Greenhouse Polycarbonate | Thickness, Multiwall Sheets & Insulation
New Zealand • Greenhouse materials guide • Bloomcabin Journal
Greenhouse polycarbonate: the complete guide
Thickness, twinwall versus multiwall sheets, insulation, UV protection, condensation and the details that determine real performance.
Start with the right question
“How thick should greenhouse polycarbonate be?” is useful, but it is not the whole decision.
Polycarbonate is used for greenhouse glazing because it combines low weight with high impact resistance and useful insulation. Multiwall sheets enclose air chambers; these still air layers 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 twinwall sheet, 10-mm triplewall sheet and 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. For an aluminium greenhouse with polycarbonate, the Bloomcabin Classic aluminium greenhouse is the key commercial starting point in New Zealand.
Begin with climate, growing season, light requirements, heating plans and frame construction. New Zealand ranges from mild coasts to colder inland and elevated areas, where winter light, frost, summer heat, snow and wind exposure vary markedly.
01 • The material
What is greenhouse polycarbonate?
Polycarbonate is a thermoplastic available as solid, corrugated, twinwall and multiwall sheet. Hollow multiwall sheets are especially relevant for greenhouses: internal ribs join the outer skins to create enclosed air chambers. The result is a light component that can be stiffer than a very thin solid sheet and offer better thermal performance.
Suitable outdoor sheets are made with UV-stabilised or co-extruded protective layers. Depending on the product, light diffusion, different transparency levels or anti-condensation surfaces may be included. “Polycarbonate” names the base material, not the complete 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 matters because greater thickness can allow deeper air chambers and more complex internal geometry. It is not, however, a universal insulation value. Use these ranges as prompts for questions, not as proof that every sheet of one thickness performs identically.
| Nominal thickness | Typical planning question | Potential benefit | What to check |
|---|---|---|---|
| 4 mm | Are minimum weight and a slim sheet the priority? | Light, simple construction | Insulation, permitted span, impact resistance and frame specification |
| 6 mm | Is a light twinwall sheet needed for seasonal use? | A useful entry into air-chamber insulation at low weight | Exact rib structure, U-value, support spacing and UV face |
| 8 mm | Is greater chamber depth or triplewall construction desired? | Potentially improved insulation in a compact sheet | Twinwall versus triplewall, light transmission and weight |
| 10 mm | Is the growing 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 main project objective? | Allows more advanced multiwall builds | Potential light loss, frame compatibility, cost and structural load assumptions |
03 • Internal geometry
Twinwall vs. triplewall vs. multiwall
Twinwall sheets have two outer skins joined by internal ribs. This essentially forms one layer of elongated air chambers. These sheets are light, comparatively simple and common in thinner greenhouse systems.
Triplewall sheets add another internal skin and create further separate air layers. Depending on the exact geometry, this can improve insulation compared with a similarly thick twinwall sheet.
Multiwall is the umbrella term for more complex cross-sections. These may include extra horizontal skins, diagonal ribs, X-structures and combinations of them. That is why “10 mm polycarbonate” alone is insufficient for a professional choice: read the cross-section and technical values too.
04 • Insulation
How polycarbonate insulates a greenhouse
Multiwall polycarbonate insulates primarily through enclosed air chambers. Relatively still air transfers heat more slowly than a solid, highly conductive path. Cell geometry, number of skins, material thickness and edge conditions can therefore produce very different results despite the same stated thickness.
University and agricultural extension guidance recommends comparing greenhouse coverings by actual heat transfer, not just the material name. A lower U-value generally means less heat flow; a higher R-value means more thermal resistance. For planning, use the rating for the exact sheet.
Where winter growing or heating cost matters, ask for the U-/R-value of the sheet actually supplied. Thermal performance is especially relevant in cold or snowy locations, but must never be considered independently of local snow loads and a 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 if too much light is lost
Every extra skin, rib, tint or solar-control layer changes light delivery. New Zealand has mild coastal regions as well as cooler inland and high-country areas, so winter light, frost, summer heat, snow and wind exposure vary by site. In low-light months, available plant radiation can be a greater limitation than a small final gain in U-value.
