Keeping Our Greenhouses Toasty After Sundown with Thermal Curtains
Picture a winter night in Chuckey, Tennessee, the thermometer dipping toward the low single digits, condensation frosting along the steel ribs of our hydroponic house. Out in the glasshouses dotted across the Mornington Peninsula or down in the Huon Valley, growers are watching the same kind of cold snap roll across their rows of butterhead and mignonette. Whatever the postcode, leafy greens hate a cold shoulder, and nights are when most of the heat budget for a greenhouse goes up the chimney.
We've spent the last few years refining the way we hold warmth in our lettuce houses, partly out of necessity in the southern Appalachians and partly because so many of our customers, from chefs sourcing microgreens in Brisbane to families picking up living lettuce at a farmers' market in Castlemaine, expect a consistent crunch twelve months a year. Cold nights mean slow growth, bitter leaves, and the kind of variability no wholesale buyer wants to see on a delivery docket. Arvo after arvo, the team walks the rows checking the overnight low, and the numbers tell us a clear story: the houses that hold their heat perform best.
For a long time we relied on unit heaters and good intentions, which works until it doesn't. Propane and electric resistance both burn money, and in our region the grid isn't always reliable when a polar vortex rolls through. After talking with growers who'd already made the switch, we landed on a piece of equipment that has quietly become the most cost-effective upgrade on the property: a system of thermal curtains, also called energy or thermal screens, pulled across the gutter line each evening and tucked away again at sunrise.
What follows is the unvarnished version of how we use them, what they cost us, what they save us, and the trade-offs a small operation should think about before installing one. If you're running a propagation house in a frost pocket of Gippsland or a hydroponic tunnel in the Adelaide Hills, the principles translate almost word for word.
What Thermal Curtains Actually Do in a Greenhouse
A thermal curtain is essentially a movable blanket suspended inside the structure, usually just below the gutter, that closes off the crop area from the upper airspace of the house. That upper zone is where the heat goes to waste, because warm air rises and the roof glazing leaks it straight back out to the night sky. By drawing a barrier across, you reduce the volume of air you're trying to keep warm and trap a layer of still, insulating air between the screen and the roof.
The fabrics themselves range from woven polyester strips with aluminium strips woven in, which reflect long-wave radiation back toward the crop, to simpler non-woven films that simply slow convective loss. At WhyNot Farm we run an aluminium-stripped woven screen on our main hydroponic house and a lighter non-woven material on our propagation tunnel, where heat retention is less critical but humidity management matters more.
The physics isn't complicated, and neither is the result. Independent research from extension services in both hemispheres consistently shows a 30 to 50 percent reduction in overnight heat loss when a properly sealed screen is deployed. For a grower in Ballarat paying through the nose for LPG, that margin is the difference between a viable winter crop and a write-off. For us, it's the reason our cos and romaine stay sweet in January rather than bolting into bitterness.
How We Installed and Configured Our System
We chose a cable-driven, motorized system that rolls the fabric onto a tube above each bay, controlled by a small panel mounted at the entry door. The hardware itself was a familiar sight to anyone who has toured a glasshouse in the Netherlands, but the installation sequence we followed owes a lot to advice from mates running similar houses near Bundaberg and in the Yarra Valley, where winter humidity behaves a bit differently than ours.
The first decision was screen height. We hung ours roughly four metres off the floor, well above the canopy, so we still have working room underneath for scissor lifts and pollination tasks. That extra headspace also lets us vent humidity on mild nights without having to retract the screen fully, which is a trick worth knowing if you've ever dealt with botrytis in a propagation house after a drizzly week. The second decision was sealing the edges. Gaps along the gutter, around the roll-up sidewalls, and at the column braces will leak more heat than the entire roof if you let them, so we added brush strips and ran a perimeter check every fortnight during the cold months.
The third decision was timing the deployment with our biosecurity measures routine. Closing the house at dusk creates a closed environment that pests love as much as plants do, so we built a quick walk-through into the same checklist our team uses for rodent control and footbath rotation. It's a small operational gain on top of the energy savings, and it keeps the curtain pulling crew honest about doing the rounds.
Automating the Curtain Schedule Year Round
Manually pulling curtains gets old fast, and on a frosty July night nobody wants to be balancing on a ladder at 9pm. We wired our motor into the same environmental controller that runs our venting and irrigation, with two temperature setpoints: one for full closure and one for partial draw to bleed off excess humidity. The screen now closes itself whenever the outside temperature drops below 8°C, and reopens an hour after sunrise once the sun has had a chance to warm the canopy.
