Winter Tops Mid layer insulation
Mid layer tops in cold weather should be insulating with easily adjustable ventilation for versatility. When the sun shines and the wind drops they can become the outer layer even if it's still cold, likewise they should quickly close up to provide vital heat retention when the weather turns again.
The mid layers of extreme cold weather clothing are more recognizable as being like "normal clothes" in a way that thermal underwear (foundation layer) and a heavyweight parka (shell layer) aren't so much.
To make them perform optimally you need to pay more attention to material and design than with your normal everyday clothes. More than one mid layer is often worn for comfort and practical purposes, too many become unwieldy and can restrict movement. Two thinner layers will be warmer than one of the same combined thickness and material due to air being trapped between as well as within layers.
The thickness of layers and level of insulation depends on the outside temperature and also on your own comfort level. For me this means two midlayers most of the time with another in my backpack in case it gets colder. If the temperature goes up you can shed the shell layer and/or a midlayer depending on wind chill. Adjustability for ventilation is important, zips, buttons etc.
Some midlayer top options for winter warmth:
Traditional style over shirt
A heavyweight long sleeved shirt to go over your base layer, very practical and versatile, long enough to tuck into your pants and prevent draughts.
Such shirts are commonly made from cotton and may have a fleece lining for extra warmth. Cotton is not good as a base layer though, so they are better used as an over shirt with a synthetic or fine merino wool foundation layer next to the skin.
"Polar fleece" as it is sometimes called is a polyester velour material, it is very breathable is soft and comfortable to wear. It doesn't restrict the movement of water vapor, dries quickly, is machine washable, and comes in light, mid, and heavy weights (sometimes called 100, 200 and 300 weights) for a choice of insulation level. The lighter weights are more versatile for layering, the heaviest will often require a somewhat over-size shell layer to go over the top comfortably.
Available in an an almost endless variety of colors and styles, not water proof or wind resistant and will need a shell garment in harsh conditions.
Wool layer - pullover, sweater etc.
A natural and traditional material for making warm winter pullover, sweaters etc. from, wool provides good warmth for the weight and remains an effective insulator even when damp or wet. Merino wool is a particularly fine and soft grade. Insist on pure wool rather than mixtures with other fibres.
Acrylic or cotton garments can look deceptively similar to those made from wool but are greatly inferior at providing warmth, cotton in particular is cold and miserable when wet, avoid these materials for the insulating layer.
Lightweight down jacket
A natural product, the best warmth for weight of any insulating material for clothing. Lightweight down jackets, sometimes called down shirts are different garments those used as outer shell jackets, they are softer, simpler, shorter and much lighter, they usually come with a rip-stop nylon outer and are very compressible (often folding into one of their own pockets) , not particularly abrasion resistant though.
New coatings are overcoming down's biggest shortfall which is performance when wet.
Lightweight synthetic insulation jacket
Similar to the down products outlined above but still warm when wet, the downside is a little heavier weight and greater bulk. There are a number of brand names for the insulation, one of the best known and most effective is thinsulate.
Knitted synthetic over shirt or sweater
Woven material rather than insulation with loft. Often made of lightweight fleece or similar, breathable and still insulating when damp or wet. A great variety of different styles, a short zip or buttons allow for a simple and quick response to changing weather conditions. Some elastic material in the fiber mix aids comfort and movement without compromising fit, as ever avoid cotton for cold climates.
Cotton hoodies and sweatshirts - Forget about that favorite heavyweight cotton sweatshirt you have, even the one advertising your allegiance to your sports team or alma mater. Cotton is a poor insulator for its weight and is a liability when damp or wet from external moisture or sweat, it hangs onto it and feels cold and clammy.
An Antarctic Scenario - layering for versatility
This is me in Antarctica on a mild and calm late winters day at about -15C (+5F). The sun and lack of wind meant it felt unusually warm for a short while, with the potential for the wind to get up and temperature to go down especially when the sun started to set as it did early in the afternoon. This is a realistic use of cold weather gear, in the worst weather it is dangerous to go far outside, so people generally stay on base.
We went out on a trip to some icebergs frozen into the sea-ice. This entailed a 3 mile very rugged overland hike to get to the edge of the sea and then more hard work over a couple of miles of broken flat sea-ice with a substantial snow covering.
This was a day of varied temperatures, changing wind speed, and different activity levels, hard uphill walking and then easier on the flat. There was always the possibility that some hardcore Antarctic weather could arrive in a pretty short time that we needed to be prepared for. The clothing had to be versatile for changing conditions and up to the job of fending off the worst the weather might bring.