4mm vs 6mm Polycarbonate Greenhouse Panels: Which Is Better?

For most growers, the 4mm vs 6mm polycarbonate greenhouse question comes down to climate and size. Go with 4mm for a small, heated, or warm-climate greenhouse where you want maximum light and a lower price. Go with 6mm for a larger or unheated structure in a cold, snowy region where heat retention and toughness matter more than a few extra percent of light. Here is the full breakdown on R-value, light, strength, and cost so you can match the right polycarbonate greenhouse panel to your yard.

TL;DR: 6mm polycarbonate has an R-value of 1.54 versus 1.43 for 4mm, giving it roughly 25 to 50% better insulation, plus more strength against snow and hail. 4mm transmits 5 to 10% more light and costs less. Pick 6mm for cold, unheated, or large greenhouses; pick 4mm for small, warm, or heated ones.

Key Takeaways

• Polycarbonate is shatterproof and strong. It is about 200 times stronger than glass and 30 times stronger than acrylic, so both 4mm and 6mm shrug off hail and falling branches.
• 6mm has a higher R-value (1.54) than 4mm (1.43), for roughly 25 to 50% better insulation and heat retention in cold climates.
• 4mm transmits 5 to 10% more light, which suits warm, sunny climates and heated greenhouses.
• 6mm is 50% thicker, giving it the edge on impact resistance, snow load, and longevity on larger greenhouses.

Benefits of polycarbonate greenhouses

Before splitting hairs over 2mm, it helps to know why polycarbonate is the default glazing for most backyard greenhouses in the first place. It hits a balance glass and film cannot: nearly unbreakable, light enough to install yourself, and insulating thanks to its hollow twin-wall build.

The main advantages of polycarbonate panels:

• Impact resistance - Polycarbonate sheets are virtually unbreakable. At roughly 200 times stronger than glass and 30 times stronger than acrylic, they resist hail, falling branches, and other impacts that would shatter a glass panel.
• Lightweight - The panels weigh far less than glass, which makes assembly easier and can reduce how much framing the structure needs.
• Insulation - The air gaps inside multiwall polycarbonate trap heat, helping hold interior temperatures steady through cold nights.
• Light diffusion - The translucent panels scatter sunlight evenly, cutting harsh shadows and hot spots inside.
• UV protection - Polycarbonate blocks over 99% of harmful UV radiation, protecting both plants and people.
• Affordable - Polycarbonate glazing costs less than glass for the same growing space, which is why most kits use it.

With that settled, here is how the two thicknesses actually compare.

Key comparison between 4mm and 6mm polycarbonate panels

The two thicknesses behave almost identically in everyday use. The differences show up at the margins: cold nights, heavy snow, and how much you pay. 4mm leans toward light and budget, 6mm toward insulation and durability. The sections below run each factor in turn.

Thickness

The difference starts with the number in the name:

• 4mm polycarbonate - Each panel is 4 millimeters thick. This is the common thickness for hobby greenhouse kits.
• 6mm polycarbonate - Each panel is 6 millimeters thick, making it 50% thicker than 4mm.

That extra 2mm is what drives the gaps in insulation, strength, and cost below.

Strength and durability

Both thicknesses are tough enough that breakage is rarely the deciding factor. Polycarbonate has around 200 times the tensile strength of standard glass, so it bends and flexes under impact instead of shattering. That toughness carries real safety and cost benefits:

• 200x stronger than glass - Polycarbonate is virtually unbreakable under normal conditions, with none of the sharp-shard risk of a broken glass pane.
• Shatterproof - Unlike glass, polycarbonate flexes rather than breaking, so a stray ball or branch leaves a dent at worst.
• Hail resistant - The panels handle hailstones well, which prevents the kind of costly damage that ruins a glass greenhouse in one storm.
• Snow loads - Multiwall polycarbonate carries heavy snow without buckling, especially over a properly braced frame.

Both thicknesses are remarkably hail and shatter resistant, but the extra 2mm gives 6mm a real edge once snow piles up or storms turn violent. On a large span, that margin is worth paying for.

Insulating properties

This is where the thicknesses genuinely diverge. Insulation is measured by R-value, which tracks how well a material resists heat transfer. A higher R-value means less heat escaping on a cold night and lower heating bills through winter.

Here are the typical R-values:

• 4mm polycarbonate - 1.43
• 6mm polycarbonate - 1.54

That jump from 1.43 to 1.54 translates to roughly 25 to 50% better heat insulation for 6mm. University extension research on greenhouse glazing notes that adding wall layers and thickness is one of the most direct ways to cut heat loss in an unheated or minimally heated structure, which is exactly what the thicker panel does.

