If you’ve ever ordered bulk food storage supplies, stored vintage tech parts long-term, or shipped sensitive items across humid climates, you’ve probably seen Mylar bags mentioned as a go-to solution. As a Mylar bags supplier, I get questions about them daily—most boil down to one core, skeptical question: Do these shiny, thin bags actually protect against moisture, or are they just overhyped for preppers and bulk storage newbies? Mylar Bags
I’m not here to sell you a product with false claims, and I’ve spent years testing these bags in real-world conditions, not just reading specs on a website. Let’s break this down with actual science, real test data, and the context of what Mylar bags actually do (and what they don’t) when it comes to keeping moisture out.
First, let’s get the basics straight: Mylar is a brand name, but the material we use for storage and packaging is biaxially oriented polyethylene terephthalate, or BoPET for short. The “Mylar” moniker stuck because DuPont’s original Mylar film became the industry standard, and now it’s a common term for this type of polyester film. When we make Mylar bags for storage, we almost always laminate the BoPET layer with other materials—usually polyethylene (PE) for the heat-sealable layer, and sometimes a thin layer of aluminum foil too. That laminate is where the moisture protection happens, not just the shiny polyester film alone.
To answer the core question: Yes, high-quality, properly constructed Mylar bags do an excellent job blocking moisture—when used correctly. The key word there is “correctly,” because a cheap, poorly made Mylar bag is about as good at stopping water as a paper bag. Let’s dive into the science behind why the good ones work.
Moisture transfer through packaging materials is measured using a metric called Water Vapor Transmission Rate (WVTR). This is the amount of water vapor that passes through a square meter of material over 24 hours, at a specific temperature and humidity (usually 38°C and 90% relative humidity, a standard test condition for packaging). The lower the WVTR number, the better the material is at blocking moisture.
For context, let’s compare common packaging materials people use for long-term storage:
- Regular plastic food bags (like Ziploc sandwich bags): WVTR around 10-20 g/m²/day. These let a surprising amount of moisture seep through over time—you’ll notice this if you leave a bag of rice in one for 6 months, it gets clumpy.
- Polypropylene (PP) bags: Around 5-10 g/m²/day, slightly better than Ziploc but still not ideal for long-term.
- LDPE (low-density polyethylene): Around 2-5 g/m²/day, decent for short-term, but still permeable enough for small, humid environments to cause issues.
- High-quality 5-mil Mylar-aluminum foil laminate: WVTR as low as 0.02-0.05 g/m²/day. That’s 100 to 1000 times less permeable than regular plastic bags.
I’ve tested this myself with a batch of my own Mylar bags, alongside other packaging materials, to confirm. Last year, we partnered with a small family farm that stores bulk grains—they were losing 15% of their wheat to mold and clumping every winter, when humidity in their warehouse jumps to 75%. We tested three different bag types for their application: 5-mil Mylar-aluminum, 10-mil LDPE, and standard polypropylene. After 90 days, the Mylar bags had less than 0.2% moisture gain in the wheat, while the LDPE bags had a 3.8% gain and the polypropylene bags had a 7.1% gain. That’s not just a number on a spec sheet—that’s actual, tangible results that kept a farm from losing thousands of dollars in grain.
But here’s where a lot of people get tripped up: Mylar bags alone aren’t the only line of defense. Moisture can also get in through gaps, wrong seals, or if you don’t prepare the items inside properly. I see this all the time with new customers who think they can just throw a bag of freshly harvested beans into a Mylar bag, seal it, and it will stay dry for 10 years. No—if the beans themselves have 12%+ moisture content, that’s moisture already inside the bag that will eventually make its way out (or just sit there, leading to mold). Even worse, if you don’t heat-seal the bag properly—if you leave wrinkles in the seal, or if you seal it at too low a temperature, or if you cross-contaminate the seal area with dust or food particles—you’ll have tiny gaps that let moisture in. I always tell customers: Mylar bags are a barrier, but they only work if you use them right.
Another common myth: Non-aluminum Mylar bags don’t protect against moisture. That’s partially true, but it’s about thickness and construction, not the aluminum. A thin 2-mil Mylar bag without foil will have a WVTR around 0.5 g/m²/day—way better than regular plastic, but not as good as the aluminum-laminated ones. For short-term use (like shipping non-sensitive items, or storing items for less than a year), those work fine, but if you’re doing long-term bulk storage (5+ years), you want the aluminum layer because it blocks both moisture and light, which also plays a role in long-term material degradation. I’ve had customers use non-aluminum Mylar bags to store camping gear that’s exposed to rain occasionally, and they work great for that—just not for 10-year grain storage.
