Dry Suits provide more insulation than wet suits by keeping you dry. They provide the most thermal protection of all suits used by recreational divers, and make a noticeable difference in how long you stay comfortable in cold temperatures (18°C/65°F). In water colder than 10°Celsius, they are the main option for a comfortable dive.
How do they work?
Air conducts heat relatively poorly, so the dry suit insulates you with a layer of air. In addition, an insulating garment such as an Under Suit (see the under suit section) may be worn for extra warmth. Unlike a wet suit, in a dry suit everything between your skin and the water (both the empty space and any material) reduces heat loss. It also means when you remove it post dive you will be bone dry, provided there has been no leaks (these are rare and only happen when it has been put on incorrectly or is damaged).
Also unlike a wet suit, they should fit relatively loosely so they can be inflated with air from your cylinder.
As they are loose fitting, dry suits create an air space which needs to be equalized just the same as the air space in your ears and mask. You also need to be able to release expanding air as you ascend. To do this, dry suits fill with air directly from your cylinder via a low pressure inflator similar to the one on your BCD. All regulators come with additional taps to add another hose for connecting to your dry suit. When not being used, this tap can easily be plugged.
Dry suits also have an exhaust valve for releasing air as you ascend.
You can find Dry suits made from several materials, including neoprene. All Dry suits have a special watertight zipper and neck and wrist seals to keep the water out. A neoprene suit gets most of its insulation from the fabric and the air inside it, while other dry suits insulate with other garments, see our section on Under suits. Because you fill them with air, you would expect dry suits to be more buoyant than wetsuits. However, with most modern suits and undergarments, they are not much more buoyant. Another advantage of a Dry suit is that with most types, you don't lose buoyancy or insulation with depth, like you would in a wetsuit. As you go deeper, you add air to the suit, maintaining both normal buoyancy and insulation.