Drysuits and Undersuits

Drysuits and Undersuits

  • Dive Life

Understanding Drysuits and Undersuits, and your decision to buy the correct ones for your diving. 


By reading this article, divers will gain a thorough understanding of:

  • Heat loss in the body, hypothermia.
  • The advantages of drysuit diving.
  • The different types of suits available.
  • Drysuit components.
  • Drysuit configuration.
  • Properly donning and removing the drysuit
  • Understand correct fit.
  • Drysuit accessories.
  • Undersuits exist and how to combine them.
  • How to control the suit and get great buoyancy and trim.
  • How to look after your suit.


As you are well aware, during any dive you are going to lose heat. With correct insulation, you can manage this so that you remain comfortable. If you don’t recognize the warning signs your body will respond and try and protect your core temperature. It will do so by restricting the circulation to the extremities such as the fingers and toes and focus on your core.


  • Uncontrollable shivering – this is your body’s attempt to generate heat.
  • Numbness in the extremities.
  • Blue lips.
  • General fatigue (sleepiness).

The real trouble begins when these warning signs start to disappear. When the body’s mechanisms begin to fail, the body will stop shivering and the warm blood that was protecting the core is released into the rest of the body. This may result in temporarily feeling warmer. However, as the warm blood cools down and returns back to the body's core, it cools the core rapidly, unconsciousness and if not treated, eventually death may follow.



Your body core temperature is approximately 37C or 96.6 Fahrenheit. Any temperature below this and your body will work hard to maintain its core temperature. On a pleasant summer day of 23C or 73 Fahrenheit you may feel quite comfortable on the surface however water conducts heat 20 times faster. Unprotected in the water you will soon get cold even in relatively warm water.

NOTE: Hypothermia can be a big safety concern as it interferes with decision-making abilities, strength, and endurance.


Your body’s muscles are heat engines, they consume oxygen and calories and create heat during exercise. The circulatory system distributes the heat generated by the muscles. If you exercise a lot, you produce a lot of heat. If you exercise little, you produce little heat. To keep warm, you must maximize the blood circulation within your body.

The important thing to remember is that each individual has separate, distinct thermal needs.

Even the same diver will require different amounts of insulation depending upon their activity in the water. There's a big difference in the heat production created by strenuous current diving and casual underwater photography.

Being warm is not just a comfort issue - it is a safety issue.


The variables that affect a diver are:

  • Insulation suit type
  • Water temperature 
  • Exposure duration
  • Body type
  • Acclimatization 
  • Activity



The most common form of in-water insulation is a wetsuit. Wetsuits are made from neoprene which is essentially synthetic rubber filled with air bubbles. The air bubbles provide insulation from the water. As the name implies a wetsuit allows water between your skin and the insulating neoprene outer layer. Your body heats the water and provided there is limited water circulation in and out the suit, the neoprene will reduce the heat loss to the water outside the suit. The amount of circulation of water is greatly dependent on the type and quality of the suit. Semi-Dry suits or suits with flow-restricting features (seals on legs, arms, and neck) will feel warmer due to less water circulation. With a wetsuit of any type, the body still loses heat to the layer of water between the neoprene and the skin, but the heat loss is reduced due to the insulating qualities of the neoprene.  

The thicker the suit the more insulation is provided. However, as you go deeper the bubbles compress so you will find that you have less insulation as you go deeper.



Drysuits exclude all water. They are sealed 100% at the arms, legs, and neck. They also have a waterproof zip and the seams are also waterproof. The drysuit shell itself often provides very little insulation, its primary job is to exclude water.

As there is no water in the suit you can wear insulation under the suit. Manufacturers of insulation will usually provide a guide on the level required for different conditions. This approach allows each diver to adjust the level of insulation for their personal comfort and requirements based on water temperature, activity, dive duration, etc.

Unlike a wetsuit, the drysuit insulation properties are not affected by depth. However, the air in the suit will compress and cause a squeeze on the body if not compensated. To achieve this your suit will be fitted with an inflator and valves for overpressure or exhaust.


Water temperature might vary at different depths. It is therefore important that you select the correct undergarments for your dive in order to provide you with enough insulation at the bottom as well as during the ascent.


