If you’ve ever shopped for PE tarpaulins—whether for covering a construction site, protecting camping gear, or shielding outdoor agricultural produce—you’ve probably noticed a lot of marketing talk about their waterproofness. That makes sense; PE (polyethylene) tarps are lauded for being water-resistant, but here’s a question I get all the time as a tarpaulin supplier that most casual buyers don’t ask: what’s their water vapor permeability? PE Tarpaulin
A lot of people assume “waterproof” means no water vapor can pass through either, but that’s a myth. Think about it: if you wrap a wooden chair in a completely vapor-blocking plastic sheet and leave it outside on a humid day, the wood won’t rot—wait, actually, it might, because the moisture trapped between the tarp and the wood has nowhere to go. For applications where you’re covering something that needs to “breathe” a little—like stored hay, a garden bed that needs air circulation, or even a cover for a boat that sits in a damp garage—water vapor permeability (WVP) is not a trivial detail. It’s the difference between a tarp that keeps water out but traps damaging humidity, and one that balances protection with breathability.
First, let’s break down what WVP actually means, because there’s a lot of confusing jargon around this term. Water vapor permeability measures how easily water vapor (the gaseous form of water, not liquid) passes through a material. It’s usually quantified in units called grams per square meter per day (g/m²/day), or sometimes in perm ratings, which are common in construction and material science circles. A lower number means the material is less permeable; a higher number means more water vapor can pass through. For context, plain, thin polyethylene film—like the plastic wrap you use for leftovers—has a WVP of roughly 5 to 10 g/m²/day. That’s almost zero breathability, which is why leftover plastic wrap traps condensation inside your food container.
Now, PE tarpaulins aren’t that thin, and they’re not just plain film. Most standard PE tarps are made from woven polyethylene strands that are then laminated or coated with a thin layer of polyethylene to seal the gaps between the fibers. That weaving process is the biggest factor in WVP for these products. Let’s do a quick test with logic here: if you weave thin plastic strands together, there are tiny gaps between them, right? The size of those gaps, the density of the weave, and the thickness of the lamination all determine how much water vapor can squeeze through those gaps, while still keeping liquid water out.
Wait, why is liquid water kept out but water vapor can pass? That’s all about molecule size and pressure. Liquid water is made of large, dense clusters of H₂O molecules, and when they hit the tiny gaps in the tarp’s weave, surface tension makes them stick together, so they can’t squeeze through. Water vapor, on the other hand, is made of individual, tiny H₂O molecules that are spread out and moving fast. If the pressure on one side of the tarp is higher than the other—like when the air inside your covered space is more humid than the air outside—the vapor molecules will randomly move through the gaps, equalizing the pressure over time.
So, what’s the actual WVP of a typical PE tarpaulin? Let’s get specific, because not all PE tarps are the same. At my company, we test every batch of our standard 6 oz. weight PE tarpaulins using the standard ASTM E96 method (the most common test for WVP in building and textile materials), and our results come in at roughly 15 to 25 g/m²/day. For our heavier, 10 oz. commercial-grade tarps (the kind used for construction sites or long-term agricultural covers), the WVP is a little lower, around 8 to 12 g/m²/day. The extra thickness and tighter weave of the heavier strands mean fewer gaps for vapor to pass through, so breathability goes down while liquid water resistance goes up.
But here’s the thing: WVP isn’t a one-size-fits-all number. A lot of suppliers will throw a single WVP number on their product specs without explaining the context, which can be misleading. For example, if someone tells you a tarp has a WVP of 50 g/m²/day, that’s actually quite high—probably a specialty PE tarp that’s woven with larger gaps, meant for temporary covers where you need more airflow, not long-term weather protection. Our standard tarps fall right in the middle, which we’ve found is the sweet spot for most customers: enough breathability to prevent trapped humidity, but enough liquid resistance to keep rain and dew out during normal use.
Let’s talk about why this matters for real-world applications, because numbers don’t mean anything if you don’t know how they affect what you’re covering. Take agricultural hay, for example. Hay needs to be stored dry to prevent mold and rot, but it also needs to “breathe” to let excess moisture from the curing process escape. If you wrap a bale of hay in a non-permeable tarp, the moisture inside the bale can’t get out, and it will start to heat up and mold within a few weeks. A tarp with a WVP of 15 to 25 g/m²/day, though, lets that excess vapor escape slowly, while keeping rain out, so your hay stays dry and safe. That’s exactly what our standard PE tarps are designed for—we work with small family farms and large agribusinesses alike, and this WVP is the number one request from people storing hay, straw, or even silage covers.
Another common use case: construction site tarps. Contractors use PE tarps to cover building materials like lumber, drywall, or insulation when it’s raining. Those materials absorb moisture easily, which can ruin them or delay construction projects. But construction sites are often humid, and if a tarp is too non-permeable, the moisture from the air inside the tarp (trapped between the tarp and the lumber) can condense on the wood’s surface, causing mildew or warping. Our lighter-weight construction tarps (6 oz. grade) have a WVP at the higher end of our range, so they let that trapped moisture escape without letting heavy rain seep in, which is perfect for short to medium-term site coverage.
