Mylar bags are made of polyester film, specifically biaxially oriented polyethylene terephthalate, or BoPET. "Mylar" is a brand name that DuPont registered in the 1950s, the way "Kleenex" is a brand name for tissue. The bags sold for cannabis, coffee and food storage are almost never a single sheet of Mylar. They're laminates: a polyester outer layer, a metallised or foil middle layer that blocks light and oxygen, and a polyethylene inner layer that heat seals. Here's what each layer does, why the layers matter more than the name, and what the numbers on a spec sheet mean.
The three layers in a mylar bag
Outer layer: polyester (PET). This is the Mylar part. It's strong, dimensionally stable, takes print well and doesn't stretch. It gives the bag its stiffness and its glossy or matte surface. Typical thickness is 12 microns, about half a thousandth of an inch.
Middle layer: metallised PET or aluminium foil. This is the barrier. Metallised film is polyester with a layer of aluminium a few hundred atoms thick vapour-deposited on it. It blocks most light and slows oxygen and moisture. Aluminium foil, usually 7 to 9 microns, blocks them almost completely and costs more. Cheaper bags skip this layer entirely, which is why some "mylar" bags let light through when you hold them up.
Inner layer: polyethylene (PE). This is the part that touches the product and the part that heat seals. Low-density or linear low-density polyethylene melts at a low enough temperature that an impulse sealer fuses the two sides together. It's food-grade, inert and doesn't transfer flavour. Typical thickness is 50 to 100 microns, which is most of the bag.
The layers are glued together with an adhesive lamination and the finished film is measured in mil (thousandths of an inch). A 4 mil bag is about 100 microns total; a 7 mil bag is about 180.
Why the layers matter more than the name
A bag labelled "mylar" could be any of these:
PET/PE, two layers, no barrier. Clear or tinted, cheap, lets light through. Fine for short-term storage, poor for anything that degrades in light.
PET/metallised PET/PE, three layers. The standard cannabis and coffee bag. Opaque, good barrier, heat sealable. This is what most people mean by a mylar bag.
PET/aluminium foil/PE. The heavy-duty version. Near-total light and oxygen block. Used for long-term food storage and premium cannabis lines.
Kraft paper/metallised PET/PE. Paper on the outside for the look, barrier inside. Same performance as the three-layer film with a different face.
When you compare bags, ask for the layer structure, not the brand word. A supplier who can only say "mylar" doesn't know what they're selling.
What the numbers mean
Mil is thickness. 3.5 to 4 mil is standard for flower that sells quickly, 5 to 7 mil for storage. More mil means more polyethylene, which means a stiffer bag and a stronger seal.
OTR is oxygen transmission rate, in cc per square metre per day. A three-layer metallised bag is around 1 or below. A foil bag is close to 0. A two-layer PET/PE bag can be 50 or more.
MVTR is moisture vapour transmission rate, in grams per square metre per day. Same pattern: metallised under 1, foil near 0.
Micron is the metric thickness, 25.4 microns to the mil.
If a spec sheet shows OTR and MVTR, the supplier has actually tested the film. If it just says "high barrier", ask.
Are mylar bags safe for food?
Yes, when the inner layer is food-grade polyethylene, which it is on any bag sold for food or cannabis. The polyester and aluminium never touch the product. The bag is inert, doesn't leach, and is the same construction used for coffee, jerky and freeze-dried food. Bags sold as "industrial" or for non-food uses may use a different inner layer, so check.
Are they recyclable?
Mostly not, in practice. A three-layer laminate can't be separated into its polyester, aluminium and polyethylene at a normal recycling facility, so it goes to landfill in most areas. Some mono-material bags (all polyethylene, with a thin barrier coating) are recyclable in store drop-off streams, and a few brands have moved to them. Kraft-faced bags aren't paper-recyclable either, because of the film inside. More on this in are mylar bags smell-proof, safe, reusable and recyclable.
Why cannabis uses mylar
Flower loses terpenes to air and THC to light. A metallised or foil bag with a heat seal blocks both, and the polyethylene inner layer doesn't absorb the smell or the oils. It's cheaper than glass, lighter than tins, and prints edge to edge. Add a child-resistant zipper and it meets the retail packaging rules in most licensed states. That combination is why the format took over. If you're choosing between bag types, best mylar bags for weed compares the seven common ones, and custom mylar bags covers what we make.
Quick answers
What is mylar made of? Polyester film, BoPET. "Mylar" is DuPont's brand name for it.
Are mylar bags plastic? Yes. Polyester outside, polyethylene inside, usually with a metallised or foil layer between.
Is mylar the same as aluminium foil? No. Foil is metal. Most mylar bags have a very thin layer of aluminium deposited on polyester, and some use a full foil layer, but the bag is mostly plastic.
What thickness is a mylar bag? 3.5 to 7 mil, or roughly 90 to 180 microns, depending on use.
Are mylar bags food safe? Yes, the inner polyethylene layer is food-grade and is the only layer that touches the contents.