Dive duration plays a big role in heat loss, no system is can completely eliminate heat loss and even the best-insulated diver will lose some heat. The longer you are in the water the more heat drains away. It’s hard to pre-determine how much this will affect us, will we be warm for 45 minutes or can we push it to 75 or even 120 minutes? There are no set times for length of exposure due to the simple fact that not one situation or condition is the same for every diver and each diver is unique.

Experience is the key here, after repeated dives in the same set up you know how long you will feel comfortable in certain temperatures.

The desire of technical or extended range divers is to dive deeper, or to extend their in-water times, sometimes both. It is vitally important that these dives are proceeded by checkout or warm-up dives. These dives will give a clear indication as to whether the drysuit and undergarments are capable of the task. Unlike recreational divers, these divers commit to a dive that has a decompression obligation. There is no point in racking up a decompression obligation if the water temperature forces you to have to abandon your plan.


Body fat and surface area to mass ratio.

It is believed that people with less body fat and more lean muscle mass will be able to tolerate cold for longer than a person with average body fat and muscle group. On average it is more common for women to show signs of cold before men which can be attributed to the average body fat percentage of women vs. men.


It is often a surprise for traveling divers, who have been diving in colder northern waters, to find divemasters or instructors who dive in warmer waters using drysuits.

This is not uncommon and is due to the simple fact that when a diver is constantly in the water the body core temperature drops and thus the diver feels the cold more resulting in the use of warmer suits.


It is important to judge your activity level and thus the thermal required, before the dive. The decision on what to wear should be based not only on water temperature but activity at depth versus activity in the shallows.

It is not uncommon for technical divers to feel colder during their decompression stops than at the bottom, due to the decreased physical activity, luckily shallow waters often tend to be warmer anyway.

The best is to find a balance between activity levels and exposure protection. The only way of finding this is through practical applications and experience.


There are 2 main types of suits on the market today:

  • Neoprene drysuits 
  • Trilaminate and membrane drysuits

Since new developments are released into the market all the time, it is difficult to keep up with the rapid changes in suit technology. Therefore, before investing in a drysuit always talk with your RAID Instructor or Dive Center for the latest advice.

  1. Neoprene Drysuits

Neoprene wetsuits and drysuits are basically made of the same fabric. However, drysuits have a watertight zip, neck and wrist seals.

The main consideration when selecting a neoprene drysuits is that they have a large buoyancy variation due to the compression of the material. When diving a neoprene suit, the diver will require a large amount of weight in shallow water and less and less as the depth increases.

Crushed neoprene suits are often the suit of choice when selecting a neoprene suit. Crushed neoprene starts life as a thick neoprene but is then crushed or compressed, which reduces the size of the bubbles. This makes the material very dense and durable. As the neoprene has been compressed, there are no inherent buoyancy effects when diving the suit.

The advantage of this material is that it provides a combination of flexibility and durability, which makes crushed neoprene popular. Crushed neoprene suits tend to be warmer than membrane or trilaminate suits with the same undergarments because the material retains some insulation.

The disadvantage to these suits is that they are heavy compared to the membrane or trilaminate types, and more so when wet. They dry slowly, which can be inconvenient when traveling or wanting to do a quick repair.


  1. Trilaminate or Membrane Suits

Trilaminate Suits are probably the most common type of drysuit found on the market today. Within this category, you will find many different types of material, from the thicker more traditional trilaminate to the thinner and lighter rip-stop type fabric. Manufacturers who use the lighter fabric will reinforce certain areas with a heavier fabric for longevity.

Since the type of fabric used may vary from one material to another, and since the build of the suit (waterproofing and seam weld sealing) may also vary these suits vary in price dramatically.

These suits are generally made from polyester or nylon or a similar trilaminate fabric, coated with either a combination of urethane, polyurethane or polyester butyl laminate or similar waterproofing.

These suits tend to be compact, lightweight and quick-drying, making them a good choice for travel and storage. These suits seldom have any inherent positive or negative buoyancy and little to no thermal protection.

Traditionally the materials used do not have any flexibility and designs needed to be quite large to allow for body movement. More modern suits panels allow a more tailored fit and in recent years the material has gained an enormous amount of flexibility.

As the material itself has no real insulating ability, you rely entirely on the undergarments and this must be taken into account when considering this type of suit.

Note: The best performing suits, whether neoprene or membrane are of course those custom made for you.



