A fabric's capacity to hold warmth is referred to as its "thermal effusivity." Two factors affect how well a fabric can hold warmth: its ability to store heat (i.e., heat capacity) and its ability to transport heat (i.e., heat conductivity).
A Fabric's Chemical and Physical Makeup
The chemical and physical makeup of a fabric determine its heat capacity and its heat conductivity. For example, the natural fibers of cotton absorb water more readily than the synthetic material used to make polyester. This is important in retaining heat because water near the skin can evaporate and cool the wearer. Additionally, if a piece of fabric is thick and loosely woven, it will hold more air than a more tightly-woven piece of fabric. This is helpful in retaining warmth because air, a poor thermal conductor, is itself an insulator. Also, the surface area of a fabric should be as small as possible to hold warmth, because more area means a greater surface for heat loss.