Glazing is the greenhouse. The frame just holds it up. Whatever covers the structure decides how much light your plants receive, how much heat you keep at 3 a.m., what the building costs, and what maintenance looks like in year ten. This guide covers every glazing family — glass, the polycarbonate grades, acrylic, fiberglass and film — with the numbers that matter and the honest trade-offs, going deeper than our earlier glass vs polycarbonate overview.
Why Glazing Choice Deserves More Thought Than Frame Choice
Buyers agonize over footprint and frame color, then accept whatever glazing the kit happens to ship — backwards priorities. The glazing determines the operating cost every heating night for the life of the structure, sets the light environment that decides what thrives, and is the component most likely to need replacement or upgrade. Two identical frames, one in 4mm twin-wall and one in 10mm, are functionally different buildings from November to March. Read this section before comparing kits, and the spec sheets start making sense.
The Three Numbers That Describe Any Glazing
Light transmission — the percentage of sunlight passing through. Glass leads around 90%+; twin-wall polycarbonate sits near 80%; each added wall layer trades light for insulation. Diffusion matters too: materials that scatter light (multiwall poly, fiberglass, diffused film) illuminate the canopy more evenly and reduce scorch, which is why measured plant growth under slightly-lower-transmission diffused glazing often equals or beats clear glass.
Insulation (R-value / U-value) — resistance to heat loss. Single glass is the worst insulator per layer (R roughly 0.9); twin-wall 8mm polycarbonate roughly doubles it; 16mm triple/five-wall grades roughly triple it. In heating-season dollars, glazing R-value is the biggest lever a winter grower controls.
Lifespan and impact resistance — glass lasts indefinitely but breaks; polycarbonate is virtually unbreakable but UV-ages over 10–15+ years (quality sheets carry 10-year warranties); films are 4–6-year consumables by design.
The Glazing Families
Horticultural and tempered glass
The traditional choice: maximum clarity, zero UV aging, cleans like new forever. Tempered glass is several times stronger than horticultural (annealed) glass and breaks into safe granules rather than shards — worth insisting on for roofs and anywhere hail, kids, or debris feature. The costs: weight (frames and foundations must be stronger), fragility relative to polymers, and the lowest insulation per layer. Single-glass houses in cold climates burn money; double-glazed glass is beautiful and priced accordingly.
Twin-wall and multiwall polycarbonate
The modern default. Two or more walls with air channels between deliver insulation glass can't match per dollar, at 200 times glass's impact resistance and a fraction of its weight. Grades run 4mm twin-wall (entry hobby kits) through 6, 8 and 10mm twin-wall to 16mm triple- and five-wall (serious winter growing). UV-coated faces are mandatory — the coating side goes out — and open flutes must be taped (solid aluminum tape up top, vented tape below) to keep condensation and algae out of the channels. Our replacement panel roundup covers sourcing.
Corrugated polycarbonate
Single-wall corrugated sheets trade the insulation of multiwall for lower cost, higher light transmission and dead-simple installation over purlins — the standard roof for three-season structures, chicken-coop runs (a favorite over at Chicken Coops), and DIY builds where winter heat retention isn't the mission.
Acrylic (PMMA)
Clearer than polycarbonate and more scratch-resistant, with excellent UV stability — but more brittle and typically pricier. It appears mostly in sunroom-class enclosures and premium double-skinned panels rather than mainstream hobby kits.
Fiberglass (FRP)
Once common, now niche: good diffusion and toughness when new, but older formulations yellow and shed fibers as the surface erodes. Modern greenhouse-grade FRP with protective films is serviceable; bargain panels age fast.
Polyethylene film
The hoop-house standard: 6-mil UV-stabilized film, typically 4-year warranted, tensioned with wiggle wire. Upgrades include infrared (IR) films that cut radiant night loss, anti-condensate coatings, diffusing grades, and double-layer inflation for tunnel-scale insulation. Film's economics — cover a huge area now, re-skin in five years — are unbeatable per square foot, as our film vs polycarbonate comparison details.
| Glazing | Light | Insulation | Lifespan | Best for |
|---|---|---|---|---|
| Tempered glass | Highest (~90%+) | Lowest per layer | Indefinite | Sheltered sites, traditional looks |
| 4mm twin-wall poly | Good (~80%) | Moderate | 10–15 yrs | Entry hobby kits |
| 8–10mm twin-wall | Good | Strong | 10–15+ yrs | Cold-climate hobby growing |
| 16mm multiwall | Moderate, diffused | Strongest | 15+ yrs | Year-round winter growing |
| Corrugated poly | High | Low (single wall) | 10+ yrs | 3-season roofs, DIY |
| 6-mil PE film | Good, diffusable | Low (double-layer helps) | 4–6 yrs | Hoop houses, max area per dollar |
Working With Each Material: Installation Notes
Polycarbonate cuts with a fine-tooth circular blade or even a sharp utility knife (score-and-snap on thin twin-wall), pre-drill oversized holes because the material expands and contracts more than the aluminum around it, and always leave the manufacturer's expansion gap at panel edges — panels installed tight in October buckle in July. Fasten with EPDM-washered screws snugged, not crushed; a dimpled panel face means over-torque. Glass is a two-person material full stop: dry-glaze systems with rubber gaskets and clips are far more forgiving than old-style putty, and tempered panels cannot be cut after manufacture, so measure before ordering. Film installs on the calm-morning rule with helpers, tensions progressively into wiggle wire from the center outward, and should never touch bare PVC or pressure-treated lumber directly — both age it chemically; a strip of poly tape or paint over contact points is the cheap fix. Across all materials, the frame's squareness decides the glazing's fate: an out-of-true structure fights every panel and pane, which is why the foundation hour matters more than the glazing brand.
Choosing for Your Situation
Winter grower in a cold climate: 8mm+ multiwall polycarbonate (or double-layer inflated film at tunnel scale) — insulation is your budget line. Three-season gardener: 4–6mm twin-wall or corrugated poly hits the value point. Display house in a sheltered garden: tempered glass earns its keep aesthetically. Maximum protected square footage: film, no contest. And whichever you choose, the structure it rides on still decides survival — pair this guide with the survivability pillar and remember that thicker glazing also stiffens roofs against the snow loads covered in our ratings explainer.
Frequently Asked Questions
What is the best glazing material for a greenhouse?
For most cold-climate growers, 8mm or thicker multiwall polycarbonate offers the best balance of insulation, toughness, and cost. Glass wins on clarity and longevity in sheltered sites; 6-mil UV film wins on cost per square foot for hoop houses.
Is thicker polycarbonate better for a greenhouse?
For winter growing, generally yes — 8, 10, and 16mm multiwall grades insulate substantially better than 4mm twin-wall and stiffen the roof against snow. The trade is somewhat lower light transmission, usually offset by better diffusion.
Which side of polycarbonate faces the sun?
The UV-coated face, which is marked on the protective film. Installing panels backwards exposes the uncoated face and dramatically shortens panel life. Tape the flutes — solid tape up, vented tape down — to keep moisture out of the channels.
How long does greenhouse film last compared to polycarbonate?
Quality 6-mil greenhouse film is typically warrantied four years and often serves five or six; polycarbonate panels commonly serve ten to fifteen-plus years. Film costs far less per square foot, which is why tunnels use it despite the replacement cycle.