Drysuit undergarments (or undersuits) are worn beneath the dry suit to provide much of the actual insulation for the scuba diver and have a large impact on the enjoyment of a dive.
Undersuits are used for providing additional warmth when diving, traditionally under a drysuit but some are now ideal for use under a wetsuit as well. Undergarments are available in a wide range of styles, thickness grades and materials, spanning from the thinnest (e.g. Northern Diver Thermalskin) up to the much thicker and more thermally insulating (e.g. Northern Diver Metalux Arctic).
For cold-water use, especially diving under ice sheets, the user will usually wear a thick undersuit in a membrane drysuit. The thickness of undersuits varies and can be chosen by the wearer according to the water temperature. Thinsulate or Fleece (e.g. Northern Diver Arctic Base) is one of the preferred fabrics for undersuits. More recently, aerogel material is being added to conventional undergarments to increase the insulating properties of those garments.
Neoprene drysuits are made from a foam-rubber sheet containing tiny air bubbles, which provide insulation by themselves, and can eliminate the need for an undersuit, or greatly reduce the thickness needed for the under fabric. A neoprene wetsuit can also be worn under a membrane drysuit for extra protection against condensation and leaks.