Greenhouse Growing for Off-Grid Properties

By Mick O'Brien · carpenter & builder · sheds-to-homes, cabins and things that stay up in a storm

“After my third greenhouse blew away in a 90km gale, I learned that 6mm polycarbonate needs at least 100mm of concrete footing to stay up.”

Introduction

I run an off-grid place and grow most of my own tucker. Nothing else beats a greenhouse for that. Australia's growing season's tight, frost rolls in overnight with no warning, and one hot day in November will wipe out a tray of seedlings before you've had your morning cuppa. A decent greenhouse sorts all three problems at once. Holds warmth through frost, gets tomatoes and capsicums in the ground early, and builds enough heat for cucumbers and the exotics — the stuff that just won't set fruit in an open bed across most of the country.

Zero Watt Greenhouse Production

Never had the greenhouse on the mains. Passive solar design does the heating, a couple of hand-operated roof vents handle the cooling, and the right siting does the rest. Keeps producing every month of the year without a single watt of input.

I run the greenhouse with the ridge east-west and the glazing facing north so winter sun hits the back wall where black-painted water drums sit on the packed earth floor. I open the ridge vents with a cord each morning through summer and close them before dark in winter; the only moving parts are the vent flaps and the door. Seedlings stay on the earth floor rather than on benches because the floor temperature swings less than the air at bench height. Shadecloth goes over the outside of the glazing once afternoon temperatures climb, and comes off when the sun drops lower in autumn.

Key Considerations

Glass, Polythene Tunnels and the Polycarbonate That Shattered

Each covering material has distinct advantages and trade-offs for Australian conditions.

Glass: Traditional glass greenhouses offer excellent light transmission and a long lifespan (20-30+ years). They're more expensive upfront, require a substantial frame and foundation, and can shatter. Toughened or horticultural glass is recommended for safety. Glass retains heat well but radiates it quickly on cold nights, requiring more thermal mass to maintain stable temperatures.

Affordable Polythene For Polytunnels

Polyethylene (polythene film): The most affordable option, typically used in polytunnel structures. Film lasts 3-6 years before UV degradation requires replacement. Polythene diffuses light well, reducing hot spots, and provides good insulation when used as a double layer with an air gap. Polytunnels are well suited to Australian conditions and can handle strong winds if well anchored. Cost per square metre is significantly lower than glass or polycarbonate.

Polycarbonate Benefits And Drawbacks

Polycarbonate: Twin-wall or multi-wall polycarbonate sheets offer excellent insulation (far better than single glass or film), good light transmission, and high impact resistance. They won't shatter like glass and last 10-15 years. The main drawbacks are initial cost (higher than film, lower than glass for comparable structures) and some light diffusion that reduces direct sunlight intensity — generally a benefit in Australian summers. Compact polycarbonate sheets are a solid mid-range choice.

Sizing the Frame to Your Block

Big doesn't mean better – not when the budget's tight and you're managing heat without a fan running. I reckon 6m x 3m, that's 18 square metres, hits the sweet spot for a family veggie patch. Enough room for seedlings, salad greens, a few climbing plants up the back wall. Go 8m x 4m if you need the space to rotate crops or handle bigger plants.

I set the eaves at 2.5 m and lift the ridge higher than that. Hot air needs a clear path out — otherwise it sits on the seedlings and boils the poor bugger trying to fix a hinge at noon. A tall frame breathes. A squat one turns into a sauna by 10 am.

Passive Solar Design

Thermal mass is how I stop the greenhouse turning into an oven by lunch and an icebox by dawn. I pour a solid concrete slab under the growing beds and stack concrete blocks tight against the north wall. They soak up the day's sun and cough it back out at night. The plants notice the difference.

Thermal mass options: Stone or concrete floors, raised beds filled with dark-coloured stone or water containers (drums, bottles), and dark-coloured water tanks positioned inside the structure. A concrete slab floor with a heat capacity of 2-3 tonnes will absorb significant solar energy on a sunny winter day and radiate it back through the night. Painted black water drums are highly effective and can double as heated propagation benches.

Greenhouse Orientation For Southern Australia

Orientation: In the southern hemisphere, a greenhouse should face north (true north, not magnetic) for maximum winter sun exposure. The long axis should run east to west. In tropical or subtropical northern Australia, east-west orientation and shade cloth become more important to reduce summer heat load.

Insulation: At night or during cold snaps, a layer of horticultural fleece draped over plants provides an extra 2-4 degrees of frost protection. Insulating the north wall with thermal mass or reflective insulation reduces heat loss.

Letting the heat out

Ventilation matters more than any other part of a healthy greenhouse, and it doesn't need power to work properly. Hot air rises, so I put ridge vents or roof vents at the highest point — that's where the heat wants to go. Down low, louvre vents or a door on each side pull the cooler air through. Stick them on opposite walls and you get cross-flow. I fit wax-piston openers to the roof vents on most builds. No wiring, no solar, no batteries. They crack the vent open when the temperature climbs past about 18‑22°C and shut it again once it cools.

Roof vents should add up to 15-20% of your floor area, and you need side vents too or the hot air just sits up there. Out past the ranges, I'd push that higher — every shed I've built west of the divide needed more opening than the plans called for, or the tomatoes cook themselves by Christmas.

What a Mid‑Summer Heatwave Did to My Shade Cloth

Summer in a polycarbonate box will cook your seedlings if you don't shade it. I staple 40–50% shade cloth over the frame — inside's fine too, depends on the build. That cuts peak temps by 5–10 °C and the veg still get enough light to do their thing. Up north in the subtropics and tropics, I switch to 70% shade for the worst stretch of the year.