Here are some of the features of the drysuits.


There are two types of zippers on the market today, the plastic waterproof zipper, and the metal zipper. The metal one is the stronger of the two and will last longer, it is also much less flexible which may restrict the suits range of motion.

Socks are pretty straight forward, they are neoprene, and overboot is used. These are often referred to as rock boots, the advantage is they may have slightly fewer buoyancy issues, and there is the ability to replace the boot easily.BOOTS
Modern suits are fitted hard boots or neoprene socks.

  • Fitted boots come in a few varieties, there is the basic wellington type boot which is quite rigid and buoyant, and the hard sole neoprene boot which has more favorable buoyancy characteristics. Once these boots wear out, they have to be removed and replaced by the manufacturer or an appointed service center.


Neck and wrist seals form an integral part of your suit, as they are the main barriers against water ingression.

There are three main types available today:

  • Neoprene
  • Latex
  • Silicone

Neoprene is by far the warmest seal type, it’s easy to repair and generally lasts quite a while. It can take some abuse before it fails. Neoprene seals are always glued and taped directly onto the suit.

Latex seals are a soft, supple, seal which are easy to slide your hand in and out of. With latex seals, there is less bulk around the wrist. New seals are sized by trimming, little by little after allowing for a stretch after use. When damaged, they are relatively easy to replace.

In regard to the neck, latex neck seals are not as popular as the neoprene seal, due to the tight fit of the latex around the neck. In colder waters, the poor insulation properties of latex create a cold area around the neck. Often a neck over collar made out of neoprene is installed to add extra warmth to the suit.

Once latex starts degrading it is key you replace it immediately before it starts breaking down into a sticky rubbery substance.

Silicone seals are the softest and most flexible seal type. They cannot be glued directly onto the suit; this seal type can only be used with a changeable seal system. Changeable seal systems can be used on the neck and wrist. Silicone has a risk of tearing if treated incorrectly. This type is the preferred option for the more elaborate suit designs due to its flexibility and quick-change function. 



As you descend, the air in the suit compresses and causes suit “squeeze”. In order to prevent this squeeze (and to act as insulation), you will need to add air to the drysuit. You will need to add and release gas from the suit as you descend and ascend to keep the suit in control and close to neutral buoyancy. Much like your BCD or wing, it will have an inflation valve and an exhaust valve.

There are three main types of valves to consider:

  • Inflator valves 
  • Exhaust or dump valves 
  • Pee valves

Inlet or inflator valves are pretty straight forward, the drysuit is equipped with a power inflator much like your BCD or wing. It is designed as a round, streamlined button design. They are fitted to the chest panel and connect to the first stage with a low-pressure hose.

Exhaust or dump valves come in two different styles. The cuff dump is the simplest and oldest design and is positioned on the drysuits cuff the diver needs to raise their hand to dump gas, these dumps are not adjustable. Cuff dumps are not as popular as shoulder or auto dump valves. The auto dump is the more modern of the two and has become a standard feature on almost all new suits. The auto dump is positioned on the shoulder and has an adjustable setting from nearly fully open (no resistance) to nearly fully closed (a lot of resistance) it also dumps when you press it down. There are various designs on the market but in essence, they all function the same.


Pee Valves

Holding back the urge to urinate for a long period of time can lead to a symptom called renal reflux. The "Pee valve" is considered a must for any drysuit diver doing moderate to long underwater exposures. Without a way to relieve themselves, the diver may intentionally attempt a state of dehydration, before or during a dive, to prevent the need to urinate. This is unhealthy and even potentially dangerous.

Hydration is almost certainly one of the main components to reducing the risk of increased DCS. There is growing evidence which indicates a clear link between decompression illness and dehydration.

There are pee valves for both men and women. The pee valve is usually installed on the inner thigh. Inside the drysuit, the urination tube runs from the pee valve to an external catheter. The diver makes the final attachment to the drysuit before donning the upper body of the suit.

External catheters may be purchased at medical supply stores that carry medical equipment. For Ladies there are a few options, the most popular is the SHE-P, speak to your local RAID Center for more information.



Standard wet or semi-dry gloves can be used when drysuit diving. For colder conditions, dry gloves have become a very popular option. The dry glove system connects to the suit with a push-on or twist-on fitting creating a seal between the glove and the suit.

