By 'outdoor trousers' I assume you mean what hikers/backpackers/winter-folk would refer to as a 'hard shell?'
Just came back from an exceptionally cold vacation to Iceland and realized that despite years of backpacking and hiking in temperate weather, I knew not nearly enough about layering for truly cold weather.
I'm honestly not sure what their proper name is. They're thin, and waterproof, is the best I could say - not particularly insulated, but I know there are some which are.
I mostly bought them to keep my dry, rather than warmer.