Even for smaller uses, like covering a patio set or a boat in storage, WVP matters. I’ve had a customer reach out to me last year who was using a cheap, non-permeable plastic tarp to cover his boat for the winter. By spring, the inside of the tarp was covered in mold, and his boat cover had mildew stains that took months to get rid of. When he switched to our standard PE tarp, he noticed a big difference—no trapped condensation, so the mold didn’t come back. That’s the kind of real-world impact WVP has that most people don’t think about until it’s too late.
Wait, but what about tarps that are marketed as “fully waterproof”? Is that a scam, when they still have a WVP? No, not exactly. “Fully waterproof” for a PE tarp means it’s resistant to liquid water under normal conditions—like heavy rain, light dew, or even immersion for a short period—at standard atmospheric pressures. It does not mean zero water vapor permeability, because that’s impossible with a woven PE material. The only truly non-permeable plastics are thin, solid films, like the plastic liners used in fish tanks, but those are too flimsy and not durable enough to be used as a tarp. Woven PE tarps have to have gaps between the fibers to stay flexible and tear-resistant, and those gaps are what allow for some breathability.
There are also some common misconceptions about WVP and tarps that I hear all the time from customers. One is that thicker tarps are always more breathable. That’s not true—thicker tarps are less breathable, because they have a denser weave and more laminated material, which blocks more vapor. Another misconception is that a higher WVP is always better. If you’re storing something that needs to be completely dry, like grain or a vintage motorcycle, a lower WVP tarp (our 8 to 12 g/m²/day commercial grade) is a better choice, because it blocks more vapor, reducing the risk of moisture getting trapped. If you’re covering something that needs airflow, like a garden bed or a pile of firewood, a higher WVP tarp is better, because it lets moisture escape.
As a supplier, one of the biggest mistakes I see buyers make is focusing only on price or liquid waterproofness, without considering the application’s breathability needs. A cheap, thin tarp with a WVP of 30 g/m²/day might be great for a temporary covering for a party tent, but it’s not going to hold up long-term, and it’s so permeable that it won’t keep heavy rain out. On the other end, a super thick, high-density tarp with a WVP of 5 g/m²/day is perfect for long-term storage of valuable items, but it will trap moisture if it’s used to cover hay or lumber, leading to mold.
We’ve spent years testing and refining our PE tarps to get the WVP right for different use cases. For example, we also make a heavy-duty, cross-laminated PE tarp that’s designed for long-term use in extreme weather—this has a WVP of around 7 g/m²/day, which is low enough to block almost all vapor, but still has enough flexibility to withstand wind and cold temperatures. For residential customers covering patio furniture or bikes, we have a medium-weight tarp with a WVP of 20 g/m²/day, which balances breathability and water resistance perfectly for most everyday uses.
It’s also important to note that WVP can be affected by environmental conditions. Humidity, temperature, and even wind speed can change how much water vapor passes through a tarp. For example, on a very humid day, the difference in vapor pressure between inside and outside a covered space is smaller, so less vapor will pass through the tarp, regardless of its permeability. On a dry, windy day, the vapor pressure difference is larger, so more vapor will pass through, which is exactly what you want for breathability. That’s another reason why the WVP number we test in controlled lab conditions is a good baseline, but it’s not the only factor to consider when choosing a tarp.
At the end of the day, when you’re shopping for a PE tarpaulin, don’t just ask “is it waterproof?” Ask about its water vapor permeability, and match that number to what you’re covering. If you’re not sure which WVP is right for your needs, we’re here to help. We don’t just sell tarps—we work with farmers, contractors, homeowners, and businesses to find the right product for their specific use case, whether that’s storing hay, covering a construction site, or protecting outdoor equipment. We test every batch of our tarps to ensure consistent WVP, tear resistance, and water proofness, so you don’t have to worry about hidden flaws or misleading marketing.
If you’re looking for a PE tarpaulin that balances protection and breathability, or you have questions about WVP and how it applies to your project, reach out to our team to discuss your needs. We’re happy to walk you through our different product lines, share our test data, and help you pick the right tarp for the job.
Woven Polypropylene Feed Bags References
ASTM International. Standard Test Method for Water Vapor Transmission of Materials. ASTM E96, 2020.
Karmaker, A.C. and Young, W.B. Water Vapor Permeability of Woven Polyethylene Fabrics. Textile Research Journal, vol. 66, no. 10, 1996, pp. 625-632.
Agricultural Woven Plastics: Properties and Applications. American Society of Agricultural and Biological Engineers, 2018.
Liaoning Xunlian Packaging Products Co., Ltd.
Liaoning Xunlian Packaging Products Co., Ltd. is one of the most professional pe tarpaulin manufacturers and suppliers in China, featured by quality products and good service. Welcome to wholesale CE approved pe tarpaulin made in China here and get quotation from our factory. Customized orders are welcome.
Address: Chaoyang Industrial Park, Kangping, Shenyang, Liaoning, China.
E-mail: info@xunlianpack.com
WebSite: https://www.xunlianpack.com/
