A practical framework for matching greenhouse cover thickness to your climate, frame, and growing season — plus how to verify the mil thickness you actually received.
Greenhouse cover thickness gets described in a single number — 3 mil, 6 mil, 10 mil — and that number gets treated like it settles the question. It doesn’t. Mil thickness is a starting point, not a decision. The real question is what that thickness has to survive: how many seasons, what wind and snow load, what kind of frame, and whether you need total blackout or maximum light.
Mil is a unit of measurement equal to one one-thousandth of an inch — not a millimeter. A 6 mil film measures .006 inches. An imperial millimeter runs about 39 mils, so the gap between “6 mil” and “6 mm” is enormous. That part hasn’t changed since growers first started asking this question. What follows is a framework for actually answering it.
Match Thickness to the Job, Not Just the Season
Four variables drive the decision: how long the cover needs to last, how exposed the site is to wind and snow, what the frame is made of, and whether the crop needs full light or blackout control. Run your project through this matrix before you order:
| Use Case | Exposure | Recommended Spec |
|---|---|---|
| Frost/freeze protection, single winter | Low-moderate | 3–5 mil, non-reinforced |
| Standard 1–2 season greenhouse cover | Low-moderate | 6 mil, non-reinforced |
| Light deprivation / blackout, daily pull | Moderate, repeated handling | 8 mil, string-reinforced |
| Multi-year, permanent structure | High wind/snow | 10 mil, scrim-reinforced |
Frame material factors in separately. Metal ribs and purlins abrade film at every contact point regardless of mil thickness — a 10 mil cover rubbing against a bare metal rib will wear through before a 6 mil cover with a protective barrier at those same points. If your frame is metal, budget for a contact barrier at rib points as part of the spec, not as an afterthought.
What Thickness Actually Costs You Over Time
The sticker price on a roll of film isn’t the real cost — replacement frequency is. A non-reinforced 6 mil cover in a high-wind location may need replacing every season or two. A scrim-reinforced 10 mil cover built for multi-year exposure costs more per roll but spreads that cost across several seasons instead of one, while also cutting the labor of re-covering the structure each time.
Americover’s own product data backs this up: one commercial grower in Wyoming reported that a standard 6 mil, non-reinforced cover failed in under two years under extreme wind. After switching to a 1000-denier scrim-reinforced 10 mil cover, the same site held up through repeated windstorms exceeding 90 mph, including a derecho with gusts to 110 mph, without a single hole. That’s the reinforcement variable in practice — not a hypothetical, a documented result from a live installation.
Run the math for your own site: count the number of times you’ve re-covered a structure in the last five years, and weigh that against a reinforced option built to hold for four seasons instead of one or two.
Verifying the Thickness You Received
Mil thickness on a spec sheet and mil thickness in your hands aren’t guaranteed to match without a check. Plastic film thickness is measured with a micrometer or handheld thickness gauge, following the methodology outlined in ASTM D6988 (Standard Guide for Determination of Thickness of Plastic Film Test Specimens). A basic gauge lets you spot-check a delivered roll against the ordered spec before installation — worth doing on any order where thickness affects a warranty claim or a code requirement, and a five-minute check that can save a return-and-reorder cycle later.
Manufacturing tolerances mean a small variance between nominal and measured thickness is normal; a large gap is not, and is worth raising with your rep before the material goes up.
| Product | Image | Key Spec | Link |
|---|---|---|---|
| Overwinter Greenhouse Film |  |
3–5 mil, clear or white [see note below] | View product → |
| Clear Greenhouse Plastic Sheeting |  |
6 mil, non-reinforced, 89% light transmission | View product → |
| BOLD® Black Out Light Deprivation Tarp |  |
8 mil, string-reinforced, 100% blackout | View product → |
| SolaScrim™ 4 Year 10 mil Reinforced Cover |  |
10 mil, 1000-denier scrim, 84% light transmission | View product → |
| Reinforced Greenhouse Repair Tape |  |
10 mil, 4in x 50ft, clear/white/black | View product → |
Frequently Asked Questions
What mil thickness should I use for a greenhouse cover?
It depends on how long the cover needs to last and how exposed the site is. Use 3–5 mil for single-season frost protection, 6 mil for a standard 1–2 season cover, and 8–10 mil reinforced for light deprivation systems or multi-year, high-wind installations.
Does a thicker greenhouse cover always last longer?
No. Reinforcement — string or polyester scrim — is what determines tear and wind resistance, not raw mil thickness alone. A reinforced film can outlast a thicker non-reinforced film in demanding conditions.
How do I check that the plastic I received matches the mil thickness I ordered?
Use a handheld thickness gauge or micrometer, following the measurement approach in ASTM D6988. Spot-check a few points on the roll and compare against the ordered spec before installation.
Do I need a barrier tape between the cover and a metal frame?
Yes, for metal frames — direct contact at ribs and purlins wears through film over time regardless of mil thickness. Ask your Americover rep about current barrier tape options for your frame type, since stock availability changes.
Can I use 3–5 mil overwinter film for a permanent greenhouse?
No. Overwinter film is built for single-season frost and freeze protection, not year-round structural use. For a permanent structure, look at 6 mil standard film at minimum, or a reinforced option for multi-year exposure.
Get the Right Cover the First Time
Whether you’re protecting one season’s crop or building for the long haul, Americover’s greenhouse plastic sheeting is manufactured in the USA and backed by 30+ years of experience matching mil thickness and reinforcement to real growing conditions. Browse greenhouse plastic sheeting and talk to a specialist → Or call 760-388-6294 to talk through your specific site conditions.
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