Water on a greenhouse air duct may be condensation on its outside, moisture inside the air path, or liquid arriving from a roof, irrigation line or air-treatment unit. Find the first wet point while the event is happening. For exterior condensation, compare the duct's outer surface temperature with the dew point of air beside it at the same time. Water at an outlet or joint needs an air-path and upstream-source check as well. A wet duct alone does not establish a dehumidifier fault or a need for more drying capacity.
This guide covers a duct serving greenhouse ventilation or air-treatment equipment. The accompanying image is an AI-created concept, not a Yakeclimate installation or diagnostic photograph. Its visible droplets illustrate a possible exterior wetting pattern; the image does not prove their origin. The whole-greenhouse humidity guide covers the wider moisture balance.
Map the air path and the first wet point
Mark the duct's inlet, equipment connection, elbows, joints, insulation changes and outlets on a simple sketch. Record whether the affected section is supply, return or exhaust, and whether it carries cooled, heated or unconditioned air. Note the unit mode, fans, ventilation, irrigation, misting, wash-down, thermal-screen position and weather when wetness first appears. Take time-stamped photographs from a safe position. Water can travel along the underside of a duct before it drips, so the landing point may be downstream of the source.
| First observation | Possible route | Next check |
|---|---|---|
| Fine beads begin on an exposed outer face or elbow | Greenhouse air condensing on a cold exterior surface; spray remains possible | Pair nearby-air temperature and RH with the same spot's surface temperature and event time; look for overhead water |
| Wetness begins at an insulation seam, hanger or termination | Exterior wetting, liquid ingress or vapor reaching a colder layer beneath a damaged jacket | Record the first wet location; ask the duct/insulation owner to inspect jacket and vapor-control continuity without assuming the layer beneath is dry |
| Water appears at an outlet or low point inside the air path | Internal condensation, water carried from an upstream coil, rain through an inlet, or another upstream source | Trace toward the equipment and intake with an authorized technician; inspect the intended condensate collection route |
| A joint blows cool air onto a neighboring surface, or draws in moist air | Air leakage changing a local surface or duct condition | Record fan state and pressure-side location; have the duct owner check the joint and measured airflow |
| Wetting follows rain, overhead irrigation or a leaking line | Liquid delivered onto the duct rather than formed there | Trace the highest first-wet point and the roof, irrigation or line path |
These are investigation branches, not fault codes. More than one route can be active. Do not open a running duct, enter a confined service space, reach a fan or disturb an electrical component to identify the source. Use the site's access and equipment isolation procedures.
Test exterior condensation at the affected section
UF/IFAS greenhouse ventilation guidance explains that humid greenhouse air can condense on a surface below the dew point of the air touching it. Measure air temperature and relative humidity beside the wet duct section and calculate dew point. Measure the duct or jacket's outer surface at the same location and time. The greenhouse psychrometric calculation guide explains the air-state calculation.
An exterior surface at or below the nearby-air dew point makes condensation plausible. It does not prove that every drop formed there: a roof drip or spray can wet the same surface. A surface above dew point during the actual event weakens that hypothesis, provided the measurement represents the first wet spot. A central greenhouse RH sensor, a later dry-period reading, or an unverified infrared reading from shiny or wet metal cannot settle it. DOE/PNNL measurement guidance explains why an infrared temperature reading depends on emissivity; have a qualified person choose a suitable surface measurement method.
Record the operating air temperature and airflow through the duct as well as local greenhouse air conditions. A section serving cooler air can create a cold outer surface; a screen, opening or stagnant pocket can make the air beside the duct different from the crop-zone sensor. Avoid a universal RH threshold or assumed insulation thickness. The relevant comparison is the actual section, air state and event.
Separate water inside the duct from outside beads
Water at an outlet may have formed inside the duct, been carried from a wet upstream component, or entered through an intake or opening. US EPA duct guidance lists precipitation through inlet louvers, excessive moisture in incoming air, poorly drained cooling-coil droplets and unsealed ducts among possible duct moisture routes. This is building HVAC guidance; the installed greenhouse duct, equipment and drainage still need site-specific inspection.
If cooling or dehumidification equipment intentionally condenses water at a coil, that water should follow the equipment's designed collection and discharge path. Liquid collecting in an ordinary air duct is a separate condition to investigate, not automatically useful moisture removal. Ask an authorized equipment or duct technician to inspect the upstream pan, outlet, inlet and duct route applicable to the installation. A wet external seam does not prove an internal leak; internal water found at a low point does not prove the external surface is condensing.
