Drysuit Diving Basics: How It Works, Training and Weight

How does a drysuit keep you warm?

The gas inside it, held by the undergarment. A 2013 study of shell drysuits in 13°C (55°F) water states the mechanism directly: an undergarment beneath a drysuit 'relies on entrapment of gas (usually air) in the interstitial spaces of the garment material for thermal insulation'. BARE's manual agrees: 'Drysuit isolation depends on the proper under garments and their thermal protection properties.' The shell keeps water out and holds that gas in place. See also our gear guides.

So the shell decides how much undergarment you need. BARE describes trilaminate as fabric whose 'only job is to keep water out', with no thermal properties of its own. Neoprene insulates by itself, so undergarments can be thinner, but standard closed-cell neoprene loses insulation and buoyancy as its cells compress. Thermal isolation drops under pressure whatever the material, the same effect our wetsuit thickness guide covers for wet neoprene.

The buoyancy consequence catches wetsuit divers out. A wetsuit only ever loses lift as you descend. A drysuit holds a gas volume you control with two valves, and BARE puts the requirement plainly: gas 'will always have to be added to the suit on descent, and removed as it begins to re-expand on ascent'.

Drysuit types, seals and boots: what each choice costs you

SDI's standards reduce the suits to three families: shell style, crushed neoprene and neoprene, with seals in latex or neoprene. The table adds what each option gives up. We have not tested any suit named here; the figures come from maker documentation and agency standards.

Boots matter more than they look. BARE offers attached heavy-duty boots, or soft neoprene socks worn inside a separate lace-up boot, and says that boot's lace front and ankle Velcro 'helps prevent any excess air or gas from entering the foot area while diving'. That is the problem two sections below.

Why a drysuit needs its own course

Because the suit becomes a second buoyancy device with its own failure modes. SDI's approved outline shows what that means: suit and seal types, undergarments, both valves, weighting, maintenance, and a section called 'Dry Suit Emergencies' covering excessive air in the suit, an inflator stuck open, an exhaust valve stuck closed, a dropped weight, excessive air in the feet and a flooded suit.

The requirements are close but not identical, and minimum age is where they part company.

No agency here calls a card a legal requirement, and BSAC opens its workshop to any qualification level. What it buys is access: BSAC says the qualification is what lets you 'hire a drysuit when you need to' at dive centres and resorts.

The two valves: adding air on the way down, dumping it on the way up

One valve on the chest lets gas in from a low pressure hose, one on the upper left arm lets it out. SDI describes the inflator as push to inflate, to maintain the air space the suit creates, and the exhaust as an adjustable open and close system. SI TECH places the exhaust at the upper arm so you can vent while controlling ascent speed, and makes it adjustable between 10 and 200 mm of water column.

The dump is a dial, not a switch. Turning the cover clockwise towards the plus sign holds more gas in the suit, counter-clockwise holds less, and a ratchet clicks so you can adjust it underwater. BARE's procedure follows: fully clockwise on the surface so you float, then empty the BCD and turn counter-clockwise until you sink. As the suit compresses, press the inflator in short bursts; if bubbles leave the dump while you do it, turn the dump clockwise until they stop.

On ascent a fully open dump vents by itself, which BARE calls a hands-free feature that keeps the ascent rate fairly constant. SI TECH says a correctly set valve on a correctly weighted diver supports an ascent of roughly 10-18 m/min (35-60 ft/min). If the suit expands faster than the valve can cope with, slow down and vent manually, which moves far more gas. The valve has to be the highest point, so lift the elbow.

Here the sources disagree, so expect to hear both. SI TECH recommends limiting the number of gas-filled compartments and says the BCD 'should normally not be used for buoyancy control during the dive'. BARE's manual says the opposite: 'A BCD system should always be worn in conjunction with a drysuit.' SDI splits it with 'Dry suit is not a substitute for a proper BCD', and DAN's commentary on a runaway ascent is firmer: 'The BCD should be used for controlling buoyancy, and the drysuit should be kept minimally inflated.'

Gas in the feet and the inverted ascent

This is the failure mode that sends drysuit divers to a chamber. DAN's medical guidance states the cause in one sentence: 'Inversion is caused by an accumulation of gas in the legs of your suit.' Once your feet are above the dump valve the gas cannot reach it, the feet rise further and the ascent accelerates.

Two DAN cases show what that looks like. A diver with about 150 dives became inverted at 27-30 m (90-100 ft); her buddy grabbed a fin, dumped his own BCD, could not stop her, and let go at 5 m (15 ft). Another, 70 dives in and on his third dive in a new trilaminate suit, could not keep his legs down and rose from 15 m (49 ft) to the surface in seconds, then needed a US Navy Table 6 about 45 hours later. He blamed rushing the details and skipping the buoyancy check.

What agencies and DAN say to do:

Sources disagree about ankle weights. DAN's medical article does not mention them and puts weighting first, then gaiters or straps. A diver writing up his own incident in Alert Diver planned to practise shallow 'with a good buddy and some ankle weights'. A BSAC instructor writing on trim says that for tall divers whose weight pockets sit too high, 'Buying ankle weights to counter this is not the best solution', preferring a weight belt or moving the cylinder down the back, though he grants that for shorter divers with a short cylinder 'ankle weights do seem a good solution'.