A few other factors stack on top of raw R-value:

• Air gaps - The hollow channels between the walls trap air and create insulated pockets. More thickness means a deeper, more effective gap.
• Light transmission trade-off - Thicker panels pass slightly less light, which also means slightly less heat loss back out through the glazing.
• Thermal mass - Thicker polycarbonate holds a touch more solar warmth during the day and releases it slowly at night.

Together these give 6mm a clear advantage for holding heat in cold-climate and unheated greenhouses.

Light transmission

Light drives plant growth, and here 4mm has the edge. The thinner panel transmits around 5 to 10% more natural light than 6mm. For seedlings, heat-loving crops, and any greenhouse in a cloudier region, that extra light can mean faster, stronger growth. Massachusetts extension guidance on greenhouse glazing points out that even small differences in light transmission add up across a full growing season, since plants respond directly to total light received.

That said, both thicknesses keep excellent light levels for a backyard greenhouse, and 6mm diffuses light more evenly, spreading it into corners and reducing harsh shadows. So 4mm wins on raw quantity, 6mm wins slightly on even distribution. If you want to dig into how thickness fits into the bigger glazing picture, our guide on how thick greenhouse plastic should be covers the full range from film to triple-wall.

Cost comparison

6mm panels cost more than 4mm because they use more material, and a larger greenhouse multiplies that difference across every panel. The thicker glazing can pay some of that back in lower heating bills over the years, but the size of that return depends on your climate and how hard you run a heater. Pricing also shifts with supplier, region, and market conditions, so compare current quotes for the exact kit you want before deciding.

Here is the comparison at a glance:

Best uses for 4mm vs 6mm polycarbonate

The right thickness follows your climate, your greenhouse size, and whether you plan to heat it. The split below covers most situations, though your local weather and growing goals should always have the final say.

Best uses for 4mm polycarbonate

• Small to mid-size hobby greenhouses up to 200 sq ft
• Warm, sunny climates that rarely freeze
• Greenhouses with supplemental heating, where insulation matters less
• Budget-friendly builds where you want to maximize light transmission

Best uses for 6mm polycarbonate

• Large greenhouses over 300 sq ft that need stronger glazing across a wide span
• Cold climates prone to heavy snow, hail, and hard freezes
• Unheated greenhouses that rely on passive solar to hold warmth
• Regions with frequent intense weather
• Greenhouses you want to last 10 to 15 years or longer

These are general guidelines. Budget, framing strength, and how much light your plants need all factor in, so weigh them against the recommendations above.

Key considerations when choosing between 4mm and 6mm

If you are still on the fence, run through these factors for your specific setup:

• Climate - Cold regions favor 6mm for heat retention; hot, sunny regions favor 4mm for light.
• Greenhouse size - Larger structures favor 6mm, which better handles snow and wind across a big span. Small hobby greenhouses do fine on 4mm.
• Budget - 4mm is cheaper upfront; 6mm costs more but can trim heating bills long term.
• Heating - Heated greenhouses can run 4mm; unheated ones benefit most from 6mm insulation.
• Lifespan - Both last well, but 6mm thickness adds margin for impact resistance and longevity over a 10 to 15 year life.

If you are still weighing polycarbonate against a glass build before settling on a thickness, our polycarbonate vs glass greenhouse comparison walks through that decision in full.

FAQs

What polycarbonate thickness is best for a small greenhouse?

For a small greenhouse, 4mm twin-wall polycarbonate is usually plenty. It is lightweight, easy to install, and lets in more light, which suits compact hobby builds. If you plan to heat the space through winter or live somewhere cold, step up to 6mm for the better insulation.

Is 10mm polycarbonate worth it over 6mm?

It can be, but only in the right setting. 10mm offers noticeably better insulation than 6mm, which pays off in cold climates or heated greenhouses where heat retention is the priority. The trade-offs are higher cost, slightly less light transmission, and more weight that may call for sturdier framing.

Which polycarbonate thickness is best for a cold climate?

For cold climates, thicker is better. 6mm is the practical minimum for an unheated greenhouse facing real winter, since its 1.54 R-value holds heat far better than 4mm. In severe cold or for year-round growing, many growers move up to 10mm or triple-wall panels for even stronger insulation.

Ready to choose?

Match the thickness to your climate and size, and you will get years of reliable growing out of your greenhouse. Browse our full range of greenhouse kits for sale to find one built with the right panel for your yard.