The Mountain Hut
Again, when temperatures are between freezing and near 20 degrees, its ok to use just shells and thermals on the upper too. this way you have good venting by just unzipping all the jacket orifices. I always make sure my shells have good vents on them. The anorak style jacket shell that I use has armpit zipper vents, front zipper vents, drawstrings on the bottom, mid waist, and hood which is high quality elastic with quick easy clips to operate with huge backpacking gloves on. If it didn't have these things, I Would Not buy it. ALong with that, the shell also has fishnet liners, velcro cuffs, internal pockets for warm-storage(such as a Powerbar, which is extremely rock hard otherwise), and batteries etc.. This jacket is equipped. Once on a hike you can unzip a few minutes after hike begins to vent, then open the other vents as you hike.
If its real cold when hiking you may need to throw on an insulator to beat the chill, but almost Always a mistake. Usually the shells comes off, and its just the thermal, with iced air over hot steamy thermals, where the chill aids and serves to make the perfect heat radiator coming off the thermal sleeves and chest while you hike in freezing air. Actually , the worst times are when you NEED to have the shell OFF. This means its getting pretty balmy, or your hiking in a pretty good sized load and your metabolism is of the easily-overheated type, such as mine. The body starts to precipitate in the middle of a 25F degree heavy-packed overheating hike. Having on just the thermal now, and the shell over that, you sweat like a horse, and the synthetic perfectly wicks the moisture out, as does the shell, conveniently leaving only the smell behind after a day of this.
The worst case would be if it was colder, and you didn't have a synthetic jacket, and had to use your down jacket to stay warm during your hike. You would just about have to set camp so that you don't get your down jacket wet or figure out a way to stay warm without it. Its imperative that you don't hike-overheated in this jacket. If the hiking is less strenuous, and its colder than about 15F, then you may be able to get away with hiking with down, but the goose-down is better left to below0F hiking.