UNDERSUITS

A membrane drysuit offers no insulation from the cold, it is only a means of keeping dry. The insulation comes from the undersuit that provides an air gap between the diver and the water. The wider the gap, the greater the insulation.
You can vary the degree of insulation by adjusting the thickness of your undersuit, perhaps with different layers or combinations.
Undersuits have developed into garments suitable for wear before, during and after the dive and this multi-purpose aspect is reflected in the materials used.
Most undersuits offer a combination of layers with different functions. The outer layer or shell may use a simple nylon or one of the more sophisticated materials developed for skiers and mountain-climbers. Its purpose is to keep you reasonably weatherproof before and after wearing your drysuit, and perhaps to make it easier to slip into it.
The next layer in may be a form of thinsulate, which gives high insulating properties even when used in very thin layers. The inner layer is most likely to be either a fur-pile or fleece lining. Both maintain a good airspace and "wick" perspiration away from the body.
The most basic suit is the traditional "woolly bear", which uses only the fur pile material to provide good insulation at a reasonable price. Even this type of suit will offer double layers of material in appropriate positions for increased insulation, and for padding in weight-bearing areas such as the shoulders, spine and knees.
The addition of an outer nylon shell can often make a suit less stretchy, particularly if it
is too heavy. Test its flexibility by crouching right down to make sure it does not restrict your movement in any way. You may need a larger size than you think.
Thermal socks will often keep your feet warm enough, but the thinsulate models offered by some manufacturers have the advantage of staying warm when damp.