06 / 07 — Life-support stack
Exposure Suit
Foam full of bubbles, fighting the water’s heat theft.
Water conducts heat about 25× better than air. Neoprene fights back with millions of trapped nitrogen bubbles — and at the extreme, drysuits give up on “wet” entirely and keep the ocean off your skin.
Lift per millimetre of neoprene at the surface
0.7kg/mm
- Thickness
- 3 / 5 / 7 mm
- Heat loss in water
- ~25× air (conduction)
- Insulation
- Trapped N₂ bubbles
- Buoyancy (7 mm)
- ~+5 kg at surface
Foam, seals and a zip
Drag or use the arrow keys to orbit. Press plus or minus to zoom, Home to reset. Use the parts manifest to move between callouts.
Drag to orbit · Tap a badgeDrag sideways to orbit
FIG 6.1 — Wetsuit 7.0 mm, Assembled
Try the variations
Suit
Drag the thickness slider — watch the suit bulk up and the buoyancy readout climb.
01Neoprene core
Synthetic rubber foamed with nitrogen micro-bubbles. Gas insulates, water conducts — so the bubbles do the actual work while the rubber just holds them in place. Thicker means warmer, stiffer and more buoyant, all at once.
Parts list5 parts
Neoprene core
Synthetic rubber foamed with nitrogen micro-bubbles. Gas insulates, water conducts — so the bubbles do the actual work while the rubber just holds them in place. Thicker means warmer, stiffer and more buoyant, all at once.
Thermal lining
Plush or metallised linings mostly cut water movement against the skin and make the suit easier to pull on; the thermal gain is small (a claimed 1–2 °C) and comes from reduced flushing, not reflected radiation — radiation is negligible underwater.
Wrist & neck seals
Smooth-skin or glideskin cuffs that press against your body and throttle water exchange. Flushing — cold water pumping through a loose suit — is the enemy; seals are the fix.
Zipper & flap
Usually a back zip with a long leash you can pull solo. Every zip is a leak path and a heat escape, so premium suits minimise, baffle or move them to the chest.
Hood
In a suit, your bare head is the one uninsulated surface left, so it leaks heat far out of proportion to its size. Below ~21 °C a hood stops being optional — and it protects the ears from cold-water bone growth (surfer’s ear).
Trapping heat with trapped gas
A controlled leak
A wetsuit lets in a thin layer of water on entry — that first cold shock — then the seals trap it. Your body pays to warm that layer once; if the suit fits it stays put, so you never pay again. The water itself insulates almost nothing — the neoprene does the work, and the seals stop cold water replacing the warm.
The buoyancy tax (Archimedes)
Neoprene is mostly gas, so it floats: roughly 0.7 kg of lift per millimetre of thickness for a full suit — about 5 kg for 7 mm, 2 kg for 3 mm — and it compresses away at depth. You carry lead to sink it at the surface, then add BCD air at depth to replace the lift you lost. Thermal protection is paid for in ballast.
Bubbles do the work
Neoprene is full of nitrogen gas cells, and gas conducts heat terribly — which is exactly what you want. The rubber matrix is just scaffolding for billions of tiny thermoses.
Conduction, not radiation
Underwater, heat leaves you by conduction into the water and by convection when that water moves. Radiation is negligible — so a shiny “heat-reflecting” lining cannot do what the label implies. The physics that works is thickness, dry gas, and stopping water from moving.
Depth crushes your warmth
Those bubbles shrink under pressure — a 7 mm suit has lost roughly half its thickness, insulation and lift by 30 m. You get colder and heavier as you descend, which is why the suit and the BCD are one system.
Why fit is thermal performance
A loose suit flushes warm water out and cold water in with every kick — you become your own heat exchanger. Snug (not constricting) fit is worth more than an extra millimetre of neoprene.
The drysuit endgame
Drysuits seal water out completely with latex or silicone gaskets and a waterproof zip; you insulate with the air inside and thermal underlayers. You add air on descent to stop the squeeze and vent it on ascent — a wearable second BCD, and one that needs its own training.
Squeeze the neoprene
Dive profile — one timeline, seven instruments
T+12 minAL80 · SAC 18 L/min · 5 mm suit
Cylinder
136bar
Wing
1.6L
Suit
2.5mm
Mask air
113mL
NDL left
10min
Ceiling
0m
Ascent
0.0m/min
Depth 30.0 m · ambient 4.00 bar. Drag the needle, or use the arrow keys.
Drag or use the arrow keys to orbit. Press plus or minus to zoom and Home to reset the view.
5 mm suit — 2.5 mm at 30 m
The compression problem
Foam crushes, air can be topped up
Neoprene is foam, and its bubbles shrink with depth: the suit gets thinner, colder and less buoyant all at once, and nothing puts the gas back. A drysuit's air space crushes too — but you can add air on the way down and hold its lift.