For plants, ask for visible light transmission or PAR data and consider light diffusion. A suitably diffusing sheet can distribute radiation more evenly through the crop and reduce harsh local peaks. Strongly tinted or solar-reflective sheets can reduce summer heat, but may also sacrifice plant light.
06 • UV protection
Which side of a polycarbonate sheet must face the sun?
Polycarbonate for permanent outdoor use is usually made with a UV-stabilised protective layer. It protects the sheet itself from long-term ultraviolet ageing. Some products have protection on one face only, others on both; check this before installation.
With one-sided UV protection, the marked protected face must face outwards towards the sun. Installing the sheet the wrong way around can compromise the weather resistance intended by the manufacturer. Remove protective films only after confirming installation direction.
Do not assume both faces are the same. Check the marking and installation instructions for the specific sheet.
07 • Condensation
Condensation on the 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 as a thin film rather than form large drops. This can reduce dripping and optical losses caused by strongly reflecting droplets.
Moisture inside hollow chambers is a different issue. Channels, edge closures and profiles must be installed so water is not needlessly trapped and contamination is limited. Polycarbonate also moves considerably with temperature changes. Edge clearances, profile cover, drillings and drainage must therefore 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 roughly ten to fifteen years for suitable polycarbonate coverings, while actual guarantees depend heavily on the product, UV protection and warranty wording. Sun exposure, cleaning, chemical contact, installation and frame condition also affect ageing.
A sheet may remain structurally intact while its appearance declines. Yellowing, surface haze, scratches, chamber contamination or damage around fixings and edges can matter to users long before mechanical failure.
For a quality greenhouse, read not only the guarantee period but also what is guaranteed: impact strength, light transmission, yellowing or another performance measure may be treated differently.
09 • Impact resistance & construction
High impact resistance does not replace sound greenhouse engineering
Polycarbonate is valued for high impact resistance, and multiwall sheets are often used where break resistance matters. Yet the sheet is only one part of the system. Wind, snow and impact loads are transferred through glazing profiles, rafters, screws, anchors and foundations.
A thicker sheet cannot compensate for a weak frame, excessive 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 one thickness.
Where severe weather is an important buying criterion, compare Bloomcabin greenhouse models for New Zealand and review the wind and snow loads specified for your project.
10 • Installation
Installation details that determine multiwall-sheet performance
Orient channels correctlyFor sloped or vertical glazing, channels generally run with the fall or downwards so moisture can drain rather than becoming trapped across the sheet.
UV face outwardsIf the sheet has UV protection on one side, its marked protected face must face the sun.
Treat sheet ends correctlyUse the closure tapes, profiles and ventilation/drainage details specified for the system. Upper and lower ends do not necessarily perform the same role.
Allow for thermal expansionPolycarbonate changes dimension with temperature. Drillings, edge gaps and profile depths must follow the technical installation instructions.
Compatible cleaners & sealantsSome chemicals can attack polycarbonate. Use only cleaners and sealants confirmed as compatible with the specific sheet.
11 • Cleaning
Clean 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 debris 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 itself remains technically sound.
For complete frame care, also read the local Bloomcabin guide to cleaning and maintaining aluminium greenhouses.
12 • Polycarbonate or glass?
Use determines which glazing suits better
Polycarbonate often suits better when …
high impact resistance and low glazing weight are important.
you want to use a hollow-sheet cell structure for insulation.
softer, less crystal-clear transparency is acceptable or desirable.
the greenhouse is primarily a plant space rather than a clear-glazed living area.
Toughened safety glass often suits better when …
maximum visual clarity is a key design goal.
you want a particularly refined, architecturally transparent effect.
scratch resistance and lasting visual clarity are highly valued.
the greenhouse will also be used as a display, dining or living space.
