The Cheapest Heat Is the Heat You Keep
Every commercial greenhouse project eventually reaches the same conclusion: heating a structure is expensive, and the single largest loss is radiation and convection through the roof at night. A screen that closes over the crop after sunset does not generate a single watt of heat — it simply stops the heat you already paid for from leaving through the roof.
Greenhouse thermal screens are one of the few energy investments on a hydroponic farm with a measurable, verifiable payback, and they interact with humidity, light transmission and crop work in ways worth understanding before ordering. This guide covers the screen types, the climate decisions, and the numbers that justify the spend.
What a Screen Actually Does
| Function | Mechanism | Where it matters most |
|---|---|---|
| Heat retention at night | Reduces radiation loss and traps a still air layer over the crop | Cold nights, high heating cost, winter production |
| Shading in summer | Reflects and absorbs incoming solar radiation | Hot climates, high DLI, sunscald risk on fruit |
| Humidity management | Changes the volume of air the climate system must condition | Screens that are permeable to water vapour vs closed screens |
| Light diffusion | Some weaves scatter light more evenly through the canopy | Tall crops, deep canopies, uneven growth |
| Structure protection | Can reduce condensation drip onto the crop | Houses with chronic drip problems |
A screen is a compromise device. Every hour it is closed it saves heat, and every hour it is closed it costs light. Getting the schedule right matters more than the headline energy saving on the datasheet.
Screen Types at a Glance
- Thermal screens (energy screens) — designed for night heat retention, usually aluminised strips woven with a clear film. The saving comes from reflecting radiant heat back into the crop and trapping a still air layer.
- Shade screens — designed to reduce incoming radiation. Measured by shade percentage; also reduce heat load on hot days.
- Combination screens — a single screen that both shades and retains heat, increasingly the default for mixed climates.
- Blackout screens — used for photoperiod control in crops that need a defined dark period. Not primarily an energy device.
- Insect screens — fixed to vents, not to a moving screen system. Different purpose entirely.
For most hydroponic projects the practical choice is a combination screen with a stated energy-saving figure and a known shade percentage, matched to the local climate and the crop’s light requirement.
The Design Questions That Decide the Specification
- What is the winter heat load, and how much of it is roof loss? Without this you cannot calculate payback, only be sold a percentage.
- How much light can the crop afford to lose? A screen that saves heat but closes too often through a low-light winter can cost more in yield than it saves in fuel.
- How humid does the house get at night? A closed screen traps moisture around the crop, and high night humidity is the main driver of botrytis and other fungal problems.
- What is the condensation behaviour of the screen? Screens that shed condensate to the edge rather than dripping on the crop are worth a premium in disease-prone crops.
- What are the structural loads? Retrofitting screens to an existing greenhouse adds load to the frame and requires a proper check, not an assumption.
- How is the screen controlled? A screen driven by a climate computer using outside temperature, wind and humidity performs very differently from one operated by hand.
Modelling the Saving Before You Buy
| Input | Why you need it |
|---|---|
| Heated area | Sizes the screen and scales the saving |
| Design night temperature and local climate data | Determines the heat load the screen is protecting against |
| Current fuel or electricity price | Converts saved kWh into money |
| Screen’s stated energy saving percentage | Supplier figure; apply conservatively |
| Hours per night the screen can realistically be closed | The saving only exists while closed |
| Light loss and its yield effect | The cost side of the same decision |
| Installed cost including structure reinforcement and controls | Usually higher than the screen material quote |
A disciplined approach is to calculate the saving only for hours you can prove the screen will be closed, discount the supplier’s percentage, subtract the light loss, and then compare against installed cost. Screens that survive that test are usually the ones with a permeable, condensation-shedding weave in a climate with a genuine cold season.
Operating a Screen Well
- Close early, open early. Most heat is lost in the first hour after sunset and the first hour after sunrise; the middle of the night matters less than the shoulders.
- Open before humidity spikes, not after. If the house has to vent moisture, it is cheaper to vent before the screen traps it.
- Modulate rather than treat it as on/off. Partially closed positions let you balance heat and humidity continuously.
- Integrate with heating, vents and CO2. A screen changes the behaviour of all three, and a climate computer that treats them independently will produce oscillation.
- Inspect the mechanism monthly. A screen that jams half open in February is worse than no screen, because you will not notice until the fuel bill arrives.
FAQ
How much energy does a thermal screen save?
Suppliers quote a range, and the achievable figure depends on climate, closing hours and how well the house is sealed. Model the saving on the hours you can realistically close, and treat the datasheet figure as a ceiling rather than a forecast.
Can I retrofit a screen to an existing greenhouse?
Usually yes, but it is a structural change. The frame, trusses and gutters must be checked for the added load, and the cost of reinforcement is part of the payback calculation.
Will a screen increase disease pressure?
It can, by trapping humid air around the crop. Choose a screen with good moisture permeability, run a humidity-driven control strategy, and keep night VPD within the range your crop tolerates.
Do screens make sense in a hot climate?
Yes, but as shade rather than heat retention. In hot climates the screen’s value is reducing incoming radiation and therefore cooling load, which is a different calculation with a different payback.
Do I still need heating if I install screens?
Yes. Screens reduce demand, they do not replace a heating system. The correct order is to insulate and screen first, then size the heater to the reduced load — which usually means a smaller, cheaper heating system.
Can a screen be combined with supplemental lighting?
It has to be, in any house running lights. The screen and luminaires must be laid out so the screen can open and close without fouling the lamps, and the light loss through the screen fabric when closed must be accounted for.
Size the Screen With the Heating System
A screen changes the heat load, and the heat load sets the heater size. Send us your climate data, house dimensions and crop plan and we will return a screen specification, a revised heat load and a payback estimate based on your local energy price. Start with the quote form.
Related reading: screens sit inside the wider climate system — see covering materials compared and farm energy loads and solar options, and check the structural implications in greenhouse structural design.