PowslayrGet a non-insulated jacket (will be marketed as rain jacket or storm jacket) and wear that over a sweatshirt. Shell jacket (not "ski shell", those have insulation) over a sweatshirt is what I do and it works great, then I can take off one of the layers if I get hot.
Why the fuck are retards saying windbreaker under sweatshirt? Windbreaker goes over the sweatshirt, guys... It keeps the sweatshirt dry and windbreakers are designed to be an outer layer
A ski shell will not have insulation that is the point of a shell it's an outer waterproof layer whic you later beneath to keep warm.
As for a windbreaker they are used as mid layers. A windbreaker often will have no waterproofing as it is designed to stop win not rain and other precipitation. So if you are wearing a sweat shirt adding a windbreaker under will keep you warmer when your moving as it will stop the cold air reaching you. However they won't often be thermal so it won't help on a generally cold day that is when down or synthetic down mid layers are excellent.
Really the bet all roun option is a good technical shell with good base layers and optional mid layers depending on the weather. That would be good for nearly all condition year round.