Buy plastic that is too thin and you will be re-covering the frame after one rough winter, paying for the whole job twice. Greenhouse plastic, the polyethylene film stretched over a frame, comes in thicknesses from about 3 to 12 mils, and the right number depends on your climate, how many seasons you need it to last, and the crops underneath it.
TL;DR: Most growers should use 6 mil polyethylene film. It transmits about 90% of light, handles moderate snow and wind, and lasts 2 to 4 years. Use 3 to 4 mil for a single season on low tunnels, and 8 to 12 mil for cold climates or commercial structures that need to last 5 years or more.
For most full-season backyard and hobby greenhouses, 6 mil polyethylene film is the right call. The usable range runs 3 to 12 mils (1 mil = 0.001 inch), but 4 to 8 mils covers nearly every home and small-farm use. Polyethylene is the most common greenhouse glazing because it transmits light well at a low cost, according to the University of Massachusetts Amherst greenhouse program.
Here is the quick map of what each band does:
The pattern is simple: thicker film lasts longer and insulates better, but costs more and is slightly less transparent. UMass Amherst notes that greenhouse-grade polyethylene is sold only in 6 mil because that is the thickness built to survive multiple years rather than one.
Thickness is a balance of four things: how cold your winters get, how many seasons you need the film to last, how much light your crops demand, and what weather the structure has to survive. Get those four straight and the right mil number falls out on its own. For choosing the whole structure, not just the covering, our greenhouse buyer guide walks through frame, size, and glazing together.
Match thickness to your lowest expected temperature, not your average. Thicker film slows heat loss, so colder regions need more of it. In moderate winters, 4 to 6 mil is enough; cool-summer regions do better with 6 to 8 mil; cold and very cold zones move up to 8 to 16 mil for real insulation on frigid nights.
Use this climate-to-thickness table as your starting point, then adjust for the other factors below.
| Climate | Recommended Thickness |
|---|---|
| Moderate | 4-6 mil |
| Cool Summer | 6-8 mil |
| Cold Winter | 8-12 mil |
| Very Cold Winter | 10-16 mil |
In genuinely cold zones, many growers run a double layer of poly with an air gap between the sheets, which adds far more insulation than thickness alone. A small inflation blower keeps the two layers separated and taut.
Pick thickness by how long you want to go between re-coverings. Thinner 3 to 4 mil film typically lasts a single growing season. Step up to 6 to 8 mil and you get 2 to 4 years; 10 to 12 mil films reach 4 to 6 years with proper care. Greenhouse-grade poly carries UV stabilizers that slow the sun damage that wrecks cheap film, which is why it is warranted for 4 years or more, per UMass Amherst.
Weigh lifespan against replacement cost. Re-covering a frame is labor as well as material, so a film that lasts three seasons instead of one often wins on total cost even at a higher sticker price. If you would rather skip re-covering altogether, our roundup of the best polycarbonate greenhouses compares twin-wall kits that last a decade or more.
Going thicker barely costs you any light. Most polyethylene greenhouse films transmit over 90% of light, and moving from 4 mil up to 8 mil reduces transmission by only 1 to 2%. That difference matters only if you grow high-light crops like tomatoes or peppers, where every percent of summer sun counts toward yield.
For those crops, lean toward the thinner end of your climate range and keep the film clean, since condensation and dust cut transmission more than thickness ever will. For leafy greens and most ornamentals, the insulation from thicker film is the better trade.
In areas with heavy snow or strong wind, 6 to 8 mil film resists tearing and puncture far better than thin sheeting. Reinforced poly with an internal scrim (a woven grid layer) adds even more tear strength, which is why woven films are used on exposed walls. UGA Extension specifies 10-mil woven polyethylene for the load-bearing south wall on its passive-solar season-extension design.
Thickness alone will not save a poorly framed greenhouse. Proper purlin spacing and tight film tension carry snow load; the film just has to resist ripping where it stretches over the frame.
Most plants grow fine under clear 4 to 6 mil film. A few, like orchids and onions, prefer diffused light, and thicker or light-diffusing films scatter direct sun more evenly across the canopy. If your crops are not light-sensitive, do not pay for diffusion you will not use; spend the budget on a thickness that survives your winters instead.
Thicker film costs more up front. Film for a small structure in the 4 to 6 mil range typically runs about $100 to $600, while commercial-scale 8 to 12 mil film for large greenhouses can reach $1,500 to $5,000 installed. Frame it as cost per season, not cost per roll: thin film replaced every year can quietly cost more over five years than a thicker film bought once.
Each thickness band has a clear job. Here is what 4, 6, 8, and 10 to 12 mil film deliver on light, lifespan, and weather resistance, so you can pin your choice to a specific use.
The takeaway holds across the range: thicker means more durable, longer-lasting, and better-insulating, at the cost of slightly lower transparency and a higher price. For the rigid-panel equivalents, see our breakdown of 4mm vs 6mm polycarbonate panels.
If you want a covering that lasts a decade instead of a few seasons, twin-wall polycarbonate is the alternative to film. These rigid multi-wall panels typically run 4 to 16 mm thick (panels are measured in millimeters, film in mils) and trade the low cost of poly film for far longer life and better insulation. UGA Extension rates 6 mm twin-wall polycarbonate at an R-value of 1.6, well above single-layer film.
What polycarbonate gives you:
What it costs you:
Polycarbonate suits permanent structures where the higher investment pays back over years, which is why most kit greenhouses ship with it rather than film. It also shrugs off hail and stray branches that would puncture poly, and it does not need re-covering every few seasons. You can browse rigid-panel options in our polycarbonate greenhouse collection. For film-covered high tunnels and seasonal hoophouses, polyethylene is still the more cost-effective choice.
The right thickness only pays off if the film is installed and maintained well. A few habits add seasons to any covering:
These habits matter more than buying the thickest film you can find. A well-supported, clean, UV-stabilized 6 mil cover routinely outlasts a neglected 10 mil one.
Right thickness, right job: 6 mil polyethylene for most full-season greenhouses, 3 to 4 mil for single-season tunnels, and 8 to 12 mil for cold climates and structures that need to last five years or more. Match the mil to your winters, your timeline, and your crops, then install and maintain the film well so it reaches its full lifespan. If you are weighing a rigid build instead, our polycarbonate vs glass comparison covers the two most common permanent options.
Ready to skip the film entirely and go with a long-lasting rigid build? Browse our greenhouse kits for sale to compare polycarbonate and glass structures built to last for years.
For most growers, single-layer polyethylene film in the 4 to 8 mil range is the better value: it costs less, installs faster, and transmits great light for plant growth. Choose twin-wall polycarbonate when you want a near-permanent structure, since it is tougher and insulates better but costs significantly more and needs a rigid frame.
Lifespan depends on thickness and installation. Thin 3 to 4 mil film usually lasts one growing season, 6 mil film lasts 2 to 4 years, and 8 to 12 mil film can reach 5 to 8 years with care. UV-stabilized greenhouse-grade poly is warranted for 4 years or more. Regular inspection, cleaning, and prompt repairs help any film reach its full lifespan.
A common rule of thumb is about 4 square feet of film per square foot of greenhouse, which leaves enough overlap to fasten and tension the covering. Measure the full surface area you need to cover, including roof, sides, and end walls, then add that overlap margin. Most growers use UV-treated polyethylene of at least 6 mil for full-season structures.
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