Water seen below a closed greenhouse thermal curtain does not show where it formed. It may have condensed on the curtain's crop-facing side, arrived from wet roof glazing or a gutter above, or traveled through a gap or a fabric that permits liquid passage. A shower when the screen retracts is an event to document, not proof that the curtain generated the water. Locate the first wet surface while the screen is still in position, then relate it to the air and surface conditions on the correct side of the screen.
The image above is a conceptual illustration, not a Yakeclimate installation or a verified water source. This article follows liquid water at and immediately below the screen. The published thermal-screen humidity guide explains how vapour and air move between the crop and roof spaces and how to compare control routes. For a first-wet-surface survey of the whole house, use the greenhouse condensation guide.
A screen separates two air states, but not every water path
With a thermal screen deployed, crop-zone air below it and roof-space air above it can differ in temperature and moisture content. Screen fabric, seams, intentional gaps and edge leaks provide different paths for air or vapour. The underside, upper side, roof cover and supports can therefore have different temperatures and water histories. A crop-zone RH reading alone cannot identify which surface first became wet.
University of Wisconsin Extension's curtain guide distinguishes porous fabrics that allow roof condensate to drip through from nonporous sheets that can hold water above the screen. UC Cooperative Extension notes that water can accumulate on or under a thermal screen and may fall when it moves. These are possible mechanisms; inspect the installed fabric before treating passage as its property.
Two studies show why the water source needs to be traced. In an AHDB thermal-screen project, water condensed on a tested mobile polythene screen and fell onto the crop as it moved. In a different Wageningen Plant Research greenhouse investigation, the researchers observed condensation in the screen material but attributed the measured drip to the cover and equipment, not the screens. These installations and materials differ. Neither result justifies assuming that all wet screens drip, or that a screen cannot be the source.
Find the first wet point before the screen moves
Observe a representative wet period before opening the screen or wiping a surface. Use the site's safe-access procedure; do not enter a moving curtain drive, reach into a roof bay, or stand under a suspected water load. From an accessible position, record the first visible bead, stain or drip on the crop-facing fabric, leading edge, seam, support wire and any nearby equipment. If the upper side is not safely visible, mark it unobserved rather than declaring it dry. Photograph the roof cover, gutter, glazing joints and any pipe or duct directly above the landing point.
Trace the sequence, not merely the final puddle. Does a bead first form on the fabric's lower face while the screen is still? Is the upper face already wet? Does water run along a seam before dripping from its lowest point? Does the drip begin only when the leading edge moves? A roof or gutter source can land on a curtain and later fall from it; the final drop location is not necessarily the source. The roof-condensation guide owns roof-face and runoff diagnosis, and the glazing guide owns moisture inside a glazing layer or joint.
| Observation during the event | Competing explanations | Next evidence to collect |
|---|---|---|
| Beads first appear on the crop-facing screen while it remains still | Direct condensation from adjacent crop-zone air; water arriving from above through a gap or a material that permits it | Same-time near-screen air temperature and RH, lower-face temperature if safely measurable, upper-face and roof observations |
| Screen upper face or fabric is wet before a lower drop appears | Roof or gutter runoff; condensation on the upper face; water carried through fabric | First wet point above, cover and drain route, upper-zone air and surface condition, fabric specification |
| Drip appears along a seam, edge or support at retraction | Water stored at that detail; runoff routed there; motion releases water formed earlier | Stationary-state photos, exact motion time, wet edge or support path, drive and screen inspection by the supplier |
| Crop is wet but screen remains dry at the checked location | Leaf dew, irrigation, overhead equipment or an unobserved water path | First wet crop point, irrigation and equipment log, leaf-origin method |
If the first water cannot be seen safely, use a qualified inspection and retain the uncertainty. Avoid assigning the problem to screen fabric solely because a droplet falls from its edge.
