My first winter jeans were a pair of thin stretch skinnies from a supermarket rail, bought because they were cheap and black and I assumed denim was denim. They were fine until the first proper cold snap, when I realised they had about as much insulation as tights and I spent a whole February pretending I wasn't freezing from the knee down.
Winter jeans only really fall short in two situations: genuinely icy, bone deep cold where nothing short of lined trousers or thermals underneath will do, and anything that needs to look a bit dressed up rather than dressed for the weather. A heavier jean, especially one with a brushed or fleece backed inside, is brilliant for the school run, the dog walk, the market, but it's not going to carry you into a smart dinner the way a dark straight leg in a crisp denim might. Over a season, the two behave very differently too. A rigid, heavier cotton denim keeps its shape well and softens gradually, getting better with wear rather than worse, while anything with a high stretch content, elastane mixed through the denim, tends to go baggy at the knee and seat faster, especially if you're sitting in a car a lot. Corduroy and moleskin, which turn up a lot in winter ranges dressed up as "jeans", wear beautifully if you brush them occasionally and keep them away from anything too hot in the wash, but they do show wear shiny at the thigh eventually. For care, washing less and washing cold is the boring but true advice, denim keeps its colour and shape far longer if it's not boiled every week, and anything fleece lined should go in inside out so the lining doesn't pill against the drum.
Fit is where most winter jeans go wrong before you've even left the shop. The lining and the extra brushing inside add bulk you don't get in summer denim, so going up half a size from your usual is often sensible rather than vanity, especially around the thigh and calf where lined styles can feel like you're wearing two pairs at once. Watch the rise too, a higher waist with a thicker jean sits very differently once you've got jumpers tucked or a coat on top, and can dig in a way a thinner pair never would. Length matters more than people admit, because a lined or heavier jean doesn't fall the same way over a boot as a lightweight one does, it tends to sit stiffer and can ride up rather than pool.
If you're trying a lined or heavier style for the first time, try it on sitting down as well as standing, since that's where the extra bulk and any snugness round the knee really shows itself.
