdery mildew start [3].

## What's happening

Every leaf sits inside a thin, still "boundary layer" of air. In still air that layer thickens; CO₂ near the leaf is used up, water vapour builds up, and the leaf can't shed heat. Moving air thins it. Under water stress and low wind, modelled leaves run 8–12 °C (14–22 °F) above air temperature [2].

## Numbers that matter

| Item | Target | Notes |
|---|---|---|
| Leaf-level air speed | 0.3–1.0 m/s (60–200 ft/min) | |
| Exhaust, lit tent (no CO₂) | One full volume every 1–3 min | Tent 1.2 × 1.2 × 2 m = 2.9 m³ → 60–175 m³/h raw; add 25–50 % for a carbon filter and ducting losses |
| Exhaust, sealed CO₂ room | Off during lights-on except emergency | Heat and humidity handled by AC and [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md) |
| Negative pressure | Tent walls slightly sucked in | Keeps odour inside |
| Intake area | ≈ 2–3 × exhaust duct area if passive | |
| Fans under canopy | Yes, in flower | Dries the lower canopy and pot surface |

## How to set it up

1. Exhaust high (hot air rises), intake low on the opposite side.
2. One oscillating or clip fan above the canopy blowing across, not down.
3. One fan below the canopy in flower, blowing across the pots.
4. In rooms, aim fans to create a circular pattern; avoid dead corners.
5. Run circulation 24 h. At lights-off is when humidity spikes.
6. Put exhaust on a speed controller tied to temperature and [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) rather than full blast.

## What to check first

- Hang a ribbon or tissue at several canopy points: it should move everywhere, gently.
- Feel the top leaves: if they are constantly bending or the edges are clawing/tacoing on the fan side, it's windburn.
- Check humidity inside dense colas vs room air.

## Common mistakes

- One big fan pointed straight at the plants: windburn on one side, stagnant air on the other.
- Exhaust too strong in a sealed CO₂ setup (throws money out the vent).
- Fans off at night to "let the plants rest".
- Undersized exhaust once a carbon filter is added.

## Evidence

Boundary-layer physics and the effect of air speed on gas exchange are *Established* in plant science; the doubling result [1] is from non-cannabis canopies in a [closed system](https://genetixbay.com/knowledge/glossary/recirculating-system.md). [Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) under low wind is *Supported* by modelling [2]. Air movement as part of disease [prevention](https://genetixbay.com/knowledge/glossary/cultural-control.md) in cannabis is *Established* extension guidance [3]. Specific air speeds and exchange rates for cannabis are *Grower practice*.

## Sources

1. Kitaya Y, Tsuruyama J, Shibuya T, Yoshida M, Kiyota M, 2003. Effects of air current speed on gas exchange in plant leaves and plant canopies. Advances in Space Research 31(1):177–182. https://www.science.gov/topicpages/a/air+current+speed
2. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, HPS and LEDs. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/
3. Punja ZK, 2021. Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science 77:3857–3870. https://pmc.ncbi.nlm.nih.gov/articles/PMC8451794/