The outer glove becomes part of the drysuit and needs to have air added and removed in order to provide insulation and prevent squeeze. An under glove is needed to provide increased thermal insulation. Different thicknesses of outer and under gloves exist to mix and match for the perfect setup. Heated under gloves have also become available in recent years. One of the disadvantages of this system is that since a connection needs to be made between the dry glove and the suit, should the glove tear or leak, it not only causes very cold and wet hands, but it can flood into the drysuit causing the suit to become waterlogged.

Most premium dry gloves systems have become very reliable.



Most heat is lost through a diver’s head. Hoods protect our heads from that heat loss. Sperate wet or semi-dry hoods may be worn, but some drysuits have a built-in hood. Presently, most divers prefer to have a separate hood.

Hoods can be fitted with and without a bib. Bibbed hoods have material around the neck and are often used by wetsuit divers in colder waters as it allows them to put the hood under their suit. Some drysuit divers prefer the bib as well as they can tuck it into their ‘warm collar’. The choice between the three different hood designs used for drysuits is entirely yours. We do advise that you speak to your RAID instructor or dive center for advice. 


Most drysuits are fitted with pockets as standard. This allows you to store your wet notes, SMB, spool, spare mask, spare light, etc. for easy access. Some budget suits come without pockets or just one pocket if you are thinking about buying a suit always buy one with pockets so that you are future-proofed.

Pockets come in many different types and sizes. Some use zips to close them, others use Velcro. The best style of pocket is those that bellow (expand when full), they are easy to access and adjust in size based on what’s in them. Look for bungee loops that come from the side or bottom of the pocket, so you can clip your items onto the loops, that way you won’t accidentally drop something when opening your pocket.



Their main purpose is to hold the suit up and prevent sagging at the waist. The waistband helps to keep the suit tight around the small of the back. The suspenders hold the suit up around your waist when wearing it partially off before and between dives.




A telescopic body is a drysuit body with a fold in it, this fold is kept neat and tidy with an elasticated crotch strap. This type of feature allows a suit to be made tighter and better fitted to the diver’s body as it allows for expansion during donning and removing the suit. It also provides increased flexibility when moving the arms above the head.

Ask your RAID Dive center to show you the difference between a suit with and without a telescopic body, and you’ll see the benefits.



Building a well-fitting drysuit is a work of art, there are many manufacturers out there, but a well-fitted suit starts with good advice and accurate measurement in the RAID Dive center.

Questions you’ll be asked:

  • Where do you want the pockets placed, standard position, a little more forward or a little more to the side?
    Migrating the pocket position slightly more backward makes them easier to reach if you have several stage cylinders on your side or if you are diving side mount.
  • Where do you want the dump valve? The traditional dump valve position does not lend itself for horizontal trim diving, you would need to come completely out of trim to properly dump your suit, instead, a position near the back of the shoulder allows for dumping gas with the slightest of movements out of trim, meaning that you can stay in nearly perfect trim, and only need to tip your shoulder a small amount.
  • Changeable wrist seals, asking to have the cuff placed about an inch out from the standard position increases the arm length by a small amount, allowing for the RAID Superhero trim position. Think about it, if your arms are out above your head (whilst standing up) you need slightly longer sleeves, especially if you bend them a little.
  • Tailoring the neoprene socks and calf area, ensuring that this area is not oversized for ‘easy entry’, as it would trap excessive air and create unwanted buoyancy in the legs.

Ask for help from your RAID instructor they are trained to understand the dynamics of good drysuit control, and what you need from your drysuit.



Before checking the fit of the suit, check the fit of the undersuit and baselayers. An ill-fitting undersuit will affect the fit of the drysuit. The drysuit must be fitted while wearing the required undergarments.

Check the fit of the seals, seals that are too loose will let water in, seals that are too tight will restrict blood flow and create unnecessary discomfort.


  • The drysuit must not restrict breathing
  • Feet must not be cramped

Complete the range of motion exercises:

Overhead reach: Reach up with both hands as if you were trying to reach the valve on your tank; you should be able to do this without the drysuit pulling up tight in the crotch.


Cross your arms reach: Reach across the chest with both arms (like as if you were hugging yourself.) You must be able to reach and operate the exhaust valve on the left shoulder


Crouch and reach: Kneel down, sit back over your heels and lean forward. This position checks the leg length and torso length at the same time.