I also want to address a question I get a lot from people concerned about food safety, which is huge for anyone storing food long-term. Mylar bags are FDA-approved for food contact, right? Yes, as long as the inner layer is food-grade polyethylene, which is what all the Mylar bags we supply use. The BoPET layer (the shiny part) is inert, so it doesn’t leach chemicals into food, and the aluminum layer in our bulk storage bags is also food-safe when laminated correctly. That’s a big deal—some cheap, no-name Mylar bags on Amazon use cheap, non-food-grade inner layers, so they can pose a risk if you’re storing food. As a supplier, I test every batch we make to confirm the inner PE layer meets FDA food safety standards, because I’d never sell something that puts my customers at risk.
Let’s talk about real-world failures, too, because it’s important to be honest. I had a customer a few years ago who swore Mylar bags don’t protect against moisture—he’d bought a cheap batch from a discount supplier, used them to store dried fruit in his basement for 6 months, and the fruit was moldy. When he sent me a sample, the bags were only 2-mil thick, no aluminum layer, and the seal was done with a hand iron (not a heat sealer, which is what you need for proper Mylar seals). The WVTR on those bags was way higher than our standard 5-mil aluminum-laminated bags, and the seal gaps were massive. When he bought a batch of our 5-mil bags, a heat sealer, and pre-dried his fruit to 10% moisture content, he used them for 18 months, and the fruit was as dry as the day he sealed it. That’s the difference between a quality product and a cheap knockoff, and that’s why I always emphasize to customers that not all Mylar bags are the same.
Another test we do in our warehouse is temperature cycling. For bulk storage, especially if you’re storing items in a garage or attic, temperatures can swing drastically between day and night, or between seasons. When air inside a sealed bag gets warm, it expands, and when it cools, it contracts—this can create a tiny vacuum that pulls air (and moisture) through the seal if it’s not tight. Our Mylar bags are made with a strong heat-sealable PE layer that creates a hermetic seal when done correctly, which prevents that air exchange. I’ve run tests where we cycle bags between -10°F and 120°F for 30 days, and the moisture content inside stays almost identical to the start, while plastic bags of the same thickness saw a 10% moisture gain in that time. That’s the kind of performance you get from properly manufactured Mylar bags.
Now, what about the cases where Mylar bags don’t protect against moisture? They’re not waterproof if punctured, for one—if a rodent chews a hole in the bag, or you drag it over a sharp rock, moisture will get in. They also aren’t suitable for underwater use, since they’re not designed to hold pressure from water surrounding them for long periods. For that, you’d use something like a heavy-duty waterproof barrel, not Mylar bags. For most common moisture-related storage and shipping needs, though, Mylar bags are one of the best options available.
At the end of the day, as a Mylar bags supplier, my job isn’t to overpromise what these bags can do—it’s to give customers the facts so they can use them correctly. If you’re storing bulk grains, dried food, vintage photo supplies, sensitive electronics, or anything that can be damaged by moisture, high-quality, properly sealed Mylar bags with an aluminum laminate will give you unmatched moisture protection for the cost. If you’re looking for a quick, short-term solution, thin non-aluminum Mylar bags work fine, but don’t expect 5-year moisture-free storage from them.
If you’re ready to pick the right Mylar bags for your needs—whether you’re a small farm storing bulk grain, a prepper building a long-term food stockpile, a business shipping sensitive parts, or anyone in between—reach out to our team to discuss your specific requirements. We can help you choose the right thickness, construction, and size for your use case, and answer any other questions you have about how Mylar bags perform in real conditions.
Tobacco Leaf Bags References
- "Water Vapor Transmission of Polymeric Films." Packaging Technology and Science, vol. 22, no. 3, 2009, pp. 121-135.
- Food and Drug Administration. "Substances Separated from Food by Coatings or Other Barriers." 21 CFR Part 175, 2023.
- "Long-Term Storage Stability of Bulk Food in Flexible Packaging." Journal of Food Protection, vol. 78, no. 11, 2015, pp. 1987-1995.
- Bieri, J. "Moisture Barrier Performance of Flexible Packaging for Cold Chain Applications." International Journal of Refrigeration, vol. 48, 2014, pp. 112-120.
Shenzhen Qiancaisheng Packing Co., Ltd.
Shenzhen Qiancaisheng Packing Co., Ltd. is one of the most professional mylar bags manufacturers and suppliers in China, specialized in providing high quality customized products. We warmly welcome you to wholesale mylar bags for sale here and get quotation from our factory.
Address: No.2, Line 7, Wuhe Road, Bantian, Longgang, Shenzhen, 518129, China
E-mail: sales@qcspacking.com
WebSite: https://www.qcspackingpouches.com/
