A drysuit keeps you warm underwater by doing something fundamentally different from a wetsuit: it keeps water out entirely. Instead of trapping a thin layer of water against your skin (the wetsuit approach), a drysuit creates a sealed air space between your body and the surrounding water. That trapped air, along with insulating undergarments, is what actually keeps you warm.
The Three-Layer System
A drysuit’s thermal protection works through three distinct layers, each handling a different job. The suit itself is the barrier layer, preventing water from ever touching your skin. This eliminates convective heat loss, which is the rapid cooling that happens when cold water flows across your body. A wetsuit slows this process down, but a drysuit stops it completely.
Underneath the suit, a midlayer of insulating clothing traps air in tiny pockets, reducing conductive heat loss. This is the same principle as a down jacket: still air is a poor conductor of heat, so the more of it you can trap close to your body, the warmer you stay. The innermost layer is a moisture-wicking base that pulls sweat away from your skin, because damp insulation loses its effectiveness quickly.
This layered approach is what gives drysuits their flexibility. You can swap undergarments depending on water temperature, wearing lightweight fleece in mild conditions or heavy insulating layers in near-freezing water. A wetsuit locks you into whatever thickness of neoprene you bought.
How the Seals Keep Water Out
The suit’s watertight barrier depends on tight seals at every opening: the neck, both wrists, and a waterproof zipper. These seals are the most critical (and most maintenance-intensive) part of the whole system.
Neck and wrist seals come in three materials. Latex seals have been the standard for years. They stretch easily, always return to their original shape, and can be trimmed to fit. The tradeoff is that natural rubber breaks down from UV exposure and ozone, and some people have latex allergies. Silicone seals are newer, softer, more comfortable, and far more resistant to UV and ozone degradation. They also conform well to the body, often creating a better seal than latex. Their one weakness is that they tear more easily than a fresh latex seal. Neoprene seals offer more warmth but tend to leak with head movement and can stretch out permanently over time.
The waterproof zipper is an engineering achievement in its own right. Metal-tooth designs like YKK’s PROSEAL use nickel elements that press tightly together, creating a seal that blocks both liquids and gases even under significant pressure. These zippers need regular care: rinsing with fresh water after every dive, brushing debris from the teeth, and applying zipper lubricant. Metal zippers should be stored partially closed to relieve pressure on the sealing surfaces, while plastic zippers do better stored fully closed.
Neoprene vs. Membrane Suits
Drysuits come in two broad categories, and each handles insulation and buoyancy differently.
Neoprene drysuits use the same foam rubber material as wetsuits, typically 7mm or 8mm thick. The neoprene itself provides insulation, so you may need fewer undergarments. The downside is that neoprene compresses at depth, just like in a wetsuit. As the gas bubbles in the foam shrink under pressure, you lose both insulation and buoyancy. This makes weighting more complicated because your buoyancy changes significantly between the surface and your deepest point.
Membrane (trilaminate) suits are made from three thin layers of material laminated together. They have almost no inherent insulation, so you rely entirely on your undergarments for warmth. The advantage is that the suit material doesn’t compress meaningfully at depth. A membrane suit has very little positive or negative buoyancy on its own and is far less affected by pressure changes during a dive, making buoyancy control simpler and more predictable.
Managing Air at Depth
Because a drysuit traps a volume of gas, the physics of pressure directly affects how the suit behaves underwater. As you descend, the increasing water pressure compresses the air inside the suit. At 10 meters (33 feet), the volume of that air is cut in half. The suit squeezes against your body, reducing insulation and becoming uncomfortable. Left uncorrected, this creates what’s called a suit squeeze.
To counteract this, drysuits have an inflation valve, usually on the chest, connected to your air supply. You add small amounts of air as you descend to maintain a comfortable air layer inside the suit. On ascent, the reverse happens: the air expands as pressure decreases. A dump valve, typically located on the upper arm, lets you vent excess air on the way up.
This is where drysuit diving requires skill that wetsuit diving doesn’t. You’re essentially managing two buoyancy systems at once: your buoyancy compensator and the air in your suit. The goal is to keep only enough air in the suit to prevent squeeze and maintain insulation, using your buoyancy compensator for the rest of your buoyancy control.
The Risk of Runaway Ascents
The most serious drysuit-specific hazard happens when air migrates to the wrong part of the suit. If you tip feet-up, even briefly, air rushes into the legs and boots. Since the dump valve is on your arm, you can no longer vent the expanding gas. The air keeps expanding as you rise, accelerating you toward the surface in an uncontrolled, feet-first ascent.
A case documented by the Divers Alert Network illustrates the danger clearly. A diver’s buddy became inverted and attempted to recover by sculling with her hands, but the expanding air in her suit made a runaway ascent inevitable. She reached the surface uncontrollably, putting her at risk of arterial gas embolism, one of the most dangerous dive injuries.
The prevention is straightforward in principle: maintain proper weighting and trim, and never carry more air in the suit than you need. A well-weighted drysuit diver should be able to rotate from head-down to head-up to horizontal without triggering an uncontrolled ascent. This is a fundamental skill taught in drysuit certification courses and the primary reason you shouldn’t dive dry without specific training.
Argon Inflation for Colder Water
Some divers in very cold water inflate their suits with argon gas instead of air from their tank. The logic is simple: argon conducts heat about 31% less efficiently than air. In theory, filling the suit with argon means the gas layer inside loses heat to the surrounding water more slowly.
In practice, this requires carrying a small, separate argon bottle with a dedicated regulator. It adds complexity and cost to the setup. The benefit is most noticeable on long dives in cold conditions, where even modest improvements in insulation compound over time. For recreational diving in moderate temperatures, standard air inflation and appropriate undergarments are sufficient.
Keeping a Drysuit in Working Condition
A drysuit is a significant investment, and its lifespan depends heavily on how you care for the seals and zipper. After every dive, rinse the entire suit with fresh water, paying particular attention to the zipper teeth where salt and grit accumulate. A soft brush helps clear debris from the zipper mechanism.
Latex seals degrade from UV exposure and ozone even when the suit is in storage, so keeping the suit out of direct sunlight and away from electric motors (which produce ozone) extends seal life. Silicone seals are far more forgiving on this front. Regardless of material, inspect your seals before each dive for cracks, tears, or areas that have lost elasticity. A failed seal during a dive means cold water flooding in, which in frigid conditions can become a serious safety issue.
If your zipper begins to show corrosion, excessive wear, or won’t seal properly even after cleaning and lubrication, it likely needs professional replacement. Zipper failure is not something you can fix in the field, and a compromised zipper turns your drysuit into an expensive wetsuit.