The drysuit should not be restrictive or binding.

Unfortunately, rental drysuits suits are never going to be a perfect fit, drysuits, much like performance clothing, are a very personal item. When you buy your own suit ensure a good fit, failing to do so will make your suit control more difficult and thus hamper your progression as a drysuit diver.

Common fitting errors:

  1. Boots or Socks too big: This allows too much air in the legs, resulting in floaty feet, poor power transfer to the fins, and risk of the boots popping off.
  2. Calf section too long or too wide: This allows is too much air in the legs, again resulting in floaty feet.
  3. Arms too short: This results in an inability to hold the RAID Superhero trim position which in turn will affect the overall trim of the diver.


Proper maintenance of your drysuit will greatly extend its useful life.

When finished diving for the day, rinse the outside of your drysuit thoroughly with fresh water. Pay particular attention to the zipper and valves. Flush the exhaust valve and the inlet valve with running water. Blow the valves dry with compressed air after flushing.

Drysuit valves must be cleaned after each use in the same way that your regulator must be cleaned. Inlet valves may stick due to a build-up of salt in the valve. Exhaust valves may stick due to lint, dirt, sand or hair in the valve.

If the inside of your drysuit becomes wet, rinse the inside of the drysuit also.


Latex Seals: Wash latex seals with a mild soap and water solution after every few dives or before storage of the drysuit. Accumulated body oils will shorten the useful life of latex seals.

Silicone Seals: Due to the nature of the material, silicone seals can attract dirt and lint. Use mild soap and water and a soft cloth to clean your silicone seals when needed.

After rinsing the drysuit, open the zipper, and hang the drysuit by the socks or boots over a line or drying rack in a shady spot to dry.

When the drysuit is completely dry outside, feel the inside of the drysuit (all the way down to the socks or boots.) If there is any moisture inside the drysuit, turn the drysuit inside out and allow the inside to dry as well.

There are specific drysuit hangers developed for safe drying and storage of your suit. Many manufacturers supply seal lubricants that not only clean but moisturize the seal, thereby extending its life.



Your waterproof zipper is the heart and soul of your drysuit. Just like eating right and getting exercise is good for your heart, follow the points outlined below to give your zipper a long and happy life:

  • Do not fight your way in or out of your drysuit - it stresses the ends of the zipper.
  • Make certain your zipper is open all the way when putting on and taking off your suit.
  • Before taking off your self-don suit, undo your crotch strap and pull the telescoping torso above your waist - this will give you plenty of room when pulling the suit over your head.
  • Rinse your zipper with fresh water after every dive day.
  • Lubricate your zipper after every dive day with manufacturer-approved lubricant on the exterior portion of the drysuit zipper.
  • Do not use silicone spray as it attracts dirt.
  • Every six months or 25 dives, gently scrub your zipper with a soft toothbrush using mild soap and water. Lubricate your zipper after this process.
  • If your zipper is fraying, remove any excess threads with a small pair of sharp scissors, be careful not to cut any of the rubber just the loose threads.
  • On steel zippers there are internal teeth, these need to be lubricated with a brush on lubricant.
  • Store your drysuit with the waterproof zipper in the open or partially open position.



Proper storage will extend the life of your drysuit. Store your drysuit in a cool dry place on a wide hanger. The storage area should be free of ozone generators, such as electric or gas appliances. If you must store the drysuit in areas with ozone generators it is best to fold the drysuit loosely with the zipper open and place it inside a sealed plastic bag.

Place suit on hanger taking care not to damage the neck seal with the hanger hook. If the suit is equipped with a changeable neck seal, hang the suit so the back of the neck ring is up against the hanger’s hook. Ensure the neck ring is hanging flat and does not have sharp bends. Any hard accessory dry glove ring installed on the wrist seals should be removed.



  1. Lay the drysuit with the zipper open face down on a clean, flat surface with the arms out to the sides. Pull the shoulders up so that the zip seal neck is laying flat.
  2. Fold the legs up so that the toes of the drysuit go just beyond the shoulder line.
  3. Fold the bottom portion of the drysuit toward the upper portion, making an approximately 20 cm/ 12 inch folded section.
  4. Fold once more so that the bottom of the last fold now rests at about the shoulder line.
  5. Tuck the wrist seals into the sleeves and fold sleeves across the entire package. Slide the folded drysuit into the drysuit bag.

