Step 5 - Greenhouses
Polycarbonate vs Glass for a Greenhouse
Glass transmits more light. Polycarbonate keeps more heat, diffuses better and does not shatter. For most gardens that settles it.
Janet H. - the hands-on owner behind Janet Media Group.
Published - last updated
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For a home greenhouse, twin-wall polycarbonate is the better default. It insulates substantially better than single glazing, it diffuses light so seedlings do not scorch, and it does not shatter. Glass transmits more light and looks better, which makes it a choice for sheltered, ornamental settings.
| Twin-wall polycarbonate | Single glass | |
|---|---|---|
| Light transmission | Good, reduced slightly by thickness | Highest |
| Light quality | Diffused -- even, no hard shadows | Direct -- hot spots and sharp shadows |
| Insulation | Good, and better with thickness | Poor unless double-glazed |
| Impact resistance | High -- flexes rather than breaks | Shatters |
| Weight | Light | Heavy, needs a stronger frame |
| Lifespan | Many years; can yellow eventually | Effectively indefinite if unbroken |
| Cost | Lower | Higher, including the frame it needs |
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  Top pickPalram Canopia Mythos GreenhouseAluminum frame, 4 mm twin-wall polycarbonate | A first real greenhouse | 9.0 | Check price on Amazon |
| 2 |  Palram Canopia Glory Greenhouse10 mm twin-wall polycarbonate throughout | Cold climates and winter growing | 8.9 | Check price on Amazon |
| 3 |  Palram Canopia Hybrid GreenhouseClear side panels, twin-wall roof | Seeing in, and light diffusion | 8.8 | Check price on Amazon |
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Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.
Why diffusion matters more than raw transmission
Diffused light scatters through the structure rather than arriving as direct beams. In practice that means more even light reaching the lower leaves of a crop and fewer hot spots scorching a tray of seedlings.
Glass transmits more total light and delivers it directly, which is why a glasshouse in strong sun often needs shading while a twin-wall structure may not. For raising seedlings specifically, diffusion is the more useful property.
Insulation is the bigger practical difference
Two skins with an air gap hold heat far better than a single pane. That shows up overnight, which is the part of the day that determines whether a winter crop survives, and it shows up in the shoulder seasons when a few degrees decides whether you can sow yet.
Thicker twin-wall -- 10 mm rather than 4 mm -- widens that advantage further, at a small cost in light. In a cold climate that trade is worth making; see greenhouse kits for winter.
Safety and practicality
Polycarbonate flexes rather than shattering, which matters if you have children, pets, a hail risk or overhanging branches. A broken greenhouse pane is both a hazard and an expensive repair.
It is also far lighter, which is why polycarbonate kits can use slim aluminum frames while glasshouses need substantially more structure -- part of the reason glass costs more overall.
The case for glass
- Appearance. A traditional glasshouse looks better than any polycarbonate kit, and that is a legitimate reason to choose one.
- Longevity. Unbroken glass does not yellow or degrade. Polycarbonate can cloud over many years, especially on the sunward side.
- Light in dark climates. Where winter light is genuinely marginal, maximum transmission may outweigh insulation.
- Cleaning. Glass cleans up more easily and stays clearer.
For kits in either material, see the best greenhouse kits.
The picks in full
Top pick
Palram Canopia Mythos Greenhouse
Palram Canopia
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The sensible first greenhouse: an aluminum frame with twin-wall panels, sized to fit a normal backyard rather than a nursery.
Twin-wall polycarbonate is the thing that separates a greenhouse from a plastic tent. Two skins with an air gap between them insulate, and the panel diffuses light rather than throwing hard shadows, which suits seedlings better than clear glass does.
The Mythos sits at the point in the range where you get a proper aluminum frame, a roof vent and a lockable door without paying for the heavy-duty glazing. For starting seeds, hardening off, and stretching the tomato season at both ends, that is the right specification.
Two warnings from the manual that people ignore. Anchor it -- every lightweight greenhouse that ends up in a neighbor's yard was not anchored. And build it with a second person; the panels flex and the frame is not square until the last bolt.
