I’ve been a serious winter hiker for close to 15 years and I’ve never owned a pair of single-layer, insulated waterproof/breathable winter hiking boots that didn’t leave my socks damp or wet from foot perspiration. I’ve used 400g and 200g insulated winter hiking boots from Salomon, Oboz, Garmont, The North Face, KEEN, Merrell, and Vasque, and not one has ever kept my socks dry on a winter hike.
Unfortunately, the accumulation of moisture inside single-layer winter hiking boots reduces their ability to trap warm air and insulate your feet. It also has consequences for overnight hikes in freezing weather, because it means your boots will freeze unless you sleep with them in a sleeping bag or wear plastic bags (vapor barrier liners) over your socks to prevent moisture from entering the boots through the liner.
Why Breathability Fails
Why does perspiration build up in an insulated winter boot, even if it has a waterproof/breathable membrane? Assuming a 4-layer boot design (liner – insulation – membrane – exterior material), here are a few potential reasons:
- Poor breathability (case 1): The rate at which the liner can transport water vapor is really slow so it backs up in the liner and leaves your socks damp.
- Poor breathability (case 2): The rate at which the insulation layer (Thinsulate, Aerogel, or Primaloft) can transport water vapor is really slow so it backs up in insulation and liner.
- Poor breathability (case 3): The rate at which the waterproof/breathable membrane in boots can transport water vapor (from perspiration) is really slow so it accumulates in the membrane, the insulation, and the liner.
- Poor breathability (case 4): The rate at which the pores in the exterior material of your boots (leather, synthetics) can transport water vapor is really slow so it backs up in the membrane, insulation, and liner.
- The water vapor freezes when it reaches the exterior material of your boots in sub-freezing temperatures closing the pores in the material, causing water vapor to back up through all the layers of your boots.
- The exterior of your boots gets wet and soaked through, preventing water vapor from getting out so it backs up through all the layers of your boots. This is called “wet-out” when talking about the same phenomenon in rain jackets. (Why Do I get Wet Inside my Rain Jacket)
- The exterior pores of your boots have been sealed with waterproofing wax (snow-seal) or oils, preventing water vapor from escaping. Similar to case 6, but for a different reason. (Should You Waterproof Gore-tex Boots?)
Frankly, I’d like to see the makers of insulated winter boots start publishing their breathability metrics to shed light on this issue. I’m not a huge fan of the current raft of breathability metrics because idealized lab tests have limited value in the real world, but it would help consumers compare insulated winter boots and foster some competitive innovation.