Keep moisture out of your winter footwear with a pair of vapor barrier liner socks, like these from from Rab ($35).

One of the bigger challenges in winter hiking is keeping your feet warm and comfortable, especially if you’re out in the chill for prolonged periods of time. Proper winter footwear is essential, of course, to keeping warm, but there’s another challenge that you may not be considering: foot sweat.

Your feet are the second sweatiest part of your body after your armpits. This means that your feet produce a considerable amount of moisture over the course of a winter hike, especially if you’re working hard. All of that moisture is then absorbed into your socks and footwear, leaving them damp and clammy by the end of your hike. And damp socks and footwear will rapidly chill your feet once you begin to to cool down.

If you’re winter camping, a common practice to address this is to bring your footwear (or the liners of your plastic mountaineering boots) into your sleeping bag with you during the night. (And put on a fresh pair of warm, dry socks while you’re at it.) This helps dry out your footwear for the morning, though it also transfers moisture from your footwear into your sleeping bag insulation, reducing its warmth as well. It can also be an uncomfortable set-up for sleeping, especially if space is tight inside your bag.

There’s a better way: vapor barrier liner socks. Their name says it all. These thin socks completely block moisture from passing through and prevent it from entering your footwear. Here’s how the system works—and how I’ve used it successfully on many a winter adventure:

First you’ll want to put on a pair of thin synthetic liner socks against your feet. Next you pull on the vapor barrier liner (VBL) socks. Then you add a thick, warm wool sock over top . Voila. No foot sweat will now enter your thick socks or footwear.

Now here’s the counter-intuitive part of the whole system. You might think that trapping moisture next to your feet would lead to a soaking, swampy, skin-pruning environment as sweat pours from your feet. But it doesn’t. Why? Even though you’re not generally aware of it, your skin is constantly producing sufficient moisture to keep it adequately moist and hydrated; this is known as insensible perspiration. However, if your skin senses that it is already adequately moist, it ceases this insensible perspiration because it’s no longer needed. And that’s exactly what happens inside your VBL socks—your feet actually start sweating less because the environment around them is already sufficiently moist.

That doesn’t mean your feet aren’t a bit damp, which is why it’s so important to wear a thin liner sock for comfort and to always take the VBL socks off at night to let your feet dry out. But in my experience, I’ve never found my (generally very sweaty) feet to be uncomfortably pruned or sopping wet after a day spent wearing VBL socks, even after a day of hard exertion. And not only do VBL socks keep your feet warmer by protecting the insulation in your footwear, having dry footwear also means that you don’t have to bring them into your sleeping bag at night to dry them out. It’s a system I highly recommend for winter use, as do others, including Philip Werner at SectionHiker and hiker-extraordinaire Andy Skurka.

So are you ready to give VBL socks a try? Despite their benefits, no major outdoor gear manufacturer makes them, though several smaller companies do, including Exped USA ($29 at Summit Hut), Rab ($35), and Stephenson’s Warmlite ($22).

Stay warm out there!