Challenge
Take the drysuit to 30 m and add air until the squeeze clears — then look at how far the wetsuit's bar has fallen behind it.
Suit
Suit lift at 30 m
Gap −0.9 kg
- Wetsuit 1.8 kg
- Drysuit 0.9 kg
Wetsuit lift
+1.8kg
Drysuit lift
+0.9kg
Neoprene now
2.5mm
Drysuit air
0.9L
Foam lift lost
1.8kg
Ambient
4.0bar
Surface lift
3.5kg
Neoprene lift
0.7kg per mm
At 30 m the 5 mm suit behaves like 2.5 mm and floats 1.8 of its 3.5 kg — the BCD has to find the other 1.8. Holding the drysuit at its trim takes 13.7 surface litres of air, 10.2 of them added on the way down.
Takeaway — locked
Hold the drysuit at its trim at 30 m to unlock what this proves.
0.7 kg of lift per mm at the surface · foam thickness ∝ 1/√P · drysuit air by Boyle · seawater at 1.025 kg/L. Underlayers and trapped air in the wetsuit are ignored.
How much rubber for your water?
Choose for the water you will actually dive most, not the coldest dive you can imagine. Every millimetre buys warmth and costs flexibility and lead. When the water drops towards 10 °C, or dives get long, the honest answer stops being a thicker wetsuit and becomes a drysuit and a course.
| Criterion | 3 mm shorty / full | 7 mm temperate | Drysuit |
|---|---|---|---|
| What it is | Tropical thickness. Mostly about scrapes, stingers and sun more than serious cold. | The temperate-water workhorse, usually with a hood and often a hooded vest underneath. | A sealed shell of trilaminate or crushed neoprene — the ocean never touches you, and you wear real clothes underneath. |
| For |
|
|
|
| Against |
|
|
|
| Best for | Tropical reefs, liveaboards, multiple dives a day in bathwater. | Temperate reefs, kelp forests, quarries — where most of the world actually dives. | Cold water, ice diving, long deco dives — anywhere a wetsuit physically cannot cope. |
3 mm shorty / full
- What it is
- Tropical thickness. Mostly about scrapes, stingers and sun more than serious cold.
- For
- Featherlight, packs tiny
- Huge flexibility
- Only ~2 kg of lift — easy to sink
- Against
- Chilly below ~26 °C on long dives
- Minimal abrasion armour
- Best for
- Tropical reefs, liveaboards, multiple dives a day in bathwater.
7 mm temperate
- What it is
- The temperate-water workhorse, usually with a hood and often a hooded vest underneath.
- For
- Real insulation down to ~10–15 °C with a hood
- Tough against wreck scrapes
- Cheap and everywhere
- Against
- ~5 kg of lift to sink with lead
- Restrictive shoulders
- Gets cold fast if it flushes
- Best for
- Temperate reefs, kelp forests, quarries — where most of the world actually dives.
Drysuit
- What it is
- A sealed shell of trilaminate or crushed neoprene — the ocean never touches you, and you wear real clothes underneath.
- For
- Warm in literally ice water
- Insulation does not compress away
- Layering tunes warmth precisely
- Against
- Requires training — air management inside the suit
- Expensive, plus seal and zip maintenance
- Bulkier topside
- Best for
- Cold water, ice diving, long deco dives — anywhere a wetsuit physically cannot cope.
What fails, and what you do
Hypothermia
What you see
It creeps: shivering, clumsy hands, bad decisions. Fine-motor skill and judgement go before you feel dangerously cold.
Response
Shivering is the end-the-dive signal, not something to push through. Warm up between dives — dry, out of the wind — and go thicker next time.
Overheating on the surface
What you see
A 7 mm suit in the sun on a boat deck is a sauna; divers have fainted before getting wet.
Response
Suit up last, or halfway — arms out — and get in the water. Drink water: dehydration also raises your decompression-sickness risk.
Flushing
What you see
A loose neck or a burst zip pumps cold water through the suit with every kick, and the insulation you paid for is gone.
Response
Fit is the fix: snug seals, a hooded vest under a 7 mm, and a zip you rinse and lubricate.
Why cold is a safety issue
Cold divers breathe more gas — shivering burns oxygen hard. Proper exposure protection literally extends your bottom time.
Hypothermia creeps: fine-motor skill and judgement degrade before you feel “dangerously cold”.
A wetsuit’s lift vanishing at depth is why you cannot set and forget your BCD — the two systems are coupled.
Peeing in your wetsuit is a brief warm lie followed by a flush of cold water. Everyone does it once.