Greenhouse Covering Materials: Polyethylene vs Polycarbonate vs Glass — What Farmers Actually Need to Know

When I visit farms and see a greenhouse that’s been up for three years with yellowed, tearing plastic and plants that look like they’re fighting for their lives, I know exactly what happened. Someone made a decision based on upfront cost and nothing else.

The covering on your greenhouse isn’t just a roof. It’s the thing that controls how much light your plants get, how much heat stays in at night, and how much you spend on heating and cooling throughout the year. Get it wrong and you’re fighting your own structure for the next decade.

I’ve seen growers spend $8,000 on a greenhouse frame and then put the cheapest polyethylene film they could find on top, only to replace it in two years and lose a winter crop to cold damage in between. The math on covering materials isn’t complicated, but most people don’t run it before they build.

The Three Options

Most commercial greenhouse coverings fall into three buckets: polyethylene film, rigid polycarbonate panels, and glass. There are niche options like acrylic and fiberglass, but for 95% of vegetable and flower growers, the decision is between these three.

Polyethylene film is the workhorse. It’s what you see on tunnel houses and most commercial vegetable greenhouses. Typically 6 mil (0.15mm) thick, sometimes double-layered with an air gap for insulation. UV-treated versions last 3-4 years. Cost runs $0.08 to $0.25 per square foot depending on quality and UV additives.

I’ve used the cheap stuff and the good stuff, and the difference in lifespan is stark. Standard 6-mil PE with no UV treatment can start degrading in 18 months in high-sun environments. The UV-stabilized versions from manufacturers like Ginegar or Plastika Kritis consistently hit the 4-year mark before you need to think about replacement. That’s a real difference, not marketing.

Polycarbonate panels come in twin-wall or multi-wall configurations, typically 8mm to 16mm thick. They look like translucent corrugated cardboard, with air channels running through them that trap heat. Cost runs $1.50 to $3.50 per square foot installed. Lifespan is 10-15 years, sometimes longer if the UV coating on the exterior face holds up.

The thing about polycarbonate that catches people off guard is the thermal performance. A twin-wall 8mm panel has an R-value around 1.6, compared to about 0.85 for a single layer of PE and 0.95 for glass. That means your heating bill drops by 30-40% compared to glass in a cold climate. I’ve talked to growers in Colorado and Ontario who switched from glass to polycarbonate and saw their propane costs drop by a third in the first winter. That’s real money.

Glass is the traditional option and still common in older commercial greenhouses, especially in the Netherlands and northern Europe. Tempered or horticultural glass runs $2.50 to $6 per square foot installed. Lifespan is effectively permanent if you don’t break a pane. Light transmission is the best of the three, at around 90% for clean glass versus 80-85% for polycarbonate and 85-90% for new PE film.

But glass is heavy. You need a stronger frame, which adds cost before you even touch the glass itself. And in hail-prone areas, one bad storm can turn your greenhouse into a repair project.

What Matters and What Doesn’t

Light transmission gets too much attention. Yes, glass lets through the most light. But for most vegetable crops: tomatoes, peppers, cucumbers, lettuce. The 80-85% you get from polycarbonate or fresh PE is plenty. Light saturation for most C3 crops happens around 600-800 μmol/m²/s, and even on a cloudy day a polycarbonate greenhouse delivers that. The grower losing yield to light limitation is growing in January in Norway, not March in Texas.

What matters more is heat retention. If you’re heating your greenhouse through a winter, every R-value point saves you money every single night. Double-layer PE with an inflation fan gets you to about R-1.5. Twin-wall polycarbonate gets you R-1.6 to R-1.8. Glass sits at R-0.95. That gap between 0.95 and 1.6 is a 40% difference in conductive heat loss. Over a 5-month heating season in Zone 5, that’s thousands of dollars.

Diffusion matters too, and nobody talks about it. Direct light creates shadows inside the greenhouse. Leaves shade other leaves, and the lower canopy gets nothing. Diffused light scatters in all directions and penetrates deeper into the plant canopy. Polycarbonate and some PE films have excellent diffusion properties. Glass, unless it’s frosted or treated, gives you harsh direct light with sharp shadows. Dutch growers have known this for decades, which is why so many of them use diffused glass or polycarbonate despite the slight transmission loss.

Condensation behavior is another thing you don’t think about until you’re standing in a dripping greenhouse. PE film forms large droplets that fall on plants, spreading disease. Polycarbonate with an anti-condensate coating keeps the water in a thin film that runs down the walls instead of dripping. Glass is the worst of the three: cold surface, instant condensation, constant dripping in humid conditions.

The Cost Picture Over 10 Years

Here’s the math that most greenhouse suppliers won’t show you up front.

A 30-foot by 96-foot greenhouse (2,880 square feet of roof and wall area):

– PE film (double-layer, UV-treated): $1,500-2,000 for materials, $500 for installation. Replace every 4 years. Over 10 years: roughly $5,000-6,250 total. – Twin-wall polycarbonate (8mm): $6,000-8,000 materials + installation. No replacement needed in 10 years. Total: $6,000-8,000. – Glass (tempered horticultural): $10,000-15,000 materials + installation (including stronger frame upgrade). Plus contingencies for breakage. Over 10 years: $12,000-18,000.

At first glance, PE looks cheapest. It is, in upfront cost. But by year 8, you’ve paid for polycarbonate twice over and you’re about to do it again. And that doesn’t account for the heating savings from polycarbonate’s higher R-value, which might net you $400-800 per winter in a cold climate.

For a small farm growing salad greens or nursery stock, I’d say polycarbonate is the sweet spot. You pay more on day one but you stop paying after that. For a large commercial tomato operation where you’re replacing covers on a schedule anyway and running supplemental lighting, PE film might make more sense. You’re not relying on the cover for insulation as much, and the lower upfront cost matters across 50 bays.

Glass, honestly, makes sense in very few new builds today unless you’re restoring a heritage greenhouse or operating in a climate where heating costs are negligible. The weight alone adds 20-30% to your frame cost.

What I’d Recommend

If you’re building a new greenhouse under 5,000 square feet and you’re heating it: twin-wall polycarbonate, 8mm minimum. The numbers work out.

If you’re putting up tunnels for seasonal vegetable production and you’re not heating them: UV-treated PE film, double-layer with an inflation fan. Don’t buy the cheapest film. The $0.08/sq ft stuff is a false economy.

If someone’s giving you glass panels for free and you have a frame that can handle the weight: sure, take them. But don’t build a new glass greenhouse in 2026 unless you have a specific reason.

The covering is the one component of your greenhouse that touches every other decision: heating, cooling, irrigation scheduling, crop selection. It’s worth getting right.