It might be the easiest mistake to make outdoors, even for expert adventurers. The benefits of wool, especially for winter recreation, are no secret. It pulls away moisture, doesn’t stink, and regulates your body temperature like a champ. But even if you knew all that and have invested in a set of top-tier merino base layers, it’s still painfully easy to pretty much throw it all down the toilet. You forgot your underwear.

Courtesy: Smartwool

Stick with me, we’re going to start with the basics of wool: While most synthetics need to wait for your sweat to liquefy before it can pull it away, merino wool transports sweat and moisture as a vapor. Instead of sweat sticking to the outside of hydrophobic materials like polyester, wool can pick up water vapor in its fibers, which makes it feel dryer and makes it easier for the moisture to leave. Because it does such a good job of getting moisture out, it makes it a lot harder for smelly bacteria to find a home there. And finally, its tiny pockets of air trap your body heat and help keep your temperature regulated. But what happens if you put a non-wool fabric, like polyester or (*gasp*) cotton between your skin and your fancy merino wool? First, sweat on your skin can’t get picked up by the wool without first passing through that other material. And if that material doesn’t wick as well as wool, it’s going to take a lot longer for it to get to the wool and off your body. That material is causing a traffic jam—a bottleneck—before the sweat can get to the smooth sailing Autobahn of merino. All the sweat that gets to your wool layer is going to disappear fast so you don’t need to worry about that getting stinky. But the layer underneath? It’s not letting go, creating a breeding ground for foul smells. And finally, if that other fabric is trapping excess body heat under the merino, there’s nothing the merino can do to save you from overheating anymore.

That material is causing a traffic jam—a bottleneck—before the sweat can get to the smooth sailing Autobahn of merino.

It’s like wearing a plastic garbage bag under a $400 Gore-Tex rainshell. It doesn’t matter how impressive that shell is or what its breathability rating says—you’re still wearing a garbage bag underneath. And no matter how breathable that jacket is, it doesn’t have the power to make a garbage bag breathable. The same goes for merino wool. No matter how well it does its job, it can’t turn your synthetic or cotton undies into merino ones. It can’t make the moisture wick through them better, and it can’t make them stink any less. We hate to break it to you, but if you’re wearing cotton or polyester underwear under your merino base layers, you’re spoiling your base layers.

And that’s a real shame considering if there’s anywhere on your body where the benefits of moisture management, smell reduction, and thermoregulation are going to be apparent, it’s going to be “down there.” Why would you wear merino wool everywhere except your underwear? Merino or not (especially not), your underwear isn’t necessarily the piece of clothing you like to stretch out for a couple of weeks. But with wool, you might be able to get away with more than you realize. Take my word for it: on a two-week expedition to the Alaska Range this summer, I brought just one pair (cut me some slack, I was already carrying 100 pounds of gear and food). No sweat getting lodged in the fibers meant they stayed comfy, didn’t stink, and worked everywhere from the blazing heat of the lower glacier to a frigid storm higher on the mountain. Was I ready to change them when I finally flew back to civilization? You bet. But if I had been wearing polyester, I’d have been forced to take that flight commando style.

I know what you’re thinking and the wool underwear of today, like the options from Smartwool, aren’t like your grandma’s itchy sweaters. Merino features much smaller fibers than standard wool, making it far softer and more comfortable against your skin, no matter where you put it. And while it might be a bigger initial investment than that 6-pack of Hanes, they’re far more durable and will last significantly longer than any other option.

Courtesy: Smartwool

One final point. What would you rather have up against your business? A petroleum-based plastic, like polyester? Or a natural, renewable, animal fiber like wool? One sheep can grow up to five pounds of wool every year (that’s a lot of underwear), and they’re happy to share. And when you are finally done with it (it’ll be a while), the proteins and amino acids that make up merino wool break down far more easily than plastics. On top of that, Smartwool is working with its merino suppliers to pioneer a new standard that helps measure and improve a sheep herder’s carbon footprint.

Want to really enjoy your merino base layers this winter? Make sure you pair them with merino. Otherwise, the investment you made on staying comfortable and safe on your next backcountry ski mission or winter hike might not be worth what you hoped. Really want to feel the benefits of merino? Start wearing merino daily. And do it where it really matters: Down under.