Short answer for the impatient: 4 mm twin-wall is the entry point, 6 mm is the sensible default for most US backyards, and 8 mm or 10 mm multi-wall is worth it only in genuinely cold winters or where hail is a regular event. 🪟
The longer answer matters because thickness controls two things that pull in opposite directions: insulation and light. More layers hold more heat and pass slightly less light. Which side of that trade you want depends entirely on what you are growing and where you live. ⚖️
🔍 What twin-wall and multi-wall actually mean
Polycarbonate sheet for greenhouses is extruded with internal channels, called flutes, running the length of the panel. Those channels trap still air, and still air is the insulator. 🌬️
- 🔹 Twin-wall: two skins with one row of channels between them. Available in 4, 6, 8 and 10 mm.
- 🔸 Triple-wall and multi-wall: three or more skins with two or more rows of channels, typically 8 mm and up.
- ⚪ Single skin: no channels at all, no insulation benefit, and generally the marker of a very cheap structure.
The rule underneath all of it: more channels means better insulation and slightly lower light transmission. Thickness is really a proxy for how many air layers you are buying.
🌡️ The numbers, given as ranges because sources vary
Manufacturers publish somewhat different figures depending on the exact structure of the sheet, so treat these as bands rather than precise values. 📊
Light transmission
- 🔆 4 mm twin-wall: roughly 80% to 82%
- 🔅 6 mm twin-wall: roughly 78% to 80%
- 🔅 8 mm twin-wall: roughly 75% to 78%
- 🔅 10 mm multi-wall: roughly 72% to 76%
- 🧊 For reference, single glass: around 90%
The important nuance: polycarbonate diffuses light where glass transmits it directly. Diffused light penetrates the plant canopy more evenly and reduces scorch, which is why the raw transmission percentage understates how well polycarbonate actually grows things. It is a real advantage, not a consolation. 🌱
Heat loss, measured against single glass
- 🧊 Single glass: the baseline, and by a wide margin the worst insulator on this list.
- 🔹 4 mm twin-wall: loses roughly a third less heat than single glass.
- 🔹 6 mm twin-wall: loses roughly half the heat of single glass.
- 🔸 8 mm twin-wall: a further modest gain over 6 mm.
- 🔸 10 mm multi-wall: the best of the four, by a smaller margin again.
Read the pattern rather than the individual figures. The jump from single glazing to 4 mm twin-wall is enormous. The jump from 4 mm to 6 mm is meaningful. From 6 mm to 8 mm it is real but modest, and beyond that you are in diminishing returns while continuing to give up light. 📉
None of this makes polycarbonate the better material outright. Glass wins decisively on clarity, on permanence and on how a building looks and feels to sit in, which is why it dominates the garden-room use case. We set the two side by side in our polycarbonate versus glass comparison. ⚖️
🥊 The thickness that matters more than any of this
Here is the part the thickness debate distracts from. The UV protection layer matters far more than a millimeter or two of sheet. 🌞
Quality greenhouse polycarbonate is co-extruded with a UV-blocking layer on the outer face. With it, panels are a ten to twenty year component. Without it, they haze, yellow and go brittle within a few seasons regardless of how thick they are.
So a 4 mm panel with proper UV protection comfortably outlives an 8 mm panel without it. If a listing specifies thickness but says nothing about UV treatment, the thickness figure is not the useful number on the page, which is one of the quality markers we set out in cheap versus premium greenhouses. 🚩
⚠️ And the corresponding assembly rule: panels must be fitted UV side out. The protected face carries a printed film. Check it before you peel, because fitting a sheet upside down is the most common assembly error in existence and it multiplies the aging rate.
🪨 Hail, snow and impact
Impact resistance is where polycarbonate is genuinely in a different category from glass, and thickness contributes to it. 🌨️
- 🔹 4 mm: handles ordinary hail without drama.
- 🔹 6 mm: noticeably stiffer, less deflection in wind, better against larger hail.
- 🔸 8 mm and above: the choice if you live somewhere hail is a recurring insurance conversation.
For snow, panel thickness is much less important than frame strength, roof pitch and the published snow load rating. Snow load is carried by the structure, not the glazing. A thicker panel on a weak frame is not a snow solution. ❄️
Thicker sheet does reduce deflection in high wind, which keeps panels seated in their clips. But again, the wind rating of the structure is the number to check, not the glazing thickness.
