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If you’re at the point of deciding what kind of growing structure to buy, you’ve almost certainly encountered three main options: polycarbonate greenhouses, glass greenhouses, and polytunnels. Each has genuine advantages. Each has real trade-offs. And the right choice depends entirely on what you want to grow, how you want to use the space, and what matters most to you in a garden structure.
This guide gives you an honest, detailed comparison of all three — the performance data, the practical realities, and the questions worth asking yourself before you buy. We’ll tell you where glass genuinely wins, where polytunnels make compelling sense, and why polycarbonate has become the dominant choice for UK gardeners across the full range of growing ambitions.
Before comparing them, it helps to understand what each option is doing and why it exists.
A glass greenhouse is the traditional growing structure — a rigid frame covered in horticultural glass, typically 3mm or 4mm thick single panes. Glass has been used for greenhouses for centuries and represents the benchmark against which everything else is measured.
A polycarbonate greenhouse uses twin-wall polycarbonate panels in place of glass. The panels are multi-layered sheets with internal air channels that provide insulation. Modern greenhouse-grade polycarbonate — such as Brett Martin sheet with a co-extruded UV-stabilised layer — is manufactured to a specification that delivers a 10-year performance warranty. The structural frame in quality greenhouses is galvanised steel, providing strength and longevity to match the panel specification.
A polytunnel uses a steel hoop framework covered in a single layer of polyethylene film. It is essentially a temporary agricultural structure adapted for garden use. The film is thin, single-layered, and provides no meaningful thermal insulation beyond windbreak effect.
Understanding this distinction — a glazed, insulated rigid structure versus a film-covered hoop — is the starting point for an honest comparison.
Glass wins here, and it’s worth being honest about it.
Standard horticultural glass transmits approximately 90–95% of available light. A new twin-wall polycarbonate panel transmits around 80–85%. Polyethylene film transmits approximately 85–90% when new, but this figure degrades meaningfully over its lifespan as the film stretches, scratches, and accumulates surface deposits.
The polycarbonate figure sounds like a disadvantage until you consider what happens to the light that does pass through. Glass transmits light as a direct beam — focused, intense, and capable of scorching tender leaves in summer and creating sharp temperature differentials within the growing space. Polycarbonate scatters light as it passes through the twin-wall structure, distributing it more evenly throughout the greenhouse interior and reaching lower leaves and shaded corners that direct sunlight would never touch.
For most commonly grown crops — tomatoes, cucumbers, peppers, aubergines, herbs, salad leaves, brassicas, flowers — the 5–10% difference in raw light transmission has no meaningful effect on yield or plant health. Research consistently shows that diffused light can actually improve photosynthesis efficiency in dense plantings because more of the plant canopy is receiving usable light simultaneously.
The light transmission gap only becomes significant in specific situations: very shaded garden positions, specialist crops with exceptionally high light requirements, or commercial growing where every percentage point of photosynthetic efficiency has economic consequences.
Verdict: Glass has the theoretical advantage; polycarbonate delivers more even light distribution in practice. For the vast majority of gardeners, this is a non-issue.
Polycarbonate wins, and it’s not particularly close.
Single-pane glass is one of the worst insulators commonly used in construction. Its R-value — a measure of resistance to heat flow — is approximately 0.95, meaning heat passes through it very easily in both directions. A glass greenhouse warms quickly in sunshine and loses that warmth equally quickly after sunset.
Twin-wall polycarbonate, by contrast, achieves this through its internal air channel structure. The trapped air between the two walls of a 4mm panel delivers an R-value of approximately 1.43; a 6mm panel achieves around 1.54. These are not dramatic numbers in absolute terms, but they represent a 50–60% improvement over glass for heat retention — and in the context of a UK climate where spring and autumn nights regularly dip below 5°C, that difference is the gap between a plant surviving the night and not.
The practical result is measurable. A polycarbonate greenhouse typically maintains overnight temperatures 2–5°C above a comparable glass structure. On a cold March night when outdoor temperature falls to 2°C, the inside of a well-sealed glass greenhouse might reach 5°C — marginal for many plants. The same night inside a polycarbonate greenhouse, temperatures could hold at 7–9°C — genuinely safe for most seedlings and tender crops.
A polyethylene polytunnel provides almost no thermal insulation at all. The single-layer film has an R-value close to zero. Wind penetration at seams and ground level is significant. Polytunnels can be 5–8°C colder overnight than a polycarbonate greenhouse of equivalent size. This is the single most important performance gap between the two structure types, and for any gardener wanting to grow through the cooler months, it is decisive.
Verdict: Polycarbonate is significantly better than glass, and dramatically better than a polytunnel, for heat retention.
Polycarbonate and glass take different approaches to durability; polytunnels are not in the same league.
Glass is hard and optically permanent — its clarity does not degrade, it cannot be scratched by rain or dust, and an unbroken pane retains its light transmission indefinitely. A glass greenhouse properly maintained can last several decades. The vulnerability of glass is mechanical: it shatters. A wayward football, a falling branch, a particularly large hailstone, or even a misplaced elbow during planting can result in a broken pane — a safety hazard, a cold gap in the greenhouse skin, and an unplanned expense.
