Formulas
Dew Point = 237.3 × α / (17.27 − α) where α = ln(RH/100) + 17.27T/(237.3+T)Absolute Humidity = (actualVP × 1000) / (0.4615 × (T + 273.15)) [g/m³]Humidity Targets by Stage
| Stage | Day RH | Night RH max | Notes |
|---|---|---|---|
| Seedling / Clone | 70–85% | 80% | Dome maintains near saturation |
| Early Vegetative | 60–70% | 70% | Begin reducing from seedling RH |
| Late Vegetative | 55–65% | 65% | Increase airflow |
| Early Flower | 50–60% | 60% | Watch dew point closely |
| Late Flower | 40–50% | 55% | Below 50% to prevent botrytis |
Accurate humidity control requires a reliable combination meter and a controllable dehumidifier. These are the tools most recommended by technical growers:
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Frequently Asked Questions
What is the dew point and why does it matter in a grow room?
The dew point is the temperature at which air becomes saturated and moisture begins to condense. In a grow room, if any surface (walls, pipes, pots) drops below the dew point, condensation forms. This creates ideal conditions for Botrytis (grey mold), powdery mildew, and root rot. Keeping the dew point well below your minimum surface temperature — typically by at least 5–6°C — prevents condensation entirely.
What is absolute humidity vs relative humidity?
Relative humidity (RH%) measures moisture as a percentage of the maximum possible at that temperature. Absolute humidity measures actual water content in grams per cubic meter (g/m³), regardless of temperature. A room at 26°C and 65% RH holds approximately 15.4 g/m³. If you cool that air to 16°C, it reaches 100% RH — the dew point — and moisture begins condensing.
How do I lower the dew point in my grow room?
Lower the dew point by reducing absolute humidity — which means removing moisture from the air. A dehumidifier is the most effective tool. Increasing temperature alone raises the dew point margin without removing moisture. The only permanent solution to condensation risk is removing water vapor with a dehumidifier or by increasing fresh air exchange.
What humidity should I keep during lights-off to avoid mold?
During lights-off, temperature drops 4–8°C and VPD falls sharply. If daytime RH is 60% at 26°C, night RH effectively increases to 75–80% at 20°C. Keep daytime RH below 55–60% in late flower so that night RH stays below 70%. A programmable dehumidifier set to activate during lights-off is the most reliable approach.
What is wet bulb temperature?
Wet bulb temperature is the lowest temperature achievable by evaporative cooling. It is always equal to or less than air temperature. In grow rooms it is used to estimate the efficiency of swamp coolers and the apparent temperature plants experience. At 26°C and 65% RH, wet bulb temperature is approximately 20.4°C.