Quick answer: Under a closed thermal screen, crop moisture leaves by three paths: vapour through the fabric, air through the fabric and gaps, and condensation on the cold screen or roof. A closed screen saves heat by trapping warm air around the crop, but it also traps moisture; measure both below and above the screen before choosing controls.
Below the screen, RH rises. Above it, the air is cold and condenses water on the roof. Vapour movement through the fabric depends on the screen’s permeability. To control humidity under a screen you can open a small gap, which releases moisture but also drops some cold air and loses heat; heat-and-gap; or dehumidify below the screen so it can stay closed.
Quick answer
- Screens trap moisture with the heat.
- Gapping releases moisture but costs heat and can drop cold air on the crop.
- Dehumidifying below the screen keeps it closed and the heat in.
Measure four places
| Location | What it tells you |
|---|---|
| Canopy air below the screen | Crop condition, dew point at the crop |
| Air just above the screen | Whether it is drier (a moisture sink) or saturated |
| Screen or roof surface temperature | Where condensation forms |
| Outdoor air | Whether venting above the screen can dry |
Convert each reading to dew point (and humidity ratio, with the site pressure); see greenhouse psychrometric calculation.
Three ways moisture crosses a screen
| Path | What drives it | What changes it |
|---|---|---|
| Vapour diffusion through fabric | Vapour-pressure difference | Screen material: open, aluminised, film |
| Air exchange through fabric and gaps | Temperature and pressure differences | Gaps, edges, deliberate gapping |
| Condensation on cold surfaces | Surface below dew point | Screen and roof temperature, outdoor cold |
A gap is not the same as permeability: closed film screens block vapour, but leaky edges can still exchange air.
Worked example (illustrative)
Below the screen 18 °C / 85 % RH → dew point about 15.4 °C. Above the screen about 10 °C. The roof glass at 5 °C will condense; the air above the screen is near saturation. Moisture moves up from the crop zone and condenses on the roof, but drips can fall back onto the screen and the crop. If the screen is a closed film, RH below it keeps rising until the screen is gapped or moisture is removed below.
Compare control routes for the same duty
| Route | Effect on humidity | Cost and risk |
|---|---|---|
| Small screen gap | Releases moist air above the screen | Heat loss; cold air drop on the crop near the gap |
| Gap plus heating | Keeps crop warm while venting | More heat |
| Roof vent above screen | Removes moisture from above-screen space when outdoor air is drier | Heat loss above the screen |
| Dehumidify below the screen | Removes water from crop air; screen stays closed | Electricity; latent heat stays in the house |
| More permeable screen | More vapour passes | Less energy saving |
See heat recovery vs dehumidification and ventilation vs dehumidification.
A practical verification sequence
- Trend the four locations for several screen-closed nights.
- Note screen position, gap, vents, heating, HAF fans, irrigation on the same timeline.
- Find when the crop-zone dew point approaches leaf temperature.
- Try one change at a time (gap size, dehumidifier on) and compare.
- Check the crop for wet leaves at dawn.
Our greenhouse dehumidifiers work under the screen as a humidity stage in the climate-computer sequence.
FAQ
Frequently asked questions
Why is humidity high under my thermal screen?
The screen traps the crop's moisture along with the heat; less air exchange means RH rises.
Should I gap my energy screen for humidity?
Small gaps release moisture but lose heat and may drop cold air; dehumidification is the alternative that keeps the screen closed.
Does a thermal screen cause condensation?
It moves the cold surface: condensation often forms above the screen on the roof, and drips can fall back.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- Wageningen University & Research greenhouse horticulture publications on energy screens and humidity.
