Beginner 8 minutes
Thermal protection: wet, semi-dry, and dry
Choosing a suit means choosing how much water you let in, and how much stays still
Two divers with the same suit, in the same water, come out: one comfortable, the other shivering. The difference is almost never the thickness of the neoprene: it's whether that suit fits well or fits loose.
In this article you'll see the three types of thermal protection, how each one works, why fit matters as much as or more than thickness, and which pieces cover the areas of greatest heat loss.
As you saw in the articles on thermoregulation and heat transfer, cold degrades dexterity, judgment, and nitrogen elimination. The suit is the main tool for controlling it.
The wetsuit
It's the most widespread and the most misunderstood. A wetsuit doesn't keep water out: its whole function is precisely to let in a minimal amount and hold it still against the skin, where the body warms it and it starts acting as insulation. The neoprene, with its gas bubbles, adds its own insulating capacity.
From this you can see why fit is decisive. If the suit is loose, water circulates freely, keeps getting renewed, and convection (which you saw in the article on thermal physics) restores the heat loss. A loose seven-millimeter suit keeps you warmer worse than a well-fitted five-millimeter one.
Typical thicknesses range from three millimeters for warm water to seven or more for temperate and cold water. There are also two-piece variants, with an additional vest or bib, that double the thickness on the torso, the area where retaining heat matters most.
One effect worth knowing: neoprene compresses with depth, so a wetsuit keeps you warmer near the surface than deep down, and it also provides less buoyancy, with the consequences for weighting you saw in the corresponding article.
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