NOTE: Suits with hard boots will roll up better from the feet upwards.



Hang the suit up as outlined above and, if the suit is equipped with changeable seals, remove the seals and store them in a plastic bag or plastic container taking care to keep the rings on the seals in their normal shapes; wrist round and neck flat. This will reduce the effects of ozone aging of the seals. It is not necessary to remove silicone Seals.

If hanging is not an option, follow the instructions for folding the suit. For long term storage, make sure the suit is completely dry and fold suit very loosely paying particular attention to the changeable seals to ensure they are kept in their normal shapes; wrist round and neck flat. After the suit is folded, place in a large plastic bag to reduce the effects of ozone aging of the seals, and store in a cool, dry space.





For easy donning follow these steps:

  1. Open up the zipper guard and the waterproof zipper completely.
  2. Open up the drysuit by folding it at the waist exposing the suspenders. The suspenders should be completely exposed and laying outside the fold. Make sure the crossover in the suspenders is in the back of the drysuit. Pick up the drysuit as if it is pair of pants.
  3. Step into the lower half of the drysuit just like a pair of pants. If footing is not stable such as on a rocking boat, it is best to sit down to start the process.
  4. Pull the drysuit completely up around the waist. Be sure to pull the suit up by the material and not the suspenders. Pull the suspenders up over the shoulders. Adjust the length of the suspenders with the slide on the two front straps. The suspenders are to keep the crotch in place and do not need to be tight.
  5. Pull the excess drysuit length up under the arms (this will give you the maximum amount of material to pull over your head).
  6. Carefully insert your left arm into the left sleeve using the appropriate method of putting the seal on for the type of seal that is on the drysuit. Adjust the seal so that it seals properly.
  7. Bring the sleeve no higher than the middle of your upper arm.
  8. Repeat the process with the right arm and adjust the wrist seal.
  9. Bring the suit up so that it is now on your shoulders.
  10. Bring the shoulders of the drysuit on top of your head and pull the seal down over the head using the appropriate method of putting the seal on for the type of seal that is on the drysuit.
  11. Fold the excess drysuit length at the hips and connect the crotch strap.
  12. With the zipper now laying smoothly around the body check that nothing is sticking out and there are no twists in the zipper.
  13. Using the left hand grasp the suit in the center of the back at the fold and, with the right hand, grasp the waterproof zipper pull handle and pull it around the side.
  14. Only a minimal amount of force should be required to close the waterproof zipper. If resistance is high or increases, stop, back up the slider and check the zipper. The zipper track should lay flat with no twist and there should be nothing caught in it. Proceed once the problem has been corrected. The waterproof zipper is one of the most important parts of the drysuit. Do not force it.
  15. Pull the waterproof zipper up flush with the stop at the end of the zipper. Give it one extra pull to make sure the zipper is completely closed.
  16. Once the waterproof zipper is closed, grasp the slider on the zipper guard on the left shoulder and pull it down until closed.
  17. If the suit is telescopic ensure that the groin bungee strap is secured in place.


  1. Open the zipper completely, then open up the drysuit by folding it down to the waist.
  2. Open up the drysuit by folding it down to the waist exposing the suspenders. The suspenders should be completely exposed and laying outside the fold.
  3. Check the boot alignment to determine the front of the drysuit.
  4. Step into the lower half of the drysuit like a pair of pants. If footing is not stable such as on a rocking boat, it is best to sit down to start the process.
  5. Pull the drysuit completely up around the waist. Be sure to pull the suit up by the material and not the suspenders. Pull the suspenders up over the shoulders. Adjust the length of the suspenders with the slide on the two front straps. The suspenders are to keep the crotch in place and do not need to be tight.
  6. Pull the excess drysuit length up under the arms (this will give you the maximum amount of material to pull over your head).
  7. Carefully insert your left arm into the left sleeve using the appropriate method of putting the seal on for the type of seal that is on the drysuit. Adjust the seal so that it seals properly.
  8. Repeat the process with the right arm and adjust the wrist seal.
  9. Pull the shoulders of the drysuit over your head and put your head through the seal using the appropriate method of putting the seal on for the type of seal that is on the drysuit.
  • Have your dive partner close the zipper while holding your arms out straight and to the side.
  • Look over your shoulder and check that the zipper pull is pulled up flush with the zipper stop.