I run shade cloth inside as a pull‑down screen, or drape it over the frame if I'm in a rush. White or aluminet — nothing else drops the heat properly. It goes up before summer hits and rolls back in autumn to grab every bit of winter sun.

Hail Protection

Hail is a proper risk across plenty of parts of Australia and can wreck crops and tear through greenhouse coverings in minutes. Polycarbonate takes a bit more of a beating than glass or film, but don't get me started on Bunnings' thin poly vs a local glass supplier's toughened options. I've seen a Bunnings polycarbonate polytunnel collapse in a Queensland storm because the cheap stuff couldn't handle the load, while a solid local glass job stayed standing. If you're sticking with glass, swap in acrylic or toughened panels for those areas where the sky turns nasty.

A heavy-duty shade cloth, 70% grade or better, acts as a shield; I've seen many off-grid growers nail up hail netting over their builds as a permanent fixture.

Getting Started

Choosing a Greenhouse

Before you spend a cent, work out your climate zone, your budget, and what you'll actually eat. My first off-grid greenhouse was a 6m x 3m polytunnel and it fed us for years. The 8m x 4m range is the sweet spot for most people starting out. Kits are available from plenty of Australian suppliers, from cheap film tunnels that'll do the job to the fancy polycarbonate jobs if you've got the coin.

I order a kit when the frame needs to go up this week and I want the invoice predictable. Salvage windows and offcuts piling up? I'll frame a custom job, spend less, and end up with a greenhouse that looks like it belongs to the property, not dropped out of a catalogue. Costs you a weekend, though. Both work. Structure's standing, job's done.

Site Selection

Face the bloody thing north on well-drained ground you can reach from the tap without a trek. Put a windbreak on the south or west so a gale doesn't rip it apart. I watched a polytunnel in Queensland fold in a storm because it sat in a frost pocket. Don't build in a valley where cold air settles; a slight slope keeps it warmer. Wind at your back, heat where you need it.

Foundation

A solid foundation stops the settling, keeps the pests out, and gives you a proper anchor when the wind hits. Concrete slabs are the toughest option but they need excavation and cement, which ain't always practical if you're off-grid. I've used pressure-treated timber bearers with star pickets and rail systems, and I've built brick and concrete block perimeter walls – both do the job if you do them right. The critical bit is getting the structure level, anchored down tight, and making the base pest-resistant. Burying chicken wire around the perimeter stops the digging animals in their tracks.

Assembly

I've rebuilt enough twisted kit greenhouses to know the look on someone's face when they realise their weekend project has become a month-long headache. The manufacturers say two people, two days. I reckon that's optimistic for a first-timer, spot on for a second build. Start with the base frame—get it square, get it level, get it bolted down to something that won't move. Then glazing goes on from the top down. Sounds backwards, but water finds any seam you've left pointing the wrong way.

Vents next, then the watering line, and only then do I bring in thermal mass—barrels, bricks, whatever's lying about. I won't put a plant in until the shell is sealed. Rush the glazing and you'll spend the next year with a torch and a garden hose, chasing drips you can hear but never quite see.

What the Bunnings Polycarbonate Cost Me

Poly tunnel kits: Budget $500-$1,500 for a 6m x 3m polytunnel kit. Materials quality and frame strength vary — in windy areas, invest in a heavier-gauge frame.

Polycarbonate greenhouse kits: 6m x 3m kits typically range from $1,500-$3,500. Premium brands with thicker polycarbonate and robust frames cost more but last longer.

Glass greenhouse kits: Traditional glass houses start from $2,500-$5,000 for comparable sizes, with custom builds running higher.

Site preparation: Foundation and site works add $200-$1,000 depending on site conditions and chosen method.

Accessories: Shade cloth ($100-$300), thermometers and hygrometers ($30-$80), watering systems ($100-$400), and thermal mass containers ($50-$200) add to the setup cost.

Council approval: In most Australian jurisdictions, a greenhouse under 10 square metres and below 2.4 metres in height doesn't require development approval. However, bushfire-prone areas, heritage overlays, and some rural zones may have specific requirements. Check with your local council before building, particularly for permanent structures on rural land.

Recommended Products

• Greenhouse kits: Sizes from 6m x 3m polytunnels to 8m x 4m polycarbonate houses — available from Australian rural suppliers and online
• Shade cloth: 40-50% white or aluminet shade cloth in various sizes, with attachment clips and ropes
• Thermometers and hygrometers: Digital min/max thermometers with remote sensors for monitoring greenhouse and outdoor temperatures simultaneously
• Watering systems: Drip irrigation kits, automatic watering timers (battery-powered for off-grid), and micro-sprinkler systems for greenhouses
• Thermal mass: Black painted water drums (200L), concrete slabs, and dark stone or gravel for raised beds
• Automatic vent openers: Wax-piston type vent openers for ridge vents and louvres — no electricity required
• Hail netting: Heavy-duty woven hail protection mesh in various sizes
• Horticultural fleece: Lightweight frost protection fabric for covering plants on the coldest nights

Shop on Amazon: View on Amazon

Conclusion

My greenhouse paid for itself in two seasons on the block. It stretched the growing window, saved seedlings from late frost, and let me raise tomatoes and capsicums that sulk in open beds across most of Australia. No power to it. I built thermal mass into the floor, fitted vents I crack by hand, and throw shade cloth over the roof when the sun gets mean. Holds steady year-round without burning a single watt.

Size it to what you can actually manage, not what you wish for. Sort the ventilation from day one—retrofitting it down the track costs more and never works as well. Thermal mass comes next, added when the budget allows. A well-sited polytunnel or polycarbonate greenhouse, a few black water drums, and shade cloth will transform your food-growing capacity from the first season. Nail the foundation and the rest is tinkering.