A deployed thermal screen creates two connected air spaces. Warm, moist air around the crop may remain below the screen while cooler air sits above it near the roof. Water vapour can cross the screen, air can pass through its fabric or its gaps, and water can condense on a cold surface. Those processes are different. Opening the screen may relieve a below-screen humidity problem, but it can also drop cold air onto the canopy and sacrifice heat. The right first move is to identify the actual moisture path and the achieved crop-zone condition.
This guide is for a grower or greenhouse engineer comparing screen materials and operating choices. The winter greenhouse heating guide covers the wider heat, ventilation and dehumidification decision; the blackout-curtain selection guide addresses light exclusion and photoperiod. A thermal screen's humidity behaviour deserves its own investigation because neither general heating nor blackout timing establishes how water leaves the crop zone.
Map the air and water boundaries
Start by drawing four measurement locations: canopy air below the screen, air immediately above it, the roof or screen surface where condensation may form, and outdoor air. Record screen position, roof-vent position, circulation fans, heating, irrigation, crop stage and any dehumidifier operation on the same time axis. A single aisle RH value cannot show whether the crop is wet or whether the space above the screen is a usable moisture sink.
Use temperature and RH to derive dew point at each air location. If humidity ratio is used, include the applicable total or barometric pressure in the calculation. RH alone can increase when air cools without any water being added. Compare the measured surface temperature with nearby air dew point when investigating condensation; an air sensor cannot prove that a leaf or screen is wet. For how to choose sensor locations and check them, use the greenhouse humidity monitoring guide.
The water balance also has an exit condition. Moving vapour from below a screen into the roof space is redistribution, not removal from the greenhouse. Identify whether it later leaves through a vent, condenses and is drained, or is captured by a dehumidifier. If it remains in the enclosure, a temporary improvement below the screen may be followed by re-evaporation or a different condensation problem.
Distinguish three paths through a screen
| Path | What it means | What evidence to request |
|---|---|---|
| Air passage through the material | Pressure and temperature differences drive air through a porous fabric or perforations, carrying sensible heat and vapour | Air-permeability test conditions, direction and pressure difference; installed pressure and actual screen position |
| Vapour movement without bulk air exchange | Water vapour diffuses through a material under a concentration difference | Dry-screen vapour-transport test conditions and units; temperature and RH on both sides |
| Wet-screen transport | Water condenses on the lower surface, is absorbed or transported by the material, and may evaporate above it | Wet and dry test results, wetting regime, drainage and repeated-use behaviour |
Wageningen University & Research's screen-material study measured air permeability and humidity transport under specified dry and wet conditions, then used the material inputs in a dynamic greenhouse model. It distinguishes convective exchange, diffusion and condensation–hygroscopic–evaporation paths. The study's simulated tomato house and operating assumptions make it useful for method and material questions; its modeled energy or RH outcomes are not a transferable target for another greenhouse.
The material data cannot be read as a simple “more breathable is better” ranking. More air exchange may help move vapour but also changes heat loss and canopy temperature. A tighter screen may retain heat yet require a separate moisture-removal path. Ask the supplier for measurements made with comparable test definitions, including wet performance and thermal-radiation properties, then evaluate the material in the intended screen system.
A gap is not the same as permeability
Air around an edge, through a tear or through a deliberately opened gap bypasses the fabric. A material test does not measure that bypass. Inspect seals, sidewalls, overlap, drive alignment and damaged areas before attributing a temperature or humidity gradient to the textile. A screen that is nominally closed can still have significant local exchange through an unintended opening.
When a gap is intentional, treat it as an actuator with a documented opening sequence, not as a universal percentage setting. Ask what air is moving between the crop and roof spaces, where the coldest air will fall, whether the roof vent is open, and whether the crop-zone target actually improves. The AHDB screen-management guide describes why above-screen conditions and unintended gaps matter to canopy climate. The University of Wisconsin greenhouse-curtain guide also treats sealing, vents and staged opening as parts of a screen system. Both are older operating guides; use their mechanism and inspection questions, not historical savings or cost figures as current project promises.