Quality polycarbonate — specifically Brett Martin twin-wall sheet with a co-extruded UV layer — is a different kind of durable. It is not optically permanent in the way glass is: panels will show some gradual reduction in light transmission over their service life, and the 10-year warranty marks the point at which the panels are designed to be performing to specification. Beyond that, panels can be replaced individually and inexpensively. What polycarbonate is, however, is exceptionally impact-resistant — rated at 200 times the impact resistance of glass. Hailstorms that would shatter glass panes leave polycarbonate panels unmarked. Accidental contact that would crack a glass pane will not damage a polycarbonate one.
The frame durability question is separate from the glazing. A CE-certified galvanised steel frame with 275gr/m² zinc coating is built for decades of outdoor service. The zinc coating provides sacrificial protection — it degrades preferentially over the underlying steel, meaning that even minor surface abrasion does not expose the frame to rust. This is a fundamentally more durable frame specification than the thin steel profiles used in budget imported greenhouses.
Polyethylene film, meanwhile, has a service life of 4–8 years under normal conditions, after which it must be replaced entirely. UV degradation and mechanical stress from wind cause the film to thin, tear, and eventually fail. Replacement film is an ongoing maintenance cost that buyers should factor into the total cost of ownership comparison.
Verdict: Glass offers indefinite glazing longevity but is fragile; polycarbonate offers a 10-year warranted performance period with exceptional impact resistance; polytunnel film needs replacement every few years.
Polycarbonate is the clear winner for family gardens.
Broken glass in a garden setting creates genuine hazards — sharp fragments in soil and on paths, risk of injury to adults and serious risk to children and pets. Horticultural glass is not toughened, and when it breaks, it breaks in the traditional dangerous way.
Polycarbonate does not shatter. When subjected to impact that would break glass, polycarbonate bends and deforms but retains its integrity. It will not produce sharp fragments, will not create sudden structural failures, and will not leave debris in growing beds. For families with children, for households with pets that access the garden freely, and for any situation where a greenhouse is close to a play area or high-traffic path, this is not a minor advantage — it is a significant one.
Polytunnels have no sharp glazing risk, but the film degrades and tears in ways that can create hazards of a different kind — particularly for wildlife that becomes entangled in loose or failing film.
Verdict: Polycarbonate is the safest glazing material for a domestic garden environment.
Polycarbonate greenhouses are considerably easier to assemble and manage than glass.
Glass greenhouse panels are heavy, fragile, and require careful handling throughout assembly. Working with glass requires two people for almost every stage. Replacement of broken panes requires sourcing correctly sized horticultural glass — increasingly difficult as older greenhouse models fall out of production — and handling it safely.
Polycarbonate panels are lightweight — a 4mm twin-wall sheet weighs less than 1kg per square metre, compared to approximately 7.5kg per square metre for 3mm glass. This makes panels genuinely manageable during assembly, reduces the risk of damage, and means a single person can handle most panels with care. A standard-sized KLASIKA greenhouse can typically be assembled in a day with one helper, using standard tools.
Polytunnels are straightforward to assemble but require re-tensioning of the film periodically, and recovering the frame with new film when the time comes is a physical and time-consuming task — typically requiring two or more people and a full day’s work.
Verdict: Polycarbonate greenhouses offer the most practical assembly experience; polytunnel re-covering is the most demanding maintenance task.
This is where the comparison becomes most nuanced, and where honest accounting matters.
Surface-level price comparisons between greenhouse types can be deeply misleading unless they account for the full cost of ownership over time.
A polyethylene polytunnel appears to offer the most space per pound at the point of purchase — you can cover a large growing area relatively cheaply. But this calculation changes substantially when you account for film replacement every 4–8 years, the ongoing cost of accessories needed to compensate for the lack of insulation, and the ultimate replacement of a structure that has a finite mechanical lifespan. A polytunnel that costs £400 and needs a £120 film replacement at year 5 and another at year 10 has cost £640 over a decade — and that excludes the labour and effort of re-covering.
A low-cost imported polycarbonate greenhouse — the kind manufactured to minimal specifications with unbranded polycarbonate panels and lightweight steel — is typically priced to attract buyers on headline cost. In reality, panels that are not co-extruded with a proper UV layer will yellow and degrade significantly within 3–5 years. The greenhouse does not fail dramatically — it simply becomes progressively less effective, less attractive, and eventually not worth repairing. A buyer who has spent £300–£400 on a budget greenhouse and replaces it after 5 years has spent more over a decade than someone who bought a quality greenhouse once.
A CE-certified galvanised steel greenhouse with Brett Martin polycarbonate panels sits at a higher initial price point than these options — deliberately so, because it is designed and warranted to perform for a full decade without the degradation issues that affect cheaper alternatives. The steel frame is built to outlast the panels significantly. When panels eventually reach the end of their warranted service life, they can be replaced individually at moderate cost, returning the greenhouse to full performance without replacing the entire structure.
