Greenhouse polycarbonate | Thickness, multiwall sheets & insulation
South Africa • Greenhouse materials guide • Bloomcabin Journal
Greenhouse polycarbonate: the complete guide
Thickness, twin-wall versus multiwall sheets, insulation, UV protection, condensation and the details that determine real-world performance.
Start with the right question
“How thick should polycarbonate be for a greenhouse?” is useful, but it is not the entire decision.
Polycarbonate is used for greenhouse glazing because it combines low weight with high impact resistance and useful thermal insulation. Multiwall sheets contain air chambers; the trapped air can reduce heat flow compared with a single thin skin, while internal ribs add stiffness.
Two sheets with the same nominal thickness may therefore perform differently. A 10-mm twin-wall sheet, a 10-mm triple-wall sheet and a more complex multiwall sheet can have different air chambers, U-values, light transmission, weights and permitted spans.
This guide therefore reads a sheet as a complete system rather than following a thickness figure. If you want to combine a quality aluminium greenhouse with polycarbonate, the Bloomcabin Classic aluminium greenhouse is the key commercial starting point in South Africa.
The best choice starts with climate, growing period, light needs, heating plans and frame design. Across South Africa, seasonal light, frost risk, summer heat and weather loads vary greatly by location.
01 • The material
What is greenhouse polycarbonate?
Polycarbonate is a thermoplastic available as solid, corrugated, twin-wall and multiwall sheet. Multiwall sheets are especially relevant for greenhouses: internal ribs join the outer skins and create enclosed air chambers. The result is a lightweight component that can be stiffer than a very thin solid sheet and offer better thermal properties.
Suitable outdoor sheets are made with UV-stabilised or co-extruded protective layers. Depending on the product, they may also provide light diffusion, varying transparency or anti-condensation surfaces. “Polycarbonate” therefore names the base material, not the full technical performance of a greenhouse sheet.
02 • Thickness
4 mm vs. 6 mm vs. 8 mm vs. 10 mm vs. 16 mm polycarbonate
Sheet thickness is useful because greater thickness can allow deeper air chambers and more complex internal geometry. It is not, however, a universal insulation rating. These ranges are prompts for the questions to ask, not a claim that every sheet of one thickness performs identically.
| Nominal thickness | Typical planning question | Possible benefit | What to check |
|---|---|---|---|
| 4 mm | Are minimum weight and a slim sheet the priority? | Light, simple construction | Thermal rating, permitted span, impact resistance and frame specification |
| 6 mm | Is a lightweight twin-wall sheet wanted for seasonal use? | A good entry point for air-chamber insulation at low weight | Exact wall structure, U-value, support spacing and UV side |
| 8 mm | Is extra chamber depth or a triple-wall structure needed? | Potentially better insulation in a compact sheet | Twin-wall versus triple-wall, light transmission and weight |
| 10 mm | Is the season being extended and heat retention becoming more important? | More insulation and stiffness potential | Light transmission, cell geometry, profile compatibility and thermal expansion |
| 16 mm+ | Is stronger thermal performance a central project goal? | Allows more demanding multiwall structures | Possible light loss, frame compatibility, cost and structural load assumptions |
03 • Internal geometry
Twin-wall vs. triple-wall vs. multiwall
Twin-wall sheets have two outer skins connected by internal ribs. This creates essentially one layer of long air chambers. These sheets are light, relatively simple and common in thinner greenhouse configurations.
Triple-wall sheets add another internal skin and create further separated air layers. Depending on the exact geometry, this can improve thermal insulation compared with a similarly thick twin-wall sheet.
Multiwall is the umbrella term for more complex cross-sections. They may include extra horizontal skins, diagonal ribs, X-structures or combinations. That is why “10 mm polycarbonate” is not enough for a professional choice: read the cross-section and technical values too.
04 • Thermal insulation
How polycarbonate insulates a greenhouse
Multiwall polycarbonate insulates mainly through its enclosed air chambers. Relatively still air transfers heat more slowly than a solid, highly conductive route. Cell geometry, number of skins, material thickness and conditions can therefore create markedly different performance at the same nominal thickness.
Universities and agricultural extension services recommend comparing greenhouse covers by their actual heat transfer rather than their material name alone. A lower U-value generally means less heat flow; a higher R-value means greater thermal resistance. The value for the specific sheet is what matters for planning.
If winter growing or heating cost matters, request the U-/R-value for the sheet that will actually be supplied. In cold or snow-prone areas, thermal performance is especially relevant, but it cannot be considered separately from local wind and snow loads or the frame designed for them. For a greenhouse used mainly from spring to autumn, the balance of light, price, ventilation and simple construction may matter more than maximum insulation.
05 • Light
More insulation is not automatically better when too much light is lost
Every extra skin, rib, tint or solar-control layer changes the way light enters. Across South Africa, seasonal light, frost risk, summer heat and weather loads vary considerably by location. In lower-light periods, plant-available radiation can become a greater limitation than a small final gain in U-value.