Compare control routes under the same duty
Do not compare a closed-screen night with an open-screen sunny period and call the difference a screen effect. Hold the crop stage, target, weather window and operating schedule as comparable as practical. Record achieved canopy conditions, heat input and any actual water removal for each route.
- Closed screen with an above-screen vent. A material that permits useful vapour transport may allow above-screen venting while the screen remains closed. This only works if the material and pressure/temperature conditions deliver the path and the roof-space air can discharge moisture. Verify below-screen RH, dew point and surface risk rather than inferring success from a vent command.
- Controlled screen gap. A gap gives a direct air path, but cold roof-space air can descend near the opening. Test the opening sequence and crop-zone uniformity, including the transition when the screen first closes or opens. An intentional gap should not be confused with a damaged seal.
- Mechanical dehumidification with screen closed. A dehumidifier can remove water from the controlled air volume without relying on an outdoor moisture sink. It needs capacity at the actual air condition, air distribution, drainage and an explicit heat/electricity budget. See the greenhouse dehumidification project-data guide for the required inputs.
- Ventilation with heat recovery or heat replacement where designed. If outside air is drier in absolute terms, ventilation may remove moisture, but its heat and CO₂ effects must be counted. A heat exchanger or heater changes the cost and control boundary; it does not make the moisture balance disappear.
The routes can be combined. No route guarantees a particular RH, energy saving, disease reduction or yield outcome without site measurements. Screen closure also has a light-duty cost during daytime use; a crop-light decision and a night humidity decision should not be collapsed into one fixed schedule.
A practical verification sequence
Before changing the control program, trend a representative operating period with the existing screen. Check sensor calibration and timing, then annotate irrigation, lights, heating, vents and screen movement. Identify which event precedes below-screen humidity rise and whether condensation appears at the canopy, screen or roof.
Next, inspect the fabric and installation. Separate material transport from air leakage at seals and gaps. If the property sheet reports only an energy-saving percentage, request the underlying air, vapour and thermal measurements with test conditions. A modeled target RH is not the same as the achieved RH distribution around a real crop.
Finally, trial one operating change at a time where crop safety permits. Compare the before and after period with the same measured objects: canopy air and surface risk, above-screen air, removed water or vented air, and heating/electrical demand. Revert or change route if the crop-zone condition deteriorates, even when a roof-space sensor looks drier. Keep an alarm and manual-response plan for failed drives, vents or sensors.
What can be concluded without a project trial?
Material physics explains possible moisture paths. Published tests show how to measure those paths and published operating guides show what to inspect. They do not establish a universal screen opening, crop RH target or payback for a particular house. A credible project decision needs the crop and climate duty, screen test data, installed geometry, comparable trend logs and a defined way for water to leave the greenhouse.
For the broader application context, start at agriculture and greenhouse climate control. For a specific winter heat or blackout schedule, return to the corresponding existing guide rather than using this page as a general heating or photoperiod manual.
FAQ
Frequently asked questions
Does a breathable thermal screen remove moisture from the greenhouse?
Not by itself. It may allow water vapour to move above the screen, but water is removed only if a vent, drainage or dehumidification path carries it out of the controlled enclosure. Confirm the actual route and crop-zone result.
Is the best way to lower humidity to open a screen gap?
There is no universal answer. A gap can move air but may bring colder roof-space air down onto the crop and lose heat. Compare a controlled gap with closed-screen venting or mechanical drying under the same duty and measured crop-zone condition.
Can dry-screen permeability alone predict condensation behaviour?
Not by itself. Air permeability, vapour diffusion and wet-screen condensation or hygroscopic transport are different properties. Ask for wet and dry tests with temperatures, RH and pressure conditions before applying a material result.
What should be measured above and below the screen?
Record temperature and RH at both air locations. Derive dew point from those measurements; if you calculate humidity ratio, include the applicable total or barometric pressure. Observe relevant leaf, screen and roof surfaces. Log screen position, vents, heating, irrigation and any dehumidifier together so a change can be interpreted.
