Why Humidity Spikes at Lights-Off

When the lights go off, the room cools faster than the moisture leaves it, and cooler air holds less water. So RH climbs without a drop of water being added. Air at 82°F and 60% RH reads about 84% RH once it cools to 72°F, if nothing pulls moisture out. The fix is dehumidification that keeps up at the night temperature, a night temperature that doesn't drop further than the chart, and air moving through the canopy.

What we aim for at night

• Night room temperature from our chart: 81–82°F in veg; 77°F in flower weeks 1–3; 72°F in weeks 4–5; then 71, 70, 68 and 65°F for weeks 6–9. The night number is room temperature, not leaf.
• Night VPD: we aim to match the daytime VPD at night. In a cooler room that means a lower RH, not a higher one.
• Never raise humidity at night to chase a VPD number. Keep lights-off humidity lower.

A good daytime VPD doesn't mean nights are fine. As the room cools, RH rises and leaves can get close to the dew point. Check night temperature and RH on their own.

Some rebound is normal

A packed room rebounds quickly when the dehumidifier cycles off. Say you're set at 60% and it spikes to 66%: set the dehumidifiers so they cycle a little below 60%. As long as they bring it back to 60% in a short time, you're good.

What isn't fine is RH that climbs and stays up. Log temperature and RH through several cycles and measure leaf temperature at the same times. Look at how long the plants sit at the high end, not just the peak.

How to bring it down

1. Set the dehumidifiers to cut in a little below target. Use the controls the unit supports; don't shorten its compressor off-time.
2. Check capacity at the night temperature. Making the room colder can reduce what the dehumidifiers remove. If they run nonstop and RH still climbs, you need more moisture removal; a lower setpoint won't create it.
3. Keep air moving through dense canopy, not just over the top.
4. Get rid of standing water and make sure dehumidifier drains are working.
5. Check more than one spot. A sensor in a stream of conditioned air can read fine while a packed corner sits much wetter.

Run the night readings, with measured leaf temperature, through our VPD calculator to see where you actually are. For rooms you're building out, we prefer separate AC and dehumidifiers for serviceability and redundancy; more on that in Choosing Environmental Setpoints.

Check the pots in the morning

Humid nights slow the plants down, so pots are often wetter at lights-on. Check pot weight before the first feed and push it later if they're still heavy. A stronger CropSalt feed won't fix a humidity problem. Matching the schedule to the room is covered in How Temperature and Humidity Change Your Watering Needs.