The Bloomcabin glazing guide explains the wider 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 light hollow-sheet glazing perform complementary roles. Aluminium enables precise profiles, lasting dimensional stability and corrosion-resistant load-bearing structures; polycarbonate reduces glazing weight while providing high impact resistance and useful insulation.
For Bloomcabin customers, the main example is the Classic aluminium greenhouse. It is the central product page referenced by this guide for classic greenhouse architecture combining an aluminium frame with polycarbonate.
The advantage is that you need not switch between completely different frame concepts. With the Classic greenhouse the architectural form remains constant; you can choose glazing according to light, thermal performance, use and preferred 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, sufficient impact resistance and secure frame compatibility come first. Maximum insulation is not automatically worthwhile if it needlessly reduces light.
Extended season in a cool climateU-/R-value, chamber geometry, edge sealing and air chambers become more important. Enough plant light must still remain available during darker months.
Hail / high impact exposureCheck impact data for the actual sheet alongside spans, profile supports and fixings. The complete assembly must carry the load.
Hot summer conditionsInclude diffusion, ventilation and shading in the plan. In hot periods, generous ventilation and shading where needed are often more effective than heavily tinting glazing. A darker or solar-reducing sheet can reduce heat gain, but should not unnecessarily cost plant light.
Garden room / presentation appearanceWhere crystal-clear views and architectural transparency matter more than low sheet weight, safety glass can be the more suitable glazing.
15 • Common mistakes
Seven polycarbonate mistakes to avoid
1. Buying by thickness aloneCompare wall structure, thermal data and light data too.
2. Installing the UV face inwardsFollow the marking for the protected face exactly.
3. Sealing both ends identically and completelyMany multiwall systems need controlled drainage or ventilation at the lower edge.
4. Ignoring thermal movementMaintain the joints and profile overlaps required by the manufacturer.
5. Using aggressive cleanersChemical compatibility matters with polycarbonate.
6. Optimising for insulation onlyPlants still need sufficient light.
7. Considering glazing separately from its frameSpan, fixing, expansion and structural loads must work as a system.
16 • FAQ
FAQ: polycarbonate in a greenhouse
Is 6 mm polycarbonate sufficient for a greenhouse?
For some seasonal greenhouse systems, 6 mm can be suitable. Whether it is enough depends on wall structure, frame spacing, wind and snow load, thermal target and approval by the greenhouse manufacturer. Sheet thickness must not be chosen independently of the frame.
Is 10 mm automatically better than 6 mm?
A 10-mm multiwall sheet may offer better insulation, but the result depends on its internal cross-section. It may also transmit less light or require different profiles. Technical data are therefore more meaningful than nominal thickness alone.
Can polycarbonate 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 guarantee are decisive.
Why is there water in the chambers?
Moisture can enter hollow chambers over time. Correct channel direction, edge treatment, suitable closing profiles and controlled drainage help limit this issue. The installation manual for the specific system is authoritative.
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 basic glazing principles while saving garden area. See Bloomcabin lean-to greenhouse solutions for New Zealand.
17 • Decision
The best greenhouse polycarbonate is the sheet that suits the whole greenhouse system
First define 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 guarantee for the specific sheet.
For many home growers, a lighter twinwall sheet can be entirely suitable. For a cooler project with a longer season, a thicker or more complex multiwall sheet may justify its extra cost. In cold or snowy locations, thermal performance is especially relevant but must never be separated from local snow loads and a frame designed for them. Where crystal-clear architecture comes first, 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 the central commercial destination for this guide.
View the Classic aluminium greenhouseSources & further information
- German Weather Service — German Climate Atlas
- 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 by sheet structure, thickness, formulation, coating and production specification. This guide uses universities, public weather and climate services, and agricultural extension sources—not competitors. For installation, always follow the technical data for the sheet purchased and the Bloomcabin frame.
