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) | being reviewed |
| 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 |
| 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
- Exhaust high (hot air rises), intake low on the opposite side.
- One oscillating or clip fan above the canopy blowing across, not down.
- One fan below the canopy in flower, blowing across the pots.
- In rooms, aim fans to create a circular pattern; avoid dead corners.
- Run circulation 24 h. At lights-off is when humidity spikes.
- Put exhaust on a speed controller tied to temperature and RH 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. Leaf temperature under low wind is Supported by modelling [2]. Air movement as part of disease prevention in cannabis is Established extension guidance [3]. Specific air speeds and exchange rates for cannabis are Grower practice.
