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Wet Mushroom Caps & Condensation
A target humidity reading cannot tell you where surface water came from. Use dew point, surface temperature and the timing of wetting to separate condensation from direct spray and overhead drips.
Why can caps be wet at 90% RH?
Relative humidity describes the air at the sensor. Condensation depends on the temperature of the surface in contact with that air. If a mushroom cap, upper rack, ceiling panel or supply diffuser is at or below the local dew point, moisture can collect there. Direct spray and overhead drips can also wet caps at the same room RH. The first diagnostic question is where and when the water appears.
Penn State Extension notes that a mushroom cap can run cooler than the surrounding room air. Its guidance also links persistent surface moisture to bacterial blotch risk. A wet cap is a reason to check conditions and farm hygiene; its appearance alone does not establish a disease diagnosis.
How small is the dew-point margin?
These examples hold room air at 60°F. Raising the RH brings the dew point closer to air temperature and leaves less room for a cold cap or overhead surface.
| Room RH at 60°F | Approximate dew point | Air-to-dew-point margin |
|---|---|---|
| 85% | 55.5°F | 4.5°F |
| 90% | 57.1°F | 2.9°F |
| 95% | 58.6°F | 1.4°F |
Example: At 60°F and 90% RH, a 56°F shelf is below the 57.1°F dew point. A 59°F shelf is above it. Use an independent sensor to check local air conditions and compare a representative surface temperature at the same time. Calculate the dew point in your room.
Trace the wetting pattern before adjusting humidity
| What the grower sees | What to check first |
|---|---|
| Moisture on cold caps or walls after a cooling cycle | Compare local surface temperature to dew point during the event. Inspect cooling discharge, cold bridges and changes between day and night settings. |
| Wet patches concentrated in front of a humidifier | Record when the unit runs. Check available evaporation distance, nozzle direction, air path and whether an adjacent rack intercepts the spray. |
| Wet caps confined to one strip under a pipe or ceiling | Inspect overhead surfaces for condensate or leaks and trace drainage. Check again after washdown and when the cooling coil cycles. |
| Dry shelf edges and wet corners in the same room | Map temperature and RH at multiple rack heights. Compare supply and return paths before changing the whole-room setpoint. |
What should you log during a problem shift?
Record the time of first wetting, room and rack-level temperature/RH, a surface temperature, humidifier output, exhaust status, cooling-coil operation and washdown or watering times. Compare a wet location and a dry location during the same interval. An event that follows nozzle operation suggests a different investigation from one that begins after the humidifier shuts off.
Once the source is identified, correct the delivery pattern, drainage or control sequence as appropriate. Keep crop-stage humidity and fresh-air requirements in view. Drying the entire room can damage exposed pins while leaving the source of a drip in place.
Check feedwater and upkeep when marks persist
A mark left after a surface dries calls for a different check from fresh condensation. Look at the source-water and treated-water readings, compare filter pressures under load, and record where deposits recur. Use the grow-room feedwater and maintenance checklist to organize the inspection. If upper and lower tiers behave differently, follow the rack-level verification steps.
Review humidity distribution across your racks
Send Smart Fog the room layout, sensor locations and a description of the wetting pattern.
Request a grow-room review → Check fresh-air humidification load