Glass greenhouses at a similar quality level are typically more expensive than polycarbonate equivalents — primarily because glass is heavier (requiring a stronger, more costly frame), more expensive to manufacture in greenhouse-appropriate form, and more costly to install safely. Premium glass greenhouses with hardwood timber frames represent the top of the market in terms of aesthetics and longevity, but at price points that are typically two to three times higher than a quality polycarbonate alternative.
The honest cost summary across ten years:
| Structure | Typical initial cost | Likely 10-year additional costs | Ten-year total |
|---|---|---|---|
| Budget imported polycarbonate | Low | Full replacement at ~year 5 | High total |
| Polytunnel | Low–medium | Film replacement × 1–2 | Medium total |
| Quality polycarbonate (Brett Martin / steel frame) | Medium | Panel refresh at year 10+ | Medium total — best value |
| Premium glass greenhouse | High | Minimal if unbroken | High total |
Verdict: Quality polycarbonate greenhouse offers the best long-term value for most buyers. Cheap imports cost more over time than they appear; glass costs more upfront; polytunnels have ongoing film costs.
Glass greenhouses are widely considered more attractive; polycarbonate offers a similar aesthetic at lower cost.
This is subjective, but worth acknowledging. A traditional glass greenhouse — particularly a Victorian or apex-ridge design — has an elegant, horticultural character that many gardeners value highly. The clarity of glass and the precision of slim glazing bars create a visual appeal that polycarbonate, however good its functional performance, does not quite replicate.
Modern polycarbonate greenhouses have closed much of this gap. The range of styles available — from the elegant house-shaped KLASIKA House through the distinctive teardrop profile of the STANDART KLASIKA to the spacious working greenhouse of the KLASIKA Arched — offer genuine visual variety. In a working garden, a well-proportioned polycarbonate greenhouse looks purposeful and appropriate.
Polytunnels are functional. They make no claims to aesthetic appeal, and most gardeners accept this as part of the trade-off for the practical benefits they offer.
Verdict: Glass edges it for pure aesthetics; polycarbonate offers good aesthetics at significantly lower cost; polytunnels are purely functional.
Rather than declaring a single winner — which would be dishonest, because each option genuinely suits different situations — here is a straightforward guide based on what matters most to you.
Choose a polycarbonate greenhouse with a galvanised steel frame if:
Choose a glass greenhouse if:
Choose a polytunnel if:
Do not choose a budget imported polycarbonate greenhouse if:
One of the most important things to understand when comparing polycarbonate options is that the category covers a very wide range of quality — and headline price alone is not a reliable guide to performance.
The defining quality indicator in polycarbonate panels is the UV protection layer. Quality greenhouse polycarbonate, like Brett Martin sheet, uses a co-extruded UV-stabilised layer — a UV-absorbing compound bonded into the material during the manufacturing extrusion process, making it a permanent part of the panel structure. This cannot be worn away by rain, damaged by cleaning, or degraded by surface abrasion. It is what makes a 10-year UV warranty a credible and meaningful commitment.
Budget polycarbonate — used in the vast majority of low-cost imported greenhouse kits — either omits UV protection entirely or applies a thin surface coating that begins to fail within a few years. As the UV protection degrades, the polycarbonate itself begins to yellow and become brittle. Light transmission drops. Panels that looked clear in the first year take on a cloudy yellow cast within three or four, and the insulation properties are not affected by yellowing alone — but a greenhouse that is visually degraded and transmitting significantly less light is not performing the function it was purchased for.
The Brett Martin polycarbonate used in KLASIKA and BALTIC LT greenhouses carries a manufacturer’s 10-year UV warranty. This is not a retailer’s claim — it is a commitment from the panel manufacturer, one of Europe’s most established polycarbonate producers, that the panels will maintain their performance to specification for a full decade of outdoor use. It is the single most reliable indicator that the polycarbonate you are buying is genuinely quality sheet, not budget film dressed up as a premium product.
Polycarbonate greenhouses are not the right choice for every situation, and this guide has tried to be fair to the alternatives. Glass offers longevity and clarity that polycarbonate does not. Polytunnels offer volume and initial affordability that polycarbonate cannot match at the same price point.
But for the majority of UK gardeners — those who want to grow tomatoes and cucumbers reliably, extend their season meaningfully in both directions, keep tender plants alive through winter with minimal heating, and have a structure that works hard and lasts — a quality polycarbonate greenhouse with a CE-certified galvanised steel frame and Brett Martin panels represents the best overall answer. Not because it is the cheapest option. Because over ten years it is the most valuable one.
The question is not whether to choose polycarbonate over glass or a polytunnel. It is whether to choose quality polycarbonate — the kind that will still be performing as intended in year eight — or to spend a little less now and replace the whole structure in year four.
Which greenhouse is right for me? A practical guide to choosing the right model
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