Footprint and Headroom
Size splits the range into three. Portable shelving units and cold frames sit under about 3 feet, walk-in tunnels run 6 to 12 feet, and the larger structures reach further. The median listing states 7 feet.
Headroom is the specification that decides whether a structure is walk-in, and it is separate from footprint. A 6 by 8 foot greenhouse with a low eave is worked bent over, which is the complaint that follows most first purchases.
Check the eave height, not the peak.
Glazing and Frame Materials
Polycarbonate panels and polyethylene film are the two glazing types, and they have very different lifespans. Polycarbonate holds up for years and diffuses light. Film is cheap, tears at the grommets, and is treated as a two-to-three-season consumable rather than a permanent skin.
Frames run steel at 44 listings, aluminium at 15 and galvanised at 18. UV resistance appears on 37 listings and is the attribute that separates film that lasts a season from film that lasts three.
Anchoring and Wind
A greenhouse is a sail.
Most of the failures in this category are wind failures rather than material failures, and nearly all of them trace to anchoring that was never installed.
Anchor kits are sold separately more often than not. The screw-in ground anchors in plant supports cover most tunnel and frame structures, and they need soft enough ground to take them.
Cold Frames and Season Extension
Cold frames sit low and hold a few trays rather than a person. They extend a growing season at both ends without heating and cost a fraction of a walk-in structure.
Vents matter more than size on a cold frame.
A closed frame in spring sun overheats within an hour, which is the failure mode that catches people who bought one without a vent or a prop.
Shelving and Portable Units
The smallest format here is a shelf unit with a zip-on cover, typically 2 to 4 tiers and stated in counts up to 20 shelves. These are seed-starting furniture rather than greenhouses and pair with the kits in seed starting.
Greenhouse Types Compared
| Type | Footprint | Glazing | Lifespan | Price band |
|---|---|---|---|---|
| Shelf unit with cover | 2 x 3 ft | PE film | 1 to 2 seasons | $29 to $78 |
| Cold frame | 3 x 4 ft | Polycarbonate | Years | $78 to $150 |
| Walk-in tunnel | 6 x 8 to 10 x 20 ft | PE film | 2 to 3 seasons | $110 to $250 |
| Walk-in hard-sided | 6 x 4 to 8 x 12 ft | Polycarbonate | Years | $193 to $850 |
Film lifespans assume the UV-stabilised grades. Unstabilised film clouds and tears within a season in full sun.
Greenhouse Questions
What size greenhouse is walk-in?
Check eave height rather than footprint. A 6 by 8 foot structure with a low eave is worked bent over, which is the usual complaint after a first purchase.
Polycarbonate or film?
Polycarbonate lasts years and costs more. Film is a two-to-three-season consumable that tears at the grommets, and the UV-stabilised grades last considerably longer.
Do greenhouses need anchoring?
Yes, and anchor kits are usually sold separately. Most failures here are wind failures, and nearly all trace to anchoring that was never fitted.
Why does a cold frame need a vent?
A closed frame in spring sun overheats within an hour. A vent or a prop is the difference between season extension and cooked seedlings.
Are shelf units real greenhouses?
They are seed-starting furniture with a zip-on cover, typically 2 to 4 tiers. They suit hardening off rather than growing on.