For plants, ask for visible light transmission or PAR data and consider diffusion too. A well-diffusing sheet can spread radiation more evenly through the crop and reduce harsh local peaks. Strongly tinted or solar-reflective sheets may reduce summer heat, but can also sacrifice useful plant light.
06 • UV protection
Which side of a polycarbonate sheet must face the sun?
Polycarbonate intended for permanent outdoor use is usually made with a UV-stabilising protective layer. It protects the sheet itself against long-term ultraviolet ageing. Some products have protection on one side only, others on both, so this must be checked before installation.
For one-sided UV protection, the marked protected side must face outward towards the sun. Installing the sheet the wrong way around can compromise the weathering resistance intended by the manufacturer. Remove protective films only after the installation direction is certain.
Do not assume both sides are the same. Check the marking and installation instructions for the specific sheet.
07 • Condensation
Condensation on a sheet differs from moisture inside its chambers
In a humid greenhouse, condensation forms when warm, moisture-rich air meets a colder surface. Special horticultural sheets may have an anti-condensation surface that lets water run away as a thin film instead of forming large drops. This can reduce dripping and optical losses from strongly reflective droplets.
Moisture inside the hollow chambers is a different issue. Channels, edge closures and profiles must be installed so water is not unnecessarily trapped and contamination remains limited. Polycarbonate also moves significantly with temperature changes, so edge clearances, profile engagement, drilling and drainage must be planned as one system.
08 • Lifespan
How long does greenhouse polycarbonate last?
There is no single lifespan for all polycarbonate. University greenhouse guides often indicate around ten to fifteen years for suitable polycarbonate covers, while specific warranties depend greatly on product, UV protection and warranty wording. Actual ageing is also affected by sun exposure, cleaning, chemical contact, installation and the mechanical condition of the frame.
A sheet can remain structurally intact while declining visually. Yellowing, surface haze, scratches, contamination in chambers or damage to fixings and edges can matter to users long before mechanical failure.
For a quality greenhouse, read not only the warranty period but also what is guaranteed: impact resistance, light transmission, yellowing or another performance measure may be treated differently.
09 • Impact resistance & construction
High impact resistance does not replace correct greenhouse engineering
Polycarbonate is valued for its high impact resistance, and multiwall sheets are often used where break resistance matters. Yet the sheet is only part of the overall system. Wind, snow and impact loads are carried through glazing profiles, rafters, screws, anchors and foundations.
A thicker sheet cannot compensate for a weak frame, overly large unsupported spans or poor fixings. Use the glazing approved for the greenhouse system instead of fitting an arbitrarily thicker sheet. Profile geometry, engagement depth and expansion gaps may be designed for a particular sheet thickness.
Where severe weather is an important buying criterion, compare the Bloomcabin greenhouse models for South Africa and consider the wind and snow loads stated for your project.
10 • Installation
Installation details that determine multiwall-sheet performance
Align channels correctlyWith sloping or vertical glazing, channels normally run so moisture can drain in the direction of fall or downward rather than becoming trapped across the sheet.
UV side outwardWhere the sheet has UV protection on one side, its marked protective side must face the sun.
Treat sheet ends correctlyUse the closure tapes, profiles and ventilation/drainage details specified for the system. The upper and lower ends do not necessarily serve the same purpose.
Allow for thermal expansionPolycarbonate changes dimension with temperature. Drilling, edge clearances and profile depths must follow the technical installation specification.
Compatible cleaners & sealantsSome chemicals can attack polycarbonate. Use only cleaners and sealants confirmed as compatible with the particular sheet.
11 • Cleaning
Cleaning polycarbonate without unnecessarily damaging the surface
Polycarbonate is impact-resistant, but its surface can scratch more easily than glass. Avoid dry rubbing, abrasive pads and unapproved solvents. A soft cloth or sponge, clean water and a mild compatible cleaner are normally a better starting point, followed by thorough rinsing.
Keep leaves and organic deposits away from sheet edges, gutters and glazing profiles. Dirt that holds water can encourage algae, staining and persistently damp edge zones even when the polycarbonate sheet remains technically sound.
For care of the complete frame, see the local Bloomcabin guide to cleaning and maintaining aluminium greenhouses.
12 • Polycarbonate or glass?
Use determines which glazing is the better fit
Polycarbonate is often better when …
high impact resistance and low glazing weight are important.
you want to use the cell structure of a multiwall sheet for insulation.
a softer, less crystal-clear transparency is acceptable or desirable.
the greenhouse is primarily a plant space, not chiefly a clear-glazed living room.
Toughened safety glass is often better when …
maximum visual clarity is a key design objective.
you want a particularly premium, architecturally transparent effect.
scratch resistance and a permanently clear appearance are highly valued.
the greenhouse will also serve as a display, dining or living space.
The Bloomcabin glazing guide explains the broader choice between glass and polycarbonate. This article deliberately goes deeper into choosing polycarbonate.
13 • Polycarbonate + aluminium
Why an aluminium frame and polycarbonate can be a strong combination
A rigid aluminium frame and lightweight multiwall glazing perform complementary jobs. Aluminium enables precise profiles, durable dimensional stability and corrosion-resistant supporting structures; polycarbonate reduces glazing weight while providing high impact resistance and useful thermal insulation.
