Long johns as a layer under trousers

Short answer: Long johns are a close fitting leg layer worn under trousers. They hold the warmth without making the trousers tight.

What they do

They create a layer of still air between skin and trousers. That air is the insulation. That is why a thin close fitting layer works better than a thick loose one under a pair of trousers.

The material

Bamboo viscose lies smooth against the skin and moves moisture away if you sweat on the way indoors. Wool is warmer but can scratch. See bamboo long johns.

When you need them

In frost, for long periods outdoors and for work where you stand still. For an ordinary commute they are often too much. See bamboo underwear for men and bamboo socks for men.

Frequently asked questions

Do they show under trousers

No, if they are close fitting and have no thick seams.

Do you get too warm indoors

Yes. That is the biggest drawback of wearing them all day.

How long should they be

To the ankle, so they overlap with the sock.