A polycarbonate greenhouse is the compromise almost everyone should make. It loses roughly 40 percent less heat than single-pane glass, it does not shatter into a bed of lettuce when a branch falls on it, and it costs a fraction of what a glass house of the same footprint does. The pick for most home growers is the Palram Canopia Mythos 6 x 6 at around $622.42, because it pairs 36 square feet of 4 mm twin-wall with something no budget kit offers: a manufacturer who still sells the clip, the channel and the panel three years from now.
What separates a good polycarbonate house from a cheap one
Polycarbonate greenhouse kits look almost identical in listing photographs, and the specification that actually differs between them is rarely printed. Three things decide whether a kit survives five winters.
The UV layer. Polycarbonate is not naturally UV stable. Quality panels carry a co-extruded UV-blocking layer on the outer face, which is what keeps the sheet clear and flexible for a decade. Panels without it yellow in three years and go brittle enough to crack when you lean a ladder against them. The layer is directional and the only marking is usually printed on the removable protective film, so a panel fitted the wrong way up fails early and there is no way to tell by looking.
The frame section. The extruded aluminum profile on a budget kit is often under 1 mm wall thickness, which racks under wind load and lets panels walk out of their channels. The tell in a listing is the stated snow load: a frame rated to 20 pounds per square foot or better has enough section to hold its geometry. Anything that does not state a snow load at all generally does not have one worth stating.
The vent area. A greenhouse with one small roof vent and no side louvres cannot ventilate itself passively. Standard horticultural practice is roughly one fifth of floor area in total vent opening, and almost no hobby kit ships with that. Budget for a second vent or a powered fan on any house you plan to use through summer. Our ventilation CFM calculator gives the forced-air figure and the fan CFM chart lists it by footprint.
| Greenhouse | Floor area | Glazing | Price at writing | Best suited to |
|---|---|---|---|---|
| 6 x 4 ft Greenhouse with Built-In Hygrometer | 24 sq ft | 4 mm twin-wall | $149.99 | A first frame on a patio |
| Glanzend 6x8 Polycarbonate Walk-In Greenhouse | 48 sq ft | 4 mm twin-wall | $143.65 | Cheapest walk-in entry |
| Palram Canopia Mythos 6 x 6 Greenhouse | 36 sq ft | 4 mm twin-wall | $622.42 | Most home growers |
| Palram Canopia Hybrid 6 x 10 Greenhouse | 60 sq ft | Clear roof, twin-wall sides | $713.99 | Winter light priority |
| Palram Canopia EcoGrow 6 x 8 Greenhouse | 48 sq ft | 4 mm twin-wall throughout | $1,049.99 | Diffused light growing |
| Palram Canopia Oasis Hex 7 x 8 Greenhouse | 56 sq ft | 4 mm twin-wall, hex plan | $1,599.99 | An ornamental focal point |
| Broivira 8x12 Polycarbonate Greenhouse | 96 sq ft | 4 mm twin-wall | $599.99 | Benching down both sides |
| 10 x 12 ft Polycarbonate Greenhouse with Roof Vent | 120 sq ft | 4 mm twin-wall | $599.99 | Year-round production |
| 10 x 18 ft Aluminum Walk-In Polycarbonate Greenhouse | 180 sq ft | 4 mm twin-wall | $759.99 | A working center aisle |
The three tiers
Beginner
Buy once
Expert
How we chose
We did not test these greenhouses in person, and any site that claims hands-on testing of nine greenhouse kits is telling you a story. What we did instead was compare published manufacturer specifications, the stated glazing thickness and frame material, listed snow and wind load ratings where the maker publishes them, the availability of replacement parts, and verified owner reviews with an emphasis on reports from the second and third season rather than the first week.
That last filter matters more than it sounds. Almost every polycarbonate greenhouse reads well in its first month, because assembly frustration and initial impressions dominate early reviews. The failures that decide whether a kit was worth buying, panels walking out of channels, frames racking, UV degradation, door hardware breaking, all surface between month twelve and month thirty-six. We weighted reviews from that window heavily and discounted the first fortnight almost entirely.
We also disqualified anything where the maker does not state glazing thickness. A listing that says "polycarbonate panels" with no millimetre figure is usually 0.7 mm corrugated single-wall, which has an R-value near 0.83, barely better than glass, with none of the light transmission that makes glass worth its heat loss.
What size do you actually need?
