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

> Lower grow-tent humidity by removing the water the plants transpire: more exhaust if the room air is drier, then a dehumidifier sized to roughly the litres you irrigate per day, running through lights-off. Targets are 55–70 % RH in veg, 50–60 % in early flower and 40–50 % in late flower. Ramp temperature down slowly at lights-off and remove standing water.

- Canonical: https://genetixbay.com/knowledge/answers/how-to-lower-humidity-in-a-grow-tent.html
- Topic: Environment · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- Also asked as: Humidity too high in my grow tent | How do I reduce RH in a grow room? | How can I lower humidity without a dehumidifier? | Why does humidity spike when the lights go off?
- 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 humidity by removing the water your plants breathe out: [exhaust](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) more air if the room is drier, and if it isn't, run a [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md) sized to roughly the litres you irrigate per day. Aim for **40–50 % [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) in late flower** and keep it at or below **55 % at lights-off**. *Supported* for the disease risk; sizing is *Grower practice*.

## Why

Almost all irrigation [water leaves](https://genetixbay.com/knowledge/glossary/fan-leaf.md) the plant as vapour. Four big plants in a 1.2 × 1.2 m (4 × 4 ft) tent can add 6–8 L a day. That water has to go somewhere.

High humidity in flower is how [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md) and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) start [2][3]. In a 2025 cannabis study, roughly 78–98 % RH delayed flowering and lowered biomass and [cannabinoids](https://genetixbay.com/knowledge/glossary/cannabinoid.md) compared with 37–58 % RH [1].

When the lights go off, air cools and relative humidity rises even though no water was added. Surfaces near the [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md) get wet [4].

## The numbers

| Stage | RH target | Air temperature |
|---|---|---|
| Veg | 55–70 % | 24–28 °C (75–82 °F) |
| Flower weeks 1–4 | 50–60 % | 23–27 °C (73–81 °F) |
| Flower week 5 to harvest | 40–50 % | 20–25 °C (68–77 °F) |
| Lights-off in flower | ≤ 55 %, surfaces ≥ 3 °C (5 °F) above dew point | |
| Dehumidifier capacity | ≈ litres irrigated per day, at real conditions | |

## What to do

1. Check the hygrometer first (salt test) and read it at canopy height.
2. Raise exhaust speed if the room air is drier than the tent.
3. Empty saucers and [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) trays; lid any reservoir.
4. Thin dense interior leaves so air reaches the middle of the canopy ("tuck, don't pluck" first).
5. Add a dehumidifier on a humidistat, inside or beside the tent, and keep it on through lights-off.
6. Ramp temperature down over 30–60 minutes at lights-off.

## Common mistakes

- Buying a dehumidifier on its 30 °C / 80 % RH rating; it removes far less at 24 °C / 50 %.
- Watching the daytime average and missing the lights-off spike.
- Turning fans off at night.
- Pulling "dry" air from a room that is just as humid as the tent.

## Related

- Full article: [Humidity by stage](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [How do I raise the humidity in my grow tent?](https://genetixbay.com/knowledge/answers/how-to-raise-humidity-in-a-grow-tent.md)
- [How do I check if my hygrometer is accurate?](https://genetixbay.com/knowledge/answers/calibrate-a-hygrometer.md)
- [VPD explained](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- Terms: [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md) · [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md)

## Follow-up questions

**How can I lower humidity without a dehumidifier?** Without a dehumidifier, lower grow-tent humidity by exhausting more air, as long as the room you pull from is drier than the tent; that is the cheapest fix. Then remove standing water and runoff trays, water early in the light period, thin dense interior leaves and keep the tent slightly warmer at lights-off. If the room itself sits above about 60 % RH, only a dehumidifier will work.

**What size dehumidifier for a grow tent?** As a first estimate, a grow-tent dehumidifier should remove about as many litres per day as the plants are irrigated, because most irrigation water is transpired into the air. Four large plants in a 1.2 × 1.2 m (4 × 4 ft) tent can add 6–8 L of vapour daily. Check the rating conditions: units rated at 30 °C and 80 % RH remove far less at 24 °C and 50 %.

**Why does humidity spike at lights-off?** Humidity spikes at lights-off because the air cools: the same water vapour in cooler air means higher relative humidity, and surfaces near the dew point collect condensation, where bud rot starts. Ramp the temperature down over 30–60 minutes instead of a sudden drop, keep the dehumidifier and circulation fans running in the dark, or add a little heat during the dark period.

## Sources

1. Corredor-Perilla IC, Kwon H, Park J, 2025. High relative humidity effects in cannabis. Frontiers in Plant Science 16:1678142. https://doi.org/10.3389/fpls.2025.1678142
2. 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/
3. Utah State University Extension. Hemp bud rot (IPM fact sheet). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot.php
4. Allen RG et al., 1998. FAO-56 Chapter 3 (vapour pressure). https://www.fao.org/4/x0490e/x0490e07.htm