# How do I lower the temperature in my grow tent?

> Lower grow-tent temperature by removing the light's heat faster: more exhaust from a cooler room, drivers or ballasts outside the tent, lights dimmed 10–20 %, and lights run in the coolest hours. Aim for 24–28 °C (75–82 °F) in veg and 22–26 °C (72–79 °F) in flower. Above about 30 °C (86 °F) without added CO₂, photosynthesis falls.

- Canonical: https://genetixbay.com/knowledge/answers/how-to-cool-a-grow-tent.html
- Topic: Environment · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)
- Also asked as: My grow tent is too hot | How do I cool down a grow room? | Heat problems in my tent | Is 30 °C too hot for cannabis?
- 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)"

## Short answer

Lower grow-tent temperature by moving the light's heat out faster than it builds: more [exhaust](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) from a cooler room, drivers outside the tent, lights dimmed **10–20 %**, and lights on during the coolest hours. Target **24–28 °C (75–82 °F) in veg** and **22–26 °C (72–79 °F) in flower**; above about **30 °C (86 °F)** without CO₂, [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) falls [1]. *Supported*.

## Why

Nearly every watt the light draws ends up as heat in the tent. The exhaust fan can only replace hot air with room air, so the room temperature sets your floor.

In cannabis, net photosynthesis and water-use efficiency drop above about 30 °C [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) unless light and CO₂ are both high [1]. CO₂ enrichment shifts the optimum warmer [2], which is why commercial enriched rooms run hotter than a tent should.

Leaf temperature is what counts. Under [LED](https://genetixbay.com/knowledge/glossary/led.md), well-watered leaves usually run a little below air; under [HPS](https://genetixbay.com/knowledge/glossary/hps.md) they run warmer [3].

Hot rooms also warm the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md). Above 24 °C (75 °F) in [DWC](https://genetixbay.com/knowledge/glossary/dwc.md), root-rot risk rises sharply [4].

## The numbers

| Item | Target |
|---|---|
| Veg, lights on | 24–28 °C (75–82 °F) |
| Flower, lights on | 22–26 °C (72–79 °F) |
| Late flower | 20–24 °C (68–75 °F) |
| Hard limit, no CO₂ | 30–32 °C (86–90 °F) sustained |
| Exhaust, lit tent | 1 full volume every 1–3 minutes, +25–50 % for filter and ducting |
| Reservoir | 18–22 °C (64–72 °F) |

## What to do

1. Measure at canopy height with a shaded probe, plus leaf temperature with an [IR thermometer](https://genetixbay.com/knowledge/glossary/ir-thermometer.md).
2. Move LED drivers or ballasts outside the tent.
3. Raise exhaust speed; pull intake air from the coolest place you can.
4. Run the lights at night in summer.
5. Dim or raise the light 10–20 % if leaves read above about 28 °C (82 °F).
6. If the room itself is hot, cool the room (air conditioning) or run less light.

## Common mistakes

- Copying a 28–30 °C CO₂-room setpoint into a tent without CO₂.
- A bigger fan pulling 30 °C air from a hot attic.
- Ignoring the reservoir in summer.
- Plugging an air conditioner into the circuit already carrying the lights.

## Related

- Full article: [Temperature by stage](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)
- [Should I run my room warmer with LEDs than with HPS?](https://genetixbay.com/knowledge/answers/led-grow-room-temperature.md)
- [Do I need an intake fan?](https://genetixbay.com/knowledge/answers/do-i-need-an-intake-fan.md)
- [Heat and light stress](https://genetixbay.com/knowledge/environment/heat-light-stress.md) · [Airflow](https://genetixbay.com/knowledge/environment/airflow.md)
- Terms: [heat stress](https://genetixbay.com/knowledge/glossary/heat-stress.md)

## Follow-up questions

**Is 30 °C (86 °F) too hot for a grow tent?** 30 °C (86 °F) is the upper edge for cannabis in a tent without added CO₂. Research shows net photosynthesis and water-use efficiency fall above about 30 °C leaf temperature unless light and CO₂ are high. Sustained air above 30–32 °C (86–90 °F) is a hard limit. CO₂-enriched rooms run 28–30 °C, but copying that into an un-enriched tent is a mistake.

**Should I run my lights at night to keep the tent cool?** Yes, running grow lights during the coolest part of the day, usually night-time, is a cheap way to keep a tent cool in summer. The heat load then lands when the intake air is coolest. Keep the dark period truly dark, and make sure the room stays quiet and light-tight during the daytime dark period.

**Does a bigger exhaust fan always cool a tent?** A bigger exhaust fan cools a grow tent only if the air it pulls in is cooler than the tent. A lit tent usually needs one full air exchange every 1–3 minutes plus 25–50 % extra for a carbon filter and ducting. If the room itself runs at 30 °C, more airflow can't get the tent below 30 °C; then you need air conditioning or less light.

## Sources

1. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
2. Chandra S, Lata H, Khan IA, ElSohly MA, 2011. Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2. Physiology and Molecular Biology of Plants 17:291–295. https://doi.org/10.1007/s12298-011-0066-6
3. 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/
4. e-GRO / Colorado State University Extension, Root disease problems and management in hydroponic systems (root-zone 68–72 °F). https://planttalk.colostate.edu/topics/miscellaneous/2043-root-disease-problems-management-hydroponic-systems/