Once the diver enters the water and the water compresses against the drysuit, it sandwiches the insulation between the drysuit and the diver. Increased pressure will also subject the insulation to a compression load. Whatever material the undergarment is made of will need to resist that compression load.All insulation known is trapped air or gas. For greater efficiency, the smaller compartment that the gas is trapped in the better. The insulation is degraded by the thermal conductivity of the insulation material. For example, heavy fibers or strong, thick fibers normally conduct much more heat than do small, lightweight fibers.

As a rule, the lower-priced materials have fewer threads per square inch, thus lower compression resistance. Some materials will lose as much as 70% of their insulating value while under the compression experienced by the average drysuit diver. High-quality materials will have more threads per square inch and are more compression resistant.

Choosing the undergarment with the correct material is not easy, and sometimes more difficult than selecting the right drysuit.

Within minutes of closing the zip on your drysuit, the air inside of your suit reaches 100% humidity. The average person gives off about one cup of water an hour even while at rest. That water will evaporate and migrate through the insulation to the inside of the drysuit. It will condense there because this part of the drysuit will be cold just as water vapor will accumulate on a cold window in the winter. When you take your drysuit off after your dive, you will find that the outer parts of the undergarment and the inner parts of the drysuit are now slightly damp. This is the natural water that came out of your skin and condensed.

If you are wearing a porous material such as any of the fleeces, you can look on the outside of the material and see little shiny beads of water. The first time the wind blows over it, they will evaporate creating cold air and the diver will feel it immediately.

Man-made insulation is sometimes made of a polypropylene fiber (ThinsulateTM) that is 1/1000th the size of a human hair or Polartec® material, called PowerStretch®. Polypropylene is a modified wax. This type of insulation is very efficient and has a high compression resistance. Fibers are matted together so closely that the natural resistance of wax to water prevents water from entering the insulation even if water did enter the drysuit.

The idea is that unless there is strong pressure on the material from a serious suit squeeze (little or no air added to the suit to compensate for the increase in pressure), the water droplets will not touch one another, and they cannot conduct heat.

The body will have to heat the water in the suit, but as long as the water droplets remain trapped in the fibers and do not touch each other, they cannot conduct heat away from the insulation. If there is any small leak, then these droplets will bond and conduct heat away from the insulation.

What keeps you warm? As we just discussed, for the most part, it is the air that is sealed within your suit and to a lesser degree the materials that your drysuit and undersuit are made of. In theory, the more air you trap the better the insulation, but trapped air spaces are not a diver’s friend. Too much air in the suit creates a migrating bubble, making suit control harder, but it also expands and compresses, so whilst the theory is that more air equals warmer the real-world practicality is that more air means more weight to compensate the buoyancy on the surface and a greater buoyancy fluctuation as you get deeper.

So, the perfect undersuit needs to be a balance between warmth, bulk, buoyancy, and compression. A tall order perhaps but don’t forget that the answer doesn’t necessarily have to reside in just one product.



Easy, let’s buy something super thick! However, going straight for the thickest undersuit isn’t necessarily the best option. There are numerous advantages to using a combination of layers. Having a range of different thicknesses and types of garments will allow you to mix and match to achieve the best choice for your dive all year round, and it will also allow you to tailor your final system for best performance.

  • The layer in contact with your skin needs to do a particular job. Even when you are relaxed, calm and comfortable your body sweats so your undersuit needs to be able to move this moisture away from the skin to prevent you from becoming damp and cold, we call this moisture-wicking. Most undersuits use materials that are fast wicking, but an independent base layer is superior as it has been specifically designed to achieve this. Something to bear in mind is that cotton is terrible at wicking moisture. It will just get caught within the fabric, getting wetter and colder so making you wetter and colder. If you aren’t going to use or need a baselayer don’t be tempted to wear a standard cotton T-shirt instead otherwise, you’ll just be making it tougher for your undersuit to its job.
  • This is a bit of a mid-layer, an optional one at that, some people might wear an additional Marino wool layer, or you may wish to wear a body vest for increased core heat, or even a heated vest. This extra layer needs to be thin, so it doesn’t restrict movement and is specifically suited for cold water conditions
  • The final layer is the actual undersuit depending on the heat requirements a 200, 300, or 400-gram suit can be selected, the material the suit is made of will have an impact on its thermal properties. More modern suits might have a hydrophobic outer layer to keep rain and sea spray from entering the insulation material.

