Climate & air
A dialled-in tent breathes — steady warmth, dry air, gentle wind, and the smell stays home.
Get the air right and the plant does the rest. Boring, stable conditions every day beat a flashy setup every time.
What good looks like
What a dialled-in environment looks like
- Temperature
Steady warmth
20-26°C with the lights on, dropping a few degrees at night. The same numbers every day, measured at canopy height where the plant actually lives.
- Humidity
Dry enough
High for seedlings, easing down as the plant grows — 45-50% in flower so dense buds stay dry and rot has nowhere to set in.
- Airflow
Gentle wind
An oscillating fan running all day, leaves rustling not flapping. Wind stress builds woody stems that hold their own buds up.
- Extraction
In and out
A fan sized to the tent pulling stale air out through a clean carbon filter, fresh air falling in low through passive vents. The tent breathes once a minute.
- Smell
Stays home
A fresh carbon filter removing up to 95% of the smell, replaced on a schedule before it saturates. Neighbours none the wiser.
The numbers that matter
The numbers that matter
Do this
The climate & air checklist
- Hang a thermometer and hygrometer at canopy height, not the floor, and check daily
- Set a heater on a thermostat so the tent holds ~24°C with the lights on
- Run an oscillating fan across the canopy all day — aim for a rustle, not a flap
- Size your extraction fan to tent volume × 60, then add for filter and ducting losses
- Keep ducting short and straight — every 90° bend costs 10-15% airflow
- Mount a carbon filter inside the tent at the top and replace it every 3-4 months
- Drop humidity to 45-50% before flower and hold it with a dehumidifier if needed
- Change one thing at a time and read the plant’s response before touching anything else
Watch for
Catch these early
The early sign, what it means, and the fix. The full stories are in the book.
Condensation running down the canvas, stems gone soft like wet spaghetti
The tent is sealed and sweating — the plant is breathing its own trapped, saturated air with nowhere for moisture to go.
Fix: Open the passive vents and run a properly sized extraction fan so air moves in, out, and once a minute through the space.
📖 The sealed-tent story that opens the chapter — a plant lost to its own exhaled breath. The full story →
Grey-brown fuzz inside the fattest cola, found only when you break it open
Bud rot. Humidity sat above 60% in flower with no airflow, and the rot spread inside the bud where you couldn’t see it.
Fix: Keep flower humidity at 45-50%, run a circulation fan, and deploy a dehumidifier during lights-on if the air won’t drop.
📖 Meet the Sleepwalker — dialled everything but humidity, lost the profile-picture cola to grey fuzz. The full story →
Hot under the light by day, freezing by night, purple stems and stalled growth
A big day-to-night temperature swing is thermally stressing the plant — a sauna followed by a cold plunge, daily.
Fix: Hold the night temperature within a few degrees of the day with a thermostat heater, and keep the room above 12°C overnight.
📖 Meet the Yo-Yo — a 20-degree daily swing the grower blamed on bad genetics. The full story →
The tent fabric sucking hard inward, the fan screaming, intake unable to keep up
The extraction fan is oversized for the tent — pulling so hard it eats growing space and adds noise and heat you can’t use.
Fix: Spec the fan to your tent size and run it at medium speed. A right-sized fan at 70% beats a giant one at full tilt.
📖 Meet the Jet Engine — a 250mm fan in a 60cm tent, and the silencer that made it worse. The full story →
Questions
Climate & air FAQ
Where should I put my thermometer and hygrometer?
At canopy height, where the plant actually lives — hung next to each other. The floor can read 5°C cooler than the canopy, so a floor reading is useless data.
What temperature and humidity am I aiming for?
20-26°C by day and 17-21°C by night, every day the same. Humidity starts high for seedlings, eases to ~60% in veg, and drops to 45-50% in flower. The plant wants boring stability, not perfection.
How do I size an extraction fan?
Tent volume in m³ × 60 gives the bare minimum m³/h. Add 20-30% for the carbon filter and more for ducting losses. For a 1.2m tent that lands around 250 m³/h — a 150mm inline fan.
Can I just open a window instead of an extraction fan?
No. An open window is uncontrolled airflow — you can’t manage humidity or temperature, and the smell goes everywhere. A properly sized fan with passive intake gives you control instead of a lucky streak.
Do I need CO2 supplementation?
Not yet. With passive intake and an extraction fan at a reasonable speed, CO2 isn’t your limiting factor — light, temperature or water is. File it under year two and fix those first.
The whole story is in the book
The full sizing maths, the VPD numbers and every mistake that costs a harvest are laid out plainly in Grow Good Bud.
The web gives you the lesson; Grow Good Bud keeps the scars. The kit to grow it is at Dublin Indoor Gardening.