Compare the correct air with the correct surface
At the wet location, record air temperature and RH close to the lower face of the screen and, where practical, separately above it. Calculate dew point for each air sample. Measure the corresponding surface temperature with a method suitable for the fabric or adjacent solid detail. A crop-facing surface at or below the dew point of air touching that face makes direct condensation plausible. It does not exclude water that also arrived from above. A warmer reading may mean the first formation happened earlier, at another spot, or on a different surface.
Do not treat an unverified surface-temperature estimate on an inaccessible or moving screen as a confirmed measurement. Record the method and its limits; ask an instrument specialist when a disputed temperature matters. Record screen position, vent and fan state, heating, irrigation, outside weather and the exact time of each image and reading. Compare the period before closure, the closed interval, the moment before opening and the opening event. Do not combine a sunrise surface reading with a night-time wet photograph as though they were simultaneous.
The published thermal-screen humidity guide gives the broader above/below-screen measurement and water-exit method. This page's narrower question is whether the visible water formed on the crop-facing screen, arrived from its upper side, or came from another component.
Route the correction by source
If direct lower-face condensation is supported, the crop-climate owner should review the actual risk window: moisture generated below the screen, local air circulation, heat delivery and a verified moisture-removal route. Changing screen gap, vent position or dehumidification without checking crop temperature and the upper air space may move the problem or introduce a cold draft. Ask the curtain supplier to review the installed material, seams and water behaviour before any material or control change. There is no transferable opening percentage or RH setpoint from these sources.
If the water starts at the roof, gutter, glazing joint or equipment above the screen, the cover, drainage or equipment owner must address that route. Once liquid has accumulated, a dehumidifier cannot repair a leaking gutter or prevent that stored water from being released by screen motion. If a wet fabric or seam stores water, the screen supplier should inspect material compatibility, support load, drive travel, intended drainage and fire requirements. Do not perforate, replace, tighten or operate the screen differently from this article alone.
Where a measured residual vapour-removal duty remains after liquid entry and drainage have been evaluated, an equipment review can use the actual crop-zone air state, moisture source, operating hours, airflow, heat/ventilation interaction, controls and condensate route. The published greenhouse humidity-control guide compares those whole-house routes. No screen observation by itself establishes a dehumidifier capacity, Yakeclimate performance result or crop outcome.
Retest the same event, not a convenient dry afternoon
Keep a short record for the affected bay: screen material and position; first wet point on each accessible face; roof and gutter condition; exact retraction time; above- and below-screen air temperature/RH; relevant surface readings; weather, irrigation, heat, vents and fans; photo timestamps; and where the water landed. Record missing observations explicitly. Preserve the supplier's inspection result and the owner of each adjustment.
After a correction, compare a similar cold or wet period with the same screen schedule and measurement locations. Verify separately whether water still first forms on the screen, whether overhead runoff still reaches it, whether motion still releases drops, and whether the crop-zone condition remains acceptable. A dry screen at one observation point is not proof that the entire roof-space water path has been corrected.
FAQ
Frequently asked questions
Does a drip from the thermal curtain prove that condensation formed on its underside?
No. Water can arrive from the cover or equipment above, travel through or along the material, then fall from a seam or edge. Find the earliest wet surface while the screen is stationary and compare both sides when safe access permits.
Can a closed thermal screen be wet without dripping onto crops?
Yes. Wetness and drip are separate observations. One Wageningen greenhouse study found screen condensation while measured drip came from the cover and equipment; an AHDB test with a different moving polythene screen observed water falling when it moved. Check the installed material and event rather than transferring either result.
Should we open a gap to stop condensation under the curtain?
Only after the crop-climate and screen owners review the measured water path and upper/lower air states. A gap changes air, heat and vapour exchange; it may also move cold air toward the crop. The appropriate control depends on the installed system and crop duty.
What should we log when water falls as the screen retracts?
Record whether the fabric, seam, roof, gutter and nearby equipment were wet before motion; photograph the first water; note the exact retraction time and position; and log both air zones, relevant surface temperatures and operating state. Give the record to the screen, structure and crop-climate owners for a joint diagnosis.