NOTE: As a rule of thumb don’t go over 3 base or undersuit layers, if you need more heat, swap a layer out.





In an ideal world buoyancy, underwater is controlled by your BCD or wing, and the drysuit is only inflated to maintain comfort and avoid suit squeeze, the 3 main reasons why this is the general recommendation are as follows:

1) There is more of a centralized air bubble in the wing than there would be in the suit, this allows the diver to hold more extreme angles in the water without losing control, they might want to hold such angles, for example, entering or exiting a wreck. It would also allow for the suit dump valve to be dived fully open, making dumping from the suit on ascents very easy.

2) Another important reason is if the suit fails and a major leak occurs the diver wouldn’t lose their primary buoyancy as well as having to deal with cold water ingress into the suit.

3) Using the BCD or wing ensures limited gas exchange in the suit, thus maintaining better insulation.

A growing number of divers inflate their drysuits with an independent gas source rather than use their limited breathing gas supply. Technical divers sometimes use Argon gas (not breathable) as it has about half the heat capacity of air, so it provides better insulation than air for a given undergarment. However, argon needs to be flushed through the suit several times to flush out all the air before it will have any significant effect. With an independent inflation system, the main advantage is that you are not using your main breathing gas supply when you inflate your drysuit.

This system consists of a small cylinder and a dedicated first stage regulator with a low-pressure hose that connects to the inflation valve on your drysuit. The system is usually mounted valve down on the left side of the main cylinder using a quick release bracket or Velcro strap. If there is no second stage, the first stage regulator must also have a high-pressure relief valve to avoid hose rupture in the event of a high-pressure leak.



It is important that you hold a near-perfect horizontal position in the water, so that the suit may support your entire body, holding this position provides the best foundation for your diving, it creates a feeling of weightlessness and gives you, with practice, great control of your position in the water. It ensures that your frog kicks are translated into only forward motion. Failing to attain a flat angle in the water could lead to some of the power of the kick to be translated in an upward motion, which in turn would need to be corrected by partially deflating the BCD or wing, on top of that the water-resistance against the body creates a further upward push, requiring more deflation on the BCD or wing. Once the diver stops swimming both upward forces will cease and the artificial positive buoyancy will now no longer exist, the diver will start sinking down. Diving like this does not promote good buoyancy control as the diver can never comfortably stop finning. Diving in a flat angle, aka the Superhero trim position, ensures that a pause between each fin kick can be made and that once the diver stops finning, they will hover motionless. This position allows the RAID diver to interact with other RAID divers, problem solve, communicate, and make controlled horizontal ascent and descents while being a strong team member. It takes a little practice, but the rewards are infinite.


When diving with a drysuit, you will find that you may need more weight than with a wetsuit. This is because you displace more water with a larger gas volume around you which creates more positive buoyancy.

However, how much more weight you need varies significantly with the type of drysuit and the material and thickness of your undergarment.

There are various ways to carry weight, such as the common weight belt, a weight integrated BCD or a stainless steel backplate and a few trim weights

An important consideration is comfort, not only underwater but also on the surface. Rather than use large weight blocks, use smaller increments to distribute the weight around the harness and integrated BCD.

Distributing the weight around helps by reducing the excessively heavy single item, such as a weight belt or integrated BCD. In the event of an emergency, you don’t have to ditch all the weight and have the resulting uncontrollable ascent.  In an emergency, it is not necessary to drop all your weight, just enough to ensure positive buoyancy. Ideally have your weight systems set so you would expect to attain positive buoyancy by releasing any weight group with a one-hand quick release.

Ankle weights are not recommended, they are a quick fix to an easily solved problem. Instead, using the correctly weighted fins will make a big impact. The weight of the fins needs to be enough to offset the amount of air in the legs, if the fins are too heavy the amount of air will be too much, if the fins are too light the amount of air may not be enough, both scenarios can result in legs pointing down or pointing up and loss of trim. Your instructor will have to assess what is right for you, so you may hold a horizontal trim position.





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