Buy one size larger than the plan you sketched. This is the most consistent regret in hobby greenhouse ownership and the reason is arithmetic rather than ambition. A 6 by 8 house has 48 square feet of floor. A 24 inch bench down one side consumes 16 of them. A 24 inch path consumes another 16. Storage for pots, bags of compost, a watering can and a hose takes 6 to 8. What is left for growing is around 8 to 10 square feet, which is two large tomato containers.
Work it the other way instead. Decide what you want to grow, then size the house from the space that crop needs. Our greenhouse size calculator works backwards from a target harvest, and the plant spacing chart gives the per-plant footprint for each crop under cover, which is consistently larger than the outdoor figure because airflow requirements are stricter.
The heat cost of every extra square foot
Floor area is not what drives heating cost, glazing surface area is, and the two do not scale together. Going from a 6 by 6 to a 6 by 10 adds 67 percent more floor but only about 45 percent more glazing surface, because the two gable ends are already paid for. Going from a 6 by 10 to a 10 by 18 adds 200 percent more floor and around 160 percent more surface. Larger houses are proportionally cheaper to heat per square foot of growing space, which is a genuine argument for buying big that most buying advice misses.
What breaks that argument is the electrical supply. A 120 V circuit tops out at 1500 W, which is 5,120 BTU per hour, and that covers a 6 by 8 twin-wall house at a 30 degree F differential with almost nothing spare. Past that you need a 240 V heater, a lower target temperature, or the acceptance that the house is frost-protection rather than heated. Run your own numbers in the heater BTU calculator before you commit to a footprint, not after.
Twin-wall, triple-wall or clear single-wall?
Twin-wall polycarbonate at 4 mm is the default on hobby kits and it is a reasonable default. It diffuses light, which is genuinely beneficial: diffuse light penetrates a plant canopy and illuminates lower leaves that direct beam light never reaches, so a diffusing glazing can produce more total photosynthesis than a clearer one despite transmitting less light overall.
Triple-wall and thicker multiwall sheets raise the R-value further, to around R-2.1 at 8 mm, at the cost of dropping photosynthetically active transmission below 75 percent. In a northern winter, where the natural daily light integral inside a greenhouse can fall under 5 mol per square meter per day against a lettuce target of 12 to 17, that transmission loss is expensive. The full comparison is in twin-wall versus triple-wall and the numbers are tabulated in the glazing R-value and light transmission chart .
Clear single-wall roof panels over twin-wall sides, which is what the Palram Hybrid 6 x 10 does, is a deliberate and sensible compromise. The roof is where the winter light comes from and where the transmission penalty hurts most; the walls are where heat is lost to wind and where insulation matters more. Splitting the specification between the two is a better answer than picking one glazing for the whole envelope.
The parts you will need that the kit does not include
Every polycarbonate greenhouse kit ships incomplete in the same four ways. None of these are scandals, they are simply not in the box, and budgeting for them at purchase avoids the pattern where a $600 greenhouse turns into a $900 project over three separate deliveries.
What to buy alongside the greenhouse
Assembly, honestly
Expect a full day for a 6 by 8 and a weekend for anything above 100 square feet, with two people. The single-person assembly figures in listings assume no wind, a perfectly level base and no mistakes. None of those hold.
Three practical points that come up in owner reports consistently. First, build the base perfectly level and perfectly square before any frame goes on it, because every subsequent tolerance stacks on that one and a frame that is 10 mm out of square will not accept its panels. Second, do not remove the protective film from the panels until they are in the channels, and check which side carries the UV printing first. Third, do not fully tighten any frame bolt until the whole frame is assembled and squared, then work around tightening in sequence, the way you would a wheel.
Who should not buy a polycarbonate greenhouse
Two groups. If your entire goal is starting transplants in spring and hardening them off, a cold frame plus an indoor light rack does that job for under $150 and needs no anchoring, no vent automation and no heating. Our beginner cold frame build is that whole setup at $421 including the lights and the propagation gear, and it will start more transplants than a 6 by 6 greenhouse will.
If, on the other hand, you want maximum ground area under cover for the money and you are growing in-ground rather than on benches, a hoop tunnel gives you three to five times the covered square footage per dollar. It insulates poorly and it will not hold a frost-free minimum, but for extending a vegetable garden's season at both ends rather than growing through winter, the economics are not close. See hoop house versus greenhouse for the full comparison, and the tunnel kit picks if that is the direction.