Divers have individual comfort needs and one of these needs is maintaining a comfortable body temperature. A shell type drysuit combined with layered insulation provides the diver with the ability to tailor the system to their comfort needs. What is layered insulation? We use layered insulation almost every day to deal with comfort needs. If we feel cool we may put on a sweater and if we become warm we take it off. In diving we are not able to add and subtract insulation during the dive. With experience, we learn to anticipate the thermal needs of the dive and can dress accordingly. This is not much different than downhill skiing. If you are going to spend 80% of your time skiing the slopes and 20% of your time in lift lines and on lifts, you would dress differently then if the mix was the other way around and you where spending 20% of the time on the slopes.
The drysuit diver should have an assortment of insulation options available, which can be used individually or in combination. A typical minimal assortment of options should include a primary garment, which has the insulation to meet the needs of most dives, a set of heavyweight long underwear (not cotton) and a vest. This allows the diver to adapt to more demanding situations by adding the long underwear and or the vest. In some cases it may only require the addition of the long underwear top or just the vest. In less demanding situations, such as a short duration summer dive, the diver may use only the long underwear or long underwear and vest. A diver who dives over a broader temperature and duration range should have a larger selection of items from which to choose
The four major things to consider when planning layering options are: (1) the water temperature, (2) the body's present metabolic rate, (3) activity planned, and (4) the time in the water whether on a single or repetitive dives. Water temperature seems obvious. Activity level and duration of the dive can have more influence on what is needed. Example: the water temperature may be 80 F° (27 C°) and for a short 30-minute dive the long underwear only may be needed. One the other hand, if the planed dive calls for 20 minutes bottom time and 1 hour hang time, the long underwear would be much too little.
The style and size of the drysuit is key to being able to do effective layering. The drysuit must be of a size so that all the potential layers can be worn under it without binding or restricting movement. Some examples of unsatisfactory situations would be where a vest is added and the diver can not take in a full deep breath; or a pile jacket is added and the diver can no longer reach the suit exhaust valve on their left upper arm with their right hand. The style of the suit is also important. The best suits for use in a layering system are shell type suits. Suits made of materials such as vulcanized rubber, trilaminate, polyurethane-coated pack cloth, and crushed foam are good examples. Why these materials? These materials have very little intrinsic buoyancy or insulation. Why is this important? Using suits made of these materials, the diver can wear a suit that is slightly oversized, a size larger than many would consider a good fit with very little impact on buoyancy. For example, the diver wears a trilaminate style suit which, by the size chart and what divers typically believe, is one size too big. The price paid for wearing this oversized suit is less than 1- pound (.45 kg) of additional weight to overcome the buoyancy of the extra suit material. What is gained is the capacity to use a variety of types of insulation layers. If the suit is made with a non-shell material such as foam neoprene, there is a greater penalty to be paid for wearing an oversized suit. When the diver moves up one size in a 6-mm suit the additional buoyancy is about 6-pounds (2.7 Kg).
The rules of thumb in layering:
There is no quick and easy guide to effective layering only - experience will help you decide how to best dress for the exposure. Every person's thermal requirements are different. The reward for mastering layering is the ability to achieve a high degree of comfort over a broad range of dive scenarios with a minimum of bulk and encumbrance.