Dustin Brooks, Director of Sales at Triton Inc., explains why differences between coatings and membranes matter.
There's a difference between a coating and a membrane, and it can be hard to draw the line at times. In my professional opinion, I consider a roofing membrane to be your primary long-term, durable, and flexible barrier to prevent water and chemicals from damaging your insulation and entering your building. Going beyond roofing, let's look at the definition of a membrane. A membrane is a "pliable sheet-like structure acting as a boundary, lining, or partition in an organism."
The cells in your body have membranes separating the inside from the outside, protecting vital interior elements from damage. These cellular membranes are two layers (lipid bilayer) working together (this is where all fans of 2-ply, 4-ply, and 6-ply roof systems start cheering...redundancy starts at the cellular level!). Not all that different from a waterproofing membrane, serving as a pliable, long-term barrier between the structure and the elements.
So, what do I mean when I say a coating is not a membrane? Let's go back to the human anatomy analogy. Your skin is a membrane. It's tough, flexible, and virtually impermeable (you can sit in the bathtub for hours and get wrinkly, but your body doesn't blow up like a balloon...skin is only permeable to lipids and fat solvents). It protects the underlying tissues and organs from "the outside".
But if you're a naturally pale guy, like me, your skin is damaged when you're in the sun too long. Your sunscreen is a coating. The purpose of sunscreen is to protect your skin, the membrane, from damage. It's permeable, so you can still sweat, and it's applied relatively thin. Unless you're a lifeguard, then you put a nice thick layer of lotion on your nose. It doesn't last very long, so you'll need to re-apply. Alright, where am I going with this crazy analogy?
Liquid roofing products are not all that dissimilar. You have those that work great as a coating (sunscreen): applied in thin layers (20-30 mils), protect from UV degradation, permeable, cool you down (reflective), but need re-coated and touched up often. They do not perform like a membrane. They are not very durable, tough, flexible, or long-lasting. But they work great for protecting membranes! Think about it, what if your cells were only protected with a couple layers of acrylic paint?
Then you have other liquid products that work great as a membrane and end up like those made into rolls at a factory. Thick (50-80 mils), durable, flexible, strong, and impermeable. They serve as a primary barrier separating the structure from the elements. Some are reinforced, some are multi-layered, but the intended purpose is the same. No seams, fully-adhered, and self-terminating. These liquid-applied, seamless membranes should act more like your skin than your sunscreen.
The reason I write this article today is because all too often any material sold in a bucket or drum is labeled a "coating" and given the same negative stereotype. I've heard it often, "if it comes in a bucket, it sucks." But they don't all suck. With so many to choose from and filter through, it gets confusing and crowded. Here are a couple charts comparing the properties of both coatings and membranes, including the VOC content of each. Which ones would you rather have acting as a skin protecting your structure?