Quick answer: Data center humidity is managed by dew point, not RH alone. The ASHRAE TC 9.9 recommended envelope for IT equipment inlets is about 18–27 °C, with moisture between about −9 °C and 15 °C dew point and no more than 60 % RH. Wider "allowable" ranges depend on the equipment class. Too much moisture risks condensation on cold surfaces and, with gaseous contaminants, corrosion. Very dry air has historically been linked to static, but ASHRAE research has relaxed low-humidity limits for most modern equipment. Measure at IT inlets by zone, watch economizer modes that bring in outdoor air, and keep cold surfaces above the dew point.
Quick answer
- Recommended: about 18–27 °C inlet; dew point about −9 to 15 °C; ≤ 60 % RH.
- Control by dew point; measure at IT inlets.
- Watch outdoor-air economizers and cold surfaces.
The recommended envelope
| Parameter | Recommended (ASHRAE TC 9.9, classes A1–A4) |
|---|---|
| Inlet temperature | About 18–27 °C |
| Low moisture limit | About −9 °C dew point |
| High moisture limit | About 15 °C dew point and 60 % RH |
Allowable ranges are wider and depend on the equipment class; check the current ASHRAE thermal guidelines and the IT makers' specifications. ASHRAE also gives guidance on gaseous and particulate contamination, which interacts with humidity in corrosion risk (ASHRAE, 2011 Gaseous and particulate contamination guidelines).
Risks at each end
| Condition | Risk |
|---|---|
| High dew point | Condensation on cold surfaces (chilled water pipes, CRAH coils, cold aisles after shutdown) |
| High RH with gaseous pollutants | Corrosion of copper and silver on boards |
| Very low humidity | Electrostatic discharge risk (mitigated by grounding and handling practice) |
| Rapid changes | Condensation during transitions, e.g. economizer switching |
Zones and sensors
- Cold aisle (IT inlet): the control point for the envelope.
- Hot aisle and return: for cooling performance, not humidity limits.
- Underfloor and plenums, near chilled water: condensation checks.
- Outdoor air intakes when economizers run.
Economizers and outdoor air
Air-side economizers save cooling energy but bring outdoor moisture inside. Control them by enthalpy or dew point so outdoor air is used only when it is within the envelope. See free cooling in data centers. ENERGY STAR describes air-side economizers (ENERGY STAR).
Control methods
| Need | Method |
|---|---|
| Lower dew point | Cooling coils below dew point; dedicated dehumidification; dew-point control of economizers |
| Raise humidity (if required) | Adiabatic or steam humidifiers; often unnecessary within current guidance |
| Avoid simultaneous humidify/dehumidify | Coordinate CRAH/CRAC controls on a shared dew-point setpoint |
What transfers to BESS and electrical rooms
Data center practice transfers the method: dew point as the control variable, zoned sensors, deliberate airflow, alarms and maintenance. It doesn't transfer the boundary. BESS containers are small, outdoor, thermally dynamic and have battery-specific safety requirements, so their targets come from the battery and enclosure suppliers. See why humidity control matters in battery cabinets and our energy storage dehumidifiers.
FAQ
Frequently asked questions
What humidity should a data center be?
Within the ASHRAE recommended envelope: dew point about −9 to 15 °C and no more than 60 % RH, at 18–27 °C inlet.
Is low humidity bad for servers?
Modern guidance allows lower humidity than older practice; follow equipment class limits and good grounding.
Why use dew point in a data center?
It shows absolute moisture and directly answers whether cold surfaces will condense.
Sources
- ASHRAE TC 9.9, Thermal guidelines for data processing environments.
- ASHRAE, Gaseous and particulate contamination guidelines for data centers (2011).
- ENERGY STAR, Use an air-side economizer.