• Weight correctly. DAN calls proper weighting the best prevention, because it cuts the gas the suit needs.
• Keep the suit minimally inflated: DAN's runaway-ascent commentary says only enough gas to stop the suit squeezing you.
• Fit gaiters or elastic straps, which DAN says 'will also minimize the migration of gas to your legs'.
• Close the ankle. BARE says the lace front and ankle Velcro on a rock boot help keep excess gas out of the foot area.
• Practise the recovery. SDI requires 'Roll from inverted position' in the pool session and on both open water dives.
• Check why the dump is slow. SI TECH lists valve position, gas pocketing above the shoulders, permeable underwear and too little lead.

How much lead does a drysuit add?

Two to five kilos over a thick wetsuit, and the undergarment decides where in that band you land. PADI's short rule is 15% of body weight or more for a drysuit against 10% for a 5-7 mm wetsuit, plus about 2 kg for an aluminium 80. Our scuba weight calculator splits the drysuit case by undergarment, and the table shows what it returns for an 80 kg diver with an aluminium 80 in salt water.

Each figure is a starting point for an in-water check, not an answer; our weighting guide has the check itself. Two things move it during the dive: SI TECH reckons a cylinder loses roughly 3 kg (6 lb) of gas weight from start to end, and BARE makes the same point about the tank gaining buoyancy as you breathe it down.

Overweighting is the trap, and it feeds the inversion problem. As DAN's cold-water article puts it, 'if you are overweighted, you will have to add more air to your suit', and more gas in the suit is more gas that can run to your feet. PADI lists the other costs: more air used, a diver who drops straight to the bottom, and worse outcomes if a BCD fails.

Undergarments, cold and decompression risk

The undergarment is the warm part, so buy it with the suit. SDI asks for insulation cut close to the skin, compression-resistant, in polyester or polypropylene. BARE's rule is to maximise insulation and minimise bulk, because a thick lofted garment traps a large volume of gas and adds buoyancy you then cancel with lead.

Does being cold change your decompression risk? The principle is accepted and the direct evidence is thin. A 2021 Frontiers in Physiology paper summarises the reasoning: 'a diver who is cold during the bottom phase and warm during decompression would desaturate more efficiently and therefore may have lower DCS risk' than the reverse, and notes that US Navy procedures require longer decompression when divers are 'exceptionally cold'. That paper's own rat experiment found no change in bubble grades after cold decompression, and its authors would not conclude either way.

DAN's advice for heated undergarments follows the same logic: keep them off during descent and the bottom phase to limit inert gas uptake, then raise the setting modestly during ascent and stops. Warmth is a health question in its own right, and BARE warns that poor thermal protection can cause trouble in both directions, hyperthermia as well as hypothermia.

Drysuit care: zips, seals, leaks and storage

A drysuit fails at the zip, the seals and the valves. One point is genuinely contested: SDI's outline says to use 'only paraffin wax, never silicon spray' on the zipper, while Santi's sheet for the TiZip plastic zip says to 'Use the original TIZIP silicone-based lubricant only' and warns that without it the sealing lips deform and leak. They are describing different zips, so follow the instruction for the zip your suit has. BARE only bans aerosol and petroleum-based lubricants.

• After every dive, rinse the outside with fresh water and rinse the neck and wrist seals inside. Wash grit out of the zip teeth.
• Flush the exhaust valve while turning the lid back and forth, so debris does not sit under the check valve (SI TECH).
• Dry the suit inside first, hung over a wide tube rather than a thin hanger, out of sunlight and fluorescent light.
• Talc the inside of latex or silicone seals; those do not need folding, fold-over neoprene ones do.
• Keep petrol, solvents and solvent-based cleaners off butyl and neoprene; warm water and detergent for stains, then rinse fully.
• Store the TiZip closed, clean, dry and unbent, never above 60°C (140°F). A damp suit hangs with the zip fully open.
• Have the valves serviced annually by an approved technician (SI TECH and BARE both say so).

What does it cost to start diving dry?

Rent first: the course usually includes a rental suit, and a season tells you whether you will use one. A Vancouver dive centre lists a drysuit package at 110 dollars per 24 hours with undergarments, hood, boots, BCD, regulator, a tank and weights, or 65 dollars for the suit with boots and undergarments alone. A southern Californian shop breaks its PADI course into a 250 dollar course fee, a 40 dollar manual, 35 dollars pool entry and 99 dollars per dive day for suit and undergarment rental.

For buying, one US dive centre's guide says you can get into 'a decent entry level suit for around $1500 (USD)' and that better suits cost at least three times that, expecting 100-500 dives from an entry-level suit and 500-1000 or more from a premium one. The same Californian shop quotes roughly 1,600-2,800 dollars for a custom suit with a four-month lead time. Prices vary by country and by retailer.

Add the parts that do not come in the box: an undergarment, a rock boot if the suit has socks, a low pressure inflator hose, and more lead than you own. Our rent or buy guide covers the rest of a kit.