This kind of automation is old hat for the big production complexes around Virginia and the big hydroponic outfits around Werribee, but it's a real step up for a small mixed farm like ours. We've also added a wind override, because nothing ruins a curtain faster than a sudden gust finding an unsealed edge. If the anemometer reads above the threshold, the controller holds the screen in a vented position rather than slamming it closed, which protects the fabric and keeps the motor from fighting a gale.
The honest truth is that we still get it wrong sometimes. A warm front in February will catch the curtain closed at dawn and spike the temperature, so we've learned to check the forecast the night before and override the schedule when a northerly is pushing through. Treat the automation as a helpful offsider rather than a set-and-forget system, and you'll avoid the worst surprises.
What We've Measured Since the Upgrade
The numbers came in faster than we expected. In the first full winter after installation, overnight propane use across our main hydroponic house dropped by roughly 38 percent, even though we pushed the thermostat two degrees higher than the previous season. That combination, more heat for less fuel, is the whole game when you're running a winter lettuce program against tight margins.
We also tracked growth. Our butterhead finished three days earlier on average in the curtain house compared with a control bay we kept running on the old setup, and the heads came in heavier and more uniform. Wholesale orders bound for restaurants in Melbourne and Brisbane tolerate very little size variability, and the curtain gave us a tighter specification without changing anything else in our nutrient recipe. On the animal side of the operation, the warm house also meant our propagation schedule for early spring transplants started a week earlier, which lined up nicely with the first cut of the year for our pasture-raised herd.
There is one trade-off worth flagging: light transmission. The aluminium-stripped screen cuts about 18 percent of incoming PAR when fully deployed, which is fine overnight but matters on dull winter days when every photon counts. We mitigate this by retracting the screen early in the morning once ambient light climbs, and we accept a small penalty on the gloomiest June days because the heat retention benefit more than pays it back.
Comparing Curtains with Other Warming Approaches
Before we picked a curtain system, we ran the numbers against three alternatives that are popular across small-scale protected cropping in both the United States and Australia. The table below summarises what we found, based on a representative 200 square metre hydroponic lettuce house.
| Warming method | Approximate installed cost (AUD) | Overnight running cost | Heat retention | Labour intensity |
|---|---|---|---|---|
| Motorised thermal curtain with sealed perimeter | 18,000 to 28,000 | Low (40-60% reduction vs unheated) | High | Low once automated |
| Unit heater only (propane or diesel) | 3,000 to 6,000 | High in cold months | Moderate, uneven | Moderate, daily checks |
| Double-layer inflated polyethylene film | 6,000 to 10,000 | Moderate (15-25% reduction) | Moderate | Low, but reduces light permanently |
| Ground-level geothermal or root-zone heat only | 35,000 to 60,000+ | Very low once installed | Targeted, not whole-house | Low, but slow response |
The curtain slot looked best for our scale, climate and crop mix, and that conclusion will hold for most growers running leafy greens through the cool months. A grower in Hobart fighting persistent fog might prefer the inflated film for its humidity-buffering side effect, while a propagation nursery in the tropics might skip supplemental heating altogether. Matching the system to the crop and the climate is the whole point.
Practical Recommendations for Growers Considering a Screen
If you're weighing up a similar upgrade, here are the lessons we'd pass on after living with our system for three winters.
- Pick a fabric with an aluminium strip content if overnight heat retention is the main goal; go non-woven if humidity management matters more than raw R-value.
- Budget for proper edge sealing, because the screen itself is only half the performance story; brush strips, perimeter rails and a fortnightly inspection will pay for themselves within a season.
- Automate the schedule through your existing climate controller rather than relying on a separate timer, so venting, irrigation and curtain pulling all respond to the same weather inputs.
- Plan the integration with your biosecurity routine from day one, since a closed house at night changes how pests and pathogens behave and you'll want a single checklist that covers both.
- Treat the first winter as a calibration period: log fuel use, growth days and humidity spikes, and be prepared to override the schedule during unseasonable warm spells.
A good thermal curtain won't turn a marginal greenhouse into a tropical paradise, but it will tighten up your overnight heat budget in a way that shows up directly on the fuel bill and the harvest record. For a small farm supplying both families and chefs, that kind of quiet, repeatable gain is what keeps the lights on through the cold months. If you'd like to see the system in person, chuck us a line and we'll walk you through the bays on the next farm visit.