Air leakage deserves its own check. EPA notes that a leaking duct can move air into or out of adjacent spaces and contribute to condensation on nearby surfaces. A supply joint that leaks cool air can chill a neighboring surface; a return-side leak may draw humid air into a cooler path. The actual direction depends on the system and operating pressure. Let the duct owner verify leakage, airflow and the intended air path before changing a joint.
If the first wet point is instead on the air-treatment cabinet or at its condensate outlet, use the equipment-surface condensation guide to separate exterior panel wetting from the designed coil-drain route and nearby liquid sources. A wet duct alone does not establish a unit fault.
Match the correction to the confirmed route
For confirmed exterior condensation, the duct designer or insulation contractor should review the affected section's thermal protection, exposed fittings, hangers, seams and vapor-control continuity. US Department of Energy duct guidance explains the role of sealing, insulation and a vapor barrier in limiting duct condensation in buildings. Its residential examples do not specify a greenhouse material, thickness or installation method. Covering a wet seam without finding the water path can conceal ongoing moisture.
For water inside the duct, inspect the responsible upstream source and drainage route before adding insulation outside. For roof, irrigation or line water, repair that liquid source with its owner; lowering greenhouse RH will not seal it. For leakage, confirm the duct connection and airflow with the equipment and duct owners. Any change must preserve access, cleanliness, fan operating point and the installed equipment's requirements.
If several greenhouse surfaces become wet during the same period, the climate owner should also examine the shared air and heat condition. The nighttime humidity guide covers the dusk-to-dawn sequence; the winter greenhouse humidity guide explains why heating changes RH without removing water. Neither replaces a duct-specific water-source check. EPA moisture guidance treats duct sealing, drainage and prevention of unwanted condensation as related but distinct goals.
Retest under a comparable operating condition
After an approved correction, revisit the same duct section during a similar unit mode, airflow, greenhouse air state, irrigation schedule, screen position and outdoor condition. Compare the first-wet time, outer surface and nearby-air measurements, visible route, upstream drain observation and outlet condition. If the conditions differ substantially, record a provisional result and observe another event. A dry duct on one morning does not prove that crop-zone humidity or leaf wetness is controlled.
For an equipment-fit discussion, bring the duct sketch, unit and duct specifications, affected section and insulation details, fan mode and airflow record, local air and surface measurements, first-wet photographs, upstream water-source checks, existing drainage and the required moisture-removal period. The ducted versus freestanding greenhouse equipment guide covers air-path and cabinet-location choices. Yakeclimate's agriculture overview is the project entry point. The site team and qualified duct or equipment specialists must confirm the actual source and any repair; no Yakeclimate field result or product performance is claimed here.
FAQ
Frequently asked questions
Why does a greenhouse duct sweat when the room RH is below 100%?
The duct exterior can be colder than the air beside it. Condensation is possible if that surface reaches or falls below the local air dew point, even when a central RH sensor reads below 100%. Check the affected section during the event and rule out liquid arriving from above.
Does water dripping from an outlet mean the duct exterior is condensing?
No. An outlet drip may come from moisture inside the air path or an upstream source. Trace the first water with an authorized technician and check equipment drainage and the inlet. Exterior beads and interior water are separate observations.
Should we add duct insulation or seal a wet joint immediately?
Identify the water source first. Insulation and vapor-control details must suit the actual duct and operating conditions; a seal cannot repair an upstream drain or roof leak. The duct and equipment owners should approve any change and verify airflow afterward.
Will a larger dehumidifier solve duct condensation?
The wet duct does not establish the greenhouse moisture-removal duty. First determine whether water formed on the outside, inside, or arrived as liquid. Then assess the remaining air-moisture load and equipment conditions for the project.
Technical sources
- UF/IFAS Extension, Greenhouse Ventilation — greenhouse surface condensation and local dew point.
- US EPA, Heating, Ventilation and Air-Conditioning Systems — duct moisture routes, leakage and insulation considerations in building HVAC.
- US DOE, HVAC Sealed and Insulated Metal Ducts — insulation, vapor barrier and duct sealing principles in buildings.
- US EPA, Moisture Control Guidance — duct, coil drainage and exterior moisture distinctions in building HVAC.
- DOE/PNNL, Infrared R-Value Tool — surface-temperature measurement and emissivity caution.
