Every greenhouse listing quotes a light transmission percentage, and almost every buyer reads it wrong. 🔆 Glass transmits around 90% and twin-wall polycarbonate around 75% to 82%, but that gap tells you far less about growing than it appears to, because the two materials deliver light in fundamentally different ways.
This guide explains what the number actually measures, why diffusion changes the picture, and when the difference genuinely matters for what you want to grow. 🌱
📊 What the percentage measures
Light transmission is the proportion of visible light passing through the glazing rather than being reflected or absorbed. Simple enough. Three things it does not tell you: 🔍
- 🌐 How the light is distributed once inside.
- 🌈 Which wavelengths get through, which is what photosynthesis actually responds to.
- 🏗️ How much of the structure is glazed at all. A frame with thick bars can lose more light than the glazing difference between two materials.
That third point is worth sitting with. A greenhouse with 90% glazing and chunky wood bars covering 25% of the surface delivers less usable light than one with 80% glazing and slim aluminum bars covering 12%. The headline number is per square inch of glazing, not per square foot of building. 🧮
🌐 Direct versus diffused light, the part that matters
This is the concept that makes sense of everything else. 💡
Glass transmits directly
Light passes through mostly unchanged in direction. You get bright, sharp light with defined shadows, and the interior looks and feels like a glass building, which is precisely why it works as a room.
The consequence for plants: the top of the canopy gets a lot and the lower leaves sit in shadow. Growth is uneven, and on very bright days leaf scorch on tender foliage is a real risk.
Polycarbonate diffuses
The internal flute structure scatters light in many directions. Shadows soften and the interior takes on an even, shadowless quality. 🌤️
The consequence for plants: light reaches lower leaves and the shaded sides of plants. Diffused light penetrates a canopy more effectively, which is well established in commercial horticulture and is a large part of why diffusing coatings are sold to professional greenhouse growers who already have 90% glass.
🧭 The honest summary: a polycarbonate greenhouse delivering 78% diffused light frequently grows a dense crop as well as a glass one delivering 90% direct light. Not because the number is wrong, but because the number was never measuring canopy penetration.
🌈 Wavelengths, briefly
Plants use mostly blue and red light for photosynthesis, the range roughly between 400 and 700 nanometers, usually referred to as PAR. 🌿
- 🧊 Glass transmits PAR well and blocks most UVB.
- 🔹 UV-protected polycarbonate also transmits PAR well and blocks UV more completely, which is the point of the protective layer.
The practical takeaway: for growing vegetables, herbs, seedlings and most ornamentals, neither material is missing wavelengths plants need. UV blocking matters only in narrow cases, such as some pigment development in certain ornamentals and specific alpine or succulent collections. 🪴
📉 What actually costs you light, in order of impact
If light is your limiting factor, these matter far more than choosing between the two glazings. ☀️
- 1️⃣ Siting and shade. A structure that loses two hours of winter sun to a conifer has thrown away more light than any material difference will ever recover. This is the whole game, and we covered it in our siting guide.
- 2️⃣ Dirty glazing. Accumulated grime, algae and pollen can cost ten percentage points or more. One wash a year recovers it, and it is free.
- 3️⃣ Frame bar coverage. Slim aluminum bars against chunky wood ones is a real difference in delivered light on the same footprint.
- 4️⃣ Panel thickness. Going from 4 mm to 10 mm polycarbonate costs on the order of eight percentage points, which is the trade we set out in our thickness guide.
- 5️⃣ Algae in unsealed flutes, which turns polycarbonate green from the inside and is permanent. Sealed edges prevent it.
- 6️⃣ Aging, at a few percentage points per decade on quality panels.
💡 Notice that the top two are free to fix and dwarf everything else. Most greenhouses are not light-limited by their glazing. They are light-limited by a tree and a year of grime.
🌱 When the difference genuinely matters
Cases where glass has a real edge
- ❄️ Midwinter propagation at high latitude, where every photon counts and there are few to begin with.
- 🌵 Cacti, succulents and alpines, which want intense direct light and often better UV transmission.
- 🏛️ Anywhere the interior is meant to feel like a room. Diffused light is excellent for plants and less appealing for people, who generally want to see out clearly.
Cases where polycarbonate's diffusion is the advantage
- 🍅 Dense summer crops, tomatoes, cucumbers, peppers, where lower-canopy light drives yield.
- 🌱 Seedling trays in spring, where even light produces even, unstretched growth.
- ☀️ Hot, high-UV climates, where direct light causes scorch and glass houses need shading anyway.
- 🌿 Leafy greens and herbs, which do not want intense direct light at all.
🧊 A fair note on insulation, because it interacts
Light and heat trade against each other, and the trade runs opposite to what the transmission number suggests. 🌡️
Single glass transmits more light and loses heat at roughly twice the rate of 6 mm twin-wall polycarbonate. ❄️
So in an unheated greenhouse in early spring, the polycarbonate structure is often the one that is actually warmer at 6 a.m., which is when a late frost does its damage. More light does not automatically mean better growing conditions, and in the shoulder seasons it frequently means the opposite. ⚖️
🧭 Choosing on the basis of light
- 🍅 Growing crops to harvest: polycarbonate's diffusion is at least a match, and the insulation is a bonus. Choose on climate and budget, not on the transmission percentage.
- 🏛️ A room to sit in: glass, without hesitation. You want to see out, and clarity is the point of the building.
- ❄️ Winter propagation in a dark climate: maximize glazed area and choose thinner polycarbonate or glass, and site for winter sun above everything else.
- 🌵 Specialist collections wanting direct light: glass.
- 🌞 Hot southern climate: polycarbonate diffusion reduces scorch, and you would be shading a glass house regardless.
A structure like The Estate is built around clarity and sightlines; the polycarbonate range is built around even growing light and thermal stability. Neither is the better glazing, and the full comparison sits in our polycarbonate versus glass guide. 🏛️
❓ FAQ: greenhouse light transmission
Does glass grow plants better than polycarbonate?
Not as a general rule. Glass transmits more light directly; polycarbonate diffuses what it transmits, which penetrates a canopy more evenly. For dense crops the outcomes are frequently comparable. 🌱
What light transmission do I actually need?
Anything above roughly 70% is sufficient for almost all backyard growing. Below that you start to notice, and above it siting and cleanliness matter far more than the last few percentage points. 🔆
Does diffused light reduce yield?
Generally the opposite for dense crops, which is why commercial greenhouse growers buy diffusing coatings for glass they already own. 🍅
How much light do dirty panels cost me?
Enough to matter: accumulated grime and algae can take ten percentage points or more. An annual wash in late winter is the cheapest growing improvement available. 🧽
Do plants need UV light?
Most do not, for growth. UV influences some pigment development and compact form in certain ornamentals, cacti and alpines, but for vegetables, herbs and seedlings its absence is not a problem. ☀️
Is more light always better?
No. In an unheated greenhouse the better-insulated structure is often warmer overnight, and in hot climates direct light causes scorch that diffusion avoids. Light and heat trade against each other. 🌡️
🌿 What to take from all this
Stop optimizing the transmission percentage. Site the greenhouse where the winter sun actually reaches, wash the glazing once a year, and choose the glazing family on what you want the building to be rather than on a number that measures one property in isolation. 🧭
Every model in the Prime Greenhouses range states glazing type and thickness per surface, so you can see exactly what light you are buying rather than inferring it from a headline figure. 📊