🧭 Choosing by climate and use
- 🌼 Mild winters, season extension only, unheated: 4 mm is sufficient and gives you the most light. Do not overspend here.
- 🍅 Most US backyards, real winter, unheated or lightly heated: 6 mm. This is the default for good reason, and it is where the insulation gain is still large and the light loss still small.
- ❄️ Cold winters, or you intend to heat: 8 mm. Every degree you are paying for is a degree worth insulating.
- 🪨 Frequent hail: 8 mm or multi-wall, for the impact rather than the heat.
- 🌱 Propagation and light-hungry seedlings in a dark climate: stay at 4 or 6 mm. Trading light for insulation is the wrong trade when light is your limiting factor.
💡 A mixed specification is often the smartest answer and is what several structures do by default: thicker glazing on the roof, where most heat is lost and where hail lands, thinner on the walls, where light entry matters more.
💰 Is the upgrade worth the money?
Two honest calculations. 🧮
If you never heat the greenhouse, the payback on thicker glazing is measured in frost protection rather than dollars. A better-insulated unheated greenhouse holds a few degrees more overnight, which can be exactly the margin that saves a crop in a late frost. That is worth something, but it is not a bill you can point at. Thermal mass and bubble insulation buy those same degrees far more cheaply.
If you do heat, the arithmetic gets concrete. Going from 4 mm to 6 mm cuts heat loss through the glazing by roughly a fifth. Against a winter heating bill in the $40 to $120 a month range, an upgrade of a few hundred dollars generally pays back within a handful of seasons and keeps paying afterward. ⚡
What we would not do is buy 10 mm for an unheated greenhouse in a mild climate. You would be paying for insulation you never use and giving up light you always need. 🚫
🔎 Five things to check on any polycarbonate listing
- 1️⃣ Is UV protection stated explicitly, and on which face? This outranks thickness.
- 2️⃣ Is thickness given per surface? Roof and walls are frequently different, and "6 mm" on a listing sometimes means the roof only.
- 3️⃣ Are the panel edges sealed? Unsealed flutes fill with dust and algae, which is what makes old panels look green from the inside, and no amount of cleaning reaches inside a flute.
- 4️⃣ What is the warranty on the panels specifically, as distinct from the frame?
- 5️⃣ Twin-wall or single skin? A very cheap structure quoting a thickness may be quoting single skin, which has no insulation value at all.
Those five questions separate quality tiers faster than any price comparison. 📊
❓ FAQ: polycarbonate thickness
Is 4 mm polycarbonate thick enough for a greenhouse?
Yes for mild climates and unheated season extension, where its higher light transmission is an advantage. It is thin for cold winters, heated growing or hail-prone regions. 🌼
What is the difference between 4 mm and 6 mm in practice?
Roughly a fifth less heat loss through the glazing, noticeably more panel stiffness, and about two percentage points less light. For most US backyards that trade is worth making. 🌡️
Does thicker polycarbonate block too much light?
Not at the thicknesses used for backyard greenhouses. The drop from 4 mm to 10 mm is on the order of eight percentage points, and polycarbonate's diffusion means canopy penetration stays good. Only propagation in genuinely dark climates is sensitive enough to notice. 🌱
Is triple-wall worth it?
Only for heated growing in a cold climate. It is the right answer for a small group of buyers and an expensive mistake for everyone else. ❄️
Can I upgrade the panels later?
Sometimes, but glazing channels and clips are sized for a specific thickness, so a thicker panel often will not seat. Treat glazing thickness as a purchase decision rather than an upgrade path. 🔩
Does thickness affect the snow load rating?
Barely. Snow load is carried by the frame and the roof geometry. Check the structure's published rating rather than assuming thicker glazing compensates for a lighter frame. 🌨️
🌿 What we would actually buy
6 mm twin-wall with a stated UV layer, sealed edges and a panel warranty, on a frame with a published wind and snow rating. That combination covers the vast majority of US backyards and neither overpays for insulation nor gives up meaningful light. 🧭
Go to 8 mm if your winters are genuinely cold, if you intend to heat, or if hail is a recurring event. Stay at 4 mm if your climate is mild and light is your limiting factor.
Our polycarbonate models are offered in 4 mm and 6 mm, with the specification stated per surface rather than as a headline. A model like The Modern lists glazing type, thickness, UV treatment and the frame's wind and snow ratings on the same page, and every structure in the Prime Greenhouses range is documented identically, so you can compare specifications rather than adjectives. 📊