What works
- Twin-wall panels diffuse light and insulate better than single-skin plastic
- Aluminum frame will not rust or need painting
- Lengths let you match the run to the space you have
- Roof vent included rather than sold as an extra
What does not
- 4 mm glazing is thin for real winter use
- Assembly is a long afternoon and genuinely needs two people
- Must be anchored -- an unanchored kit is a sail
Skip it if: you live somewhere with heavy snow load. 4 mm panels are a three-season specification; the thicker-glazed models exist for a reason.
| Frame | Rust-resistant aluminum |
|---|---|
| Glazing | 4 mm twin-wall polycarbonate |
| Sizes | 6 ft wide, several lengths including 6 x 8 and 6 x 10 |
| Ventilation | Roof vent, lockable door handle |
| Base | Integrated base rail, anchoring required |
Cold climates and winter growing
Palram Canopia Glory Greenhouse
Palram Canopia
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The heavy-glazed model in the range, and the one to buy if the plan is to keep growing through a real winter.
Glazing thickness is the whole argument for this kit. A 10 mm twin-wall panel holds substantially more heat overnight than a 4 mm one, and overnight is when a winter greenhouse either works or does not.
It is also the model to look at if you get real snow. Thin panels and light frames are a three-season specification; if you are in the Cascades or anywhere with a genuine snow load, buy the heavier structure rather than hoping.
None of that makes it heated. A greenhouse without a heat source holds a few degrees over outside, which is the difference between hardy greens surviving and not -- it is not a tropical house in January.
What works
- 10 mm glazing holds heat far better than 4 mm
- Rated for harder weather than the lighter kits
- Wider footprint gives room for benches down both sides
- Better vent provision as standard
What does not
- Substantially more expensive
- Thicker panels cut light transmission a little
- Heavy enough that assembly is a two-person, whole-day job
Skip it if: you only want a spring seed-starting space. You are paying a large premium for insulation you will use for three months of the year.
| Frame | Heavy-gauge aluminum |
|---|---|
| Glazing | 10 mm twin-wall polycarbonate |
| Sizes | 8 ft wide, several lengths |
| Ventilation | Roof vents, and a screened side vent on some models |
| Base | Integrated base, anchoring required |
Seeing in, and light diffusion
Palram Canopia Hybrid Greenhouse
Palram Canopia
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Clear walls so you can see what is happening inside, with a diffusing twin-wall roof over the plants that need protection from hard overhead sun.
The Hybrid name refers to two glazing types in one kit: crystal-clear panels on the walls, twin-wall on the roof. In practice that means high light levels from the sides and diffused light from above, which is a good compromise in a northern or cloudy garden.
It also means you can see in. That sounds trivial until you have walked out in the rain three times a week to check whether anything needs water.
In a hot, south-facing, unshaded spot the clear walls become a liability. If that is your site, choose the fully diffused model or plan on shade cloth from midsummer.
What works
- You can see the plants without opening the door
- High light transmission through the clear walls
- Twin-wall roof still protects from midday sun
- Same aluminum frame as the rest of the range
What does not
- Clear walls run hotter than fully diffused glazing
- Shows dirt and algae sooner
- Needs shading in a hot, exposed spot
Skip it if: your site bakes in afternoon sun. Clear walls transmit far more light, and heat follows light.
| Frame | Rust-resistant aluminum |
|---|---|
| Glazing | Clear polycarbonate walls, twin-wall roof |
| Sizes | 6 ft wide, multiple lengths |
| Ventilation | Roof vent plus hinged door |
| Base | Integrated base, anchoring required |
Common questions
Is polycarbonate better than glass for a greenhouse?
For most home gardens, yes. Twin-wall polycarbonate insulates far better than single glazing, diffuses light so seedlings do not scorch, does not shatter, and costs less including the lighter frame it needs. Glass transmits more light and looks better.
What thickness of polycarbonate should a greenhouse have?
Four millimeter twin-wall is a three-season specification suited to temperate climates. Ten millimeter holds substantially more overnight heat and copes better with snow, which makes it the right choice for cold climates and winter growing.
Does polycarbonate block light in a greenhouse?
It transmits slightly less total light than glass and diffuses what passes through. Diffusion is generally an advantage -- light reaches lower leaves more evenly and seedlings are less likely to scorch. Only in genuinely light-marginal climates does the loss matter.
How long does greenhouse polycarbonate last?
Many years, though panels can yellow or cloud over time, particularly on the sunward side. UV-protected panels last considerably longer, and individual panels can usually be replaced from the manufacturer rather than replacing the whole structure.
Sources
Factual claims on this page are backed by the published sources below, and product specifications are the manufacturers’ own. Where a figure is not published, we say so rather than estimating it.