For Bloomcabin customers, the leading example is the Classic aluminium greenhouse. It is the central product page referenced by this guide when combining classic greenhouse architecture, an aluminium frame and polycarbonate.
The advantage is that you do not need to change between entirely different frame concepts. With the Classic greenhouse , the underlying architectural form stays constant, while glazing can be chosen for light, thermal protection, use and desired appearance.
If you need additional width and interior space, also compare the Classic Plus 385.
14 • Choose by use
Which polycarbonate properties should different greenhouse owners prioritise?
Spring–autumn hobby growingGood light transmission, adequate impact resistance and secure frame compatibility are priorities. Maximum insulation is not automatically worthwhile if it unnecessarily reduces light.
Extended season in a cool climateU-/R-value, chamber geometry, edge sealing and air chambers become more important. Sufficient plant light must still be retained in darker months.
Hail / high impact exposureCheck impact-resistance data for the specific sheet alongside spans, profile supports and fixings. The whole assembly must carry the load.
Hot summer conditionsInclude diffusion, ventilation and shading in the plan. During hot summer periods, large ventilation areas and shading where needed are often more effective than heavily tinting the glazing. A darker or solar-reducing sheet may reduce heat gain, but should not unnecessarily cost plant light.
Garden room / statement appearanceWhere crystal-clear views and architectural transparency matter more than low sheet weight, safety glass may be the more suitable glazing.
15 • Common mistakes
Seven polycarbonate mistakes to avoid
1. Buying on thickness aloneCompare wall structure, thermal and light data too.
2. Installing the UV side inwardFollow the protected-side marking exactly.
3. Fully sealing both ends identicallyMany multiwall systems need controlled drainage or ventilation at the lower edge.
4. Ignoring thermal movementMaintain the manufacturer-required gaps and profile overlaps.
5. Using aggressive cleanersChemical compatibility matters with polycarbonate.
6. Optimising only for insulationPlants still need sufficient light.
7. Treating glazing separately from the frameSpan, fixing, expansion and structural loads must work as one system.
16 • FAQ
FAQ: greenhouse polycarbonate
Is 6 mm polycarbonate enough for a greenhouse?
For certain seasonal greenhouse systems, 6 mm can be appropriate. Whether it is sufficient depends on wall structure, frame spacing, wind and snow load, thermal goal and the greenhouse manufacturer’s approval. Sheet thickness must not be chosen independently of the frame.
Is 10 mm automatically better than 6 mm?
A 10-mm multiwall sheet can offer better insulation, but the result depends on the internal cross-section. It may also transmit less light or need different profiles. Technical data are therefore more informative than nominal thickness alone.
Can polycarbonate turn yellow?
Unprotected polycarbonate can age under UV radiation. Suitable outdoor sheets have UV stabilisation or co-extruded protection. Correct orientation, product quality, weathering and the specific warranty all matter.
Why is there water in the chambers?
Moisture can enter hollow chambers over time. Correct channel direction, edge treatment, appropriate closure profiles and controlled drainage help limit the problem. The installation instructions for the specific system are decisive.
Can I use polycarbonate in a quality aluminium greenhouse?
Yes, if the greenhouse is designed for it. The Bloomcabin Classic aluminium greenhouse is deliberately the central product reference for this combination in this guide.
What works where space is very limited?
A lean-to model can use the same fundamental glazing principles while saving garden space. See the Bloomcabin lean-to greenhouse solutions for South Africa.
17 • Decision
The best greenhouse polycarbonate is the sheet that suits the entire greenhouse system
First define the growing season, climate, desired light level, any heating and the frame. Then compare wall structure, U-/R-value, light transmission, UV protection, condensation behaviour, impact resistance, thermal expansion and warranty for the specific sheet.
For many home growers, a lighter twin-wall sheet can be entirely appropriate. For a cooler project with a longer season, a thicker or more complex multiwall sheet may justify the extra cost. In cold or snow-prone areas, thermal performance is particularly useful but must never be viewed separately from local wind and snow loads and the frame designed for them. Where clear architecture is the priority, safety glass remains a strong alternative.
To compare polycarbonate and glass options within a quality aluminium frame, start with the Bloomcabin Classic greenhouse. It remains this guide’s central commercial destination.
View the Classic aluminium greenhouseSources & further information
- German Weather Service — German Climate Atlas (Germany)
- Oklahoma State University Extension — Greenhouse Structures and Coverings
- Washington State University Extension — Greenhouse Construction and Management
- Cornell University / NRAES — Greenhouses for Homeowners and Gardeners
- Cornell University / NRAES — Greenhouse Engineering
- University of Arkansas Cooperative Extension — High Tunnel Production Guide
Technical values vary with sheet structure, thickness, formulation, coating and manufacturing specification. This guide uses universities, public weather and climate services, and agricultural extension resources as external sources—not competitors. For installation, always follow the technical data for the sheet actually purchased and the Bloomcabin frame.
