How does a regular winter boot keep you warm?

The thick sole and insulation in a regular winter boot help retain the heat your body makes. A thick sole also insulates your foot from the cold ground.

That works, but it has a price. A boot like that is often tight, and its sturdy build keeps the foot from moving. When the boot limits foot movement, the muscles in your foot do not work the same way. The insulation can only hold in the warmth that's already there. It can't make more.

How does a barefoot winter boot keep you warm?

In a barefoot winter boot, the warmth comes from you, and the boot helps keep it in.

In a flexible shoe, the foot gets to move. The muscle pumps circulate blood, and warmth with it. Space keeps you warm too. When there's room in the shoe, there's air, and air works as insulation. There's also room for a thick wool sock.

There's one thing worth knowing, though. A thin sole puts less material between you and the frozen ground. When you stand still, the cold finds your feet sooner.

On top of that, Vaistoa winter boots have a warm lining and an insulating winter insole, and some models also have 3M Thinsulate insulation. The adult models also come with the Winter Layer+ extra insole, which you can slip under the shoe's own insole when you know you'll be standing still.

"Will these boots keep me warm at −40 °C?"

You're comparing winter boots. One box says −40 °C. Ours says nothing. So which one keeps you warmer?

It's a fair question, and people ask it a lot. What temperature are the boots rated for? Which model is the warmest? Will they keep me warm at −30 °C?

I understand why you'd want a number. But a number would only tell you half the story, because the other half is you.

The same boot feels completely different on a brisk walk and at a bus stop after twenty minutes of standing still. Damp socks, a tight fit and a long day outside all change how it feels. A number on the box can't know any of that.

So it's hard to give one number that holds true in every situation. I'd rather show you how your body regulates its temperature.

Where does the warmth in your feet come from?

Shoes don't warm you. Your body does. A shoe keeps the warmth in and lets your foot move, or at least it should.

When your body meets the cold, its most important job is to keep your internal organs warm. It narrows the blood vessels in your skin and limbs so that warm blood stays in the core. Your toes are the farthest from your heart, so they are the first to give way.

This is where movement comes in. When your foot gets to move, the muscles work. Work needs oxygen and nutrients, and they travel with the blood. Work creates demand, and demand brings blood. With the blood comes warmth from the centre of your body.

Movement also warms your whole body. Once your body warms up, it no longer needs to hold back, and it lets warm blood flow all the way to your toes. That's why your toes often warm up on a walk with a little delay.

What happens when you stand still in the cold?

The movement stops, your muscles work less, and less warm blood flows to your feet.

In Kittilä, Finnish Lapland, I once spent an hour and a half walking the trails along the Ounasjoki river at −25 °C in barefoot shoes. As long as I kept walking, my feet stayed comfortable. When I stopped to take photos, the cold started rising from the ground through the soles. The same thing happened at the Levi ski resort while I waited for the kids at the bottom of the slope. Each time, the fix was the same: I walked, hopped and moved around, and the warmth came back.

Back then, though, I didn't have Vaistoa shoes or their winter features yet.

So the colder it is and the longer you stand still, the more is demanded of the shoe. The need defines the shoe.

Can you get used to the cold? Yes, but in a different way than many people think. Repeated cold exposure barely changes the circulation in your toes. What it changes is how the cold feels (Daanen and Van Marken Lichtenbelt 2016).

Cold can feel less intense even while your feet are still cooling. Cold tolerance varies from person to person.