Get the airflow right and a grow tent more or less runs itself - temperature, humidity and the mould risk that comes with stale air all settle down. Get it wrong and you're firefighting every week. The layout is simple once you see it: pull air out high, let fresh air fall in low, and stir the canopy with fans that don't move air out of the tent at all. Here's how each part fits, and the gear we stock to do each job.
How should air flow in a grow tent?
Work with gravity, not against it. Warm, used air rises, so it should exit high - that's your inline fan and carbon filter, mounted at the top. Cool, fresh air should enter low, falling in through the passive intake vents near the floor, pulled in by the suction the extraction creates. Done right, the tent gently inhales: the walls pull slightly concave. If they balloon outward you've got too much air coming in and not enough going out, and smell starts leaking from the seams.
So the order from top to bottom is: filter and extraction fan at the ceiling, plants in the middle, intake vents open at the floor. Three jobs, three zones.
Intake low, extraction high: setting up the airflow path
Extraction is the engine. An inline fan sits at the top of the tent and pulls air out through a carbon filter, taking heat, humidity and odour with it and creating the negative pressure that keeps smell from escaping. Match the fan to the tent volume and the filter to the fan b undersize either and the whole system limps. Browse our inline extraction fans sized by tent footprint.
Intake is the easy half: keep the passive mesh vents at the bottom open and unblocked so cool air can fall in to replace what's leaving. Most growers don't need a powered intake - the extraction does the pulling. Just don't bury the bottom vents behind pots or ducting.
Why does ducting routing matter?
Ducting is where a lot of airflow quietly disappears. Keep the run between fan, filter and tent port as short and straight as you can - every 90° bend costs you roughly 10 - 15% of your airflow, and a long sagging run of flexible ducting full of kinks can halve it. Mount the filter at the very top where the warm, smelly air collects, run the duct out through the tent's port to your extraction point, and resist the urge to coil the spare length up neatly - coils choke the flow. Our ducting and clamps come in the common 4-, 5- and 6-inch sizes to match your fan.
Where do circulation fans go in a grow tent?
This is the part beginners miss. Extraction exchanges the air; it doesn't stir it. For that you want clip fans pointed across and up through the canopy b never straight down like a hairdryer. The leaves should rustle, not flap. A simple tissue test works: hold a tissue near the plant and the airflow should barely move it.
Moving air across the canopy does three real jobs - it thickens stems through gentle wind stress (still air gives you weak, celery-like stalks), it breaks up the stagnant humid pockets where mould takes hold, and it refreshes the thin layer of spent air at the leaf surface so the plant keeps feeding. One small clip fan below the canopy and one above beats a single strong one blasting from one side. If leaves are visibly flapping or the plant leans away, the fan's too close - back it off.
Does the tent itself make a difference?
It does. A well-built tent gives you the mounting bars, sealed ducting ports and light-proof intake flaps that make this layout actually work - cheap tents leak air and light and fight you on every step. We stock Secret Jardin tents for exactly this reason: the Dark Room and Dark Street lines have solid bars to hang a filter and fan from, proper passive vents low down and tidy cable and ducting ports. Browse the full grow-tent range here.
Pop in and we'll spec your airflow
If you're not sure what size inline fan, filter or ducting your tent needs, measure your tent and tell us the footprint - we work this out every day. DIG is Ireland's largest grow shop, based in Dublin, with collection available in store and delivery across Ireland.
Want the full science behind why low-in, high-out works and how the canopy actually breathes? The team at Grow Good Bud have the free in-depth guide.
FAQ
Do I need both an extraction fan and a circulation fan?
Yes! They do different jobs. Extraction exchanges the tent's air, removing heat, humidity and smell. Circulation stirs the air inside to clear stagnant pockets and strengthen stems. You need both.
Where should the intake go in a grow tent?
Low down, through the passive mesh vents near the floor, so cool fresh air is pulled in by the extraction's suction and rises through the canopy. Keep those vents open and unblocked.
How much does ducting bend reduce airflow?
Every 90° bend costs roughly 10 - 15% of your airflow, and long sagging or coiled runs cost more. Keep ducting short and straight, with the filter mounted at the top of the tent.
Why are my plant's stems weak and floppy?
Usually too little air movement. Still air lets a plant skimp on stem strength. A gentle clip fan making the leaves rustle triggers it to build thicker, stronger stems.
Where can I buy grow tent fans and ducting in Ireland?
DIG (Dublin Indoor Gardening) stocks Secret Jardin tents, inline extraction fans, carbon filters, ducting and clip fans, with collection in Dublin and delivery across Ireland.