We get a lot of questions from customers about how to stay warm, moreso, we hear customers commenting not enjoying their diving because of the cold. Although being cold it's totally avoidable in 'cold' water diving, there are strategies and steps you can take to make your diving safer and more enjoyable.

I've outlined some crucial info & knowledge on the subject and then summarised it in 10 key tips to keep warm.

Where Does Body Heat Come From?

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

INSULATION SUITS
Wetsuits
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 the 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 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.

Dry suits
Dry suits 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 dry suit 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 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
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.

EXPOSURE DURATION
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 which has a decompression obligation. There is no point racking up a decompression obligation if the water temperature forces you to have to abandon your plan.

BODY TYPE
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 an 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.

ACCLIMATIZATION
It is often a surprise for travelling divers, who have been diving in colder northern waters, to find dive masters 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.

ACTIVITY
It is important to judge your activity level and thus the thermals 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.

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

Undersuits

HOW DOES THE INSULATION WORK?
All insulation known is trapped air or gas. For greater efficiency, the smaller the compartment that the gas is trapped in the better. The insulation is degraded by the thermal conductivity of the insulation material. For example, heavy, strong, or thick fibers normally conduct much more heat than  small, lightweight fibers.

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.

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. The water will evaporate and migrate through the insulation to the upper layer of the undersuit. It will condense there because this part of the drysuit will be cold, just as water vapour 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.  

LAYERS EXPLAINED
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.

DRESS TO IMPRESS - Layering Up
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 thickness and types of garments will allow you mix and match to achieve the best choice for your dive all year around, and it will also allow you to tailor your final system for best performance.

Base Layer. 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 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. The moisture 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 underneath your undersuit, wear nothing or wear something with some wicking properties. 

Mid Layer. 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

Top Layer. 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.  

Primaloft is an excellent insulator, look for how many g/m2 or even better a CLO value of the suit.

Insulation Values: CLO vs TOG
CLO Value is the amount of insulation that allows a person at rest to maintain thermal equilibrium in an environment at 21 degrees Celsiust. As divers we often dive in much colder condistions, so a CLO value of more than 1 is essential.

CLO is a measure in GSM which is the Mass of material per m2 , g/m2. Sometimes the value TOG is used instead of CLO. Posture and activity are important variables to be considered in measuring CLO.

The TOG also describes the thermal resistance of clothing: 1 TOG = 0.645 CLO and 1 CLO = 1.5 TOG.

What does this mean?
A CLO value indicates how resistant a garment is to thermal loss. For example, a CLO value of zero would mean a person is not wearing anything. Therefore, the higher the CLO value, the warmer a person will be and the colder the environment, the higher CLO value needed.

SANTI examples:
 • Flex 190 - 0.96CLO
 • BZ200    - 1.30 CLO
 • BZ400    - 1.80 CLO

10 Pro Tips

Tip 1: Don't use Cotton. Cotton is a great fabric for a warm summery day, it absorbs moisture and transpiration from your skin, and then helps it evaporate. However as part of your layering strategy it is a very poor choice, it will actually trap moisture against your skin, making you cold. Think you don't sweat? Think again, as soon as that zipper closes you are reaching relative 100% humidity pretty darn quickly.

Tip 2: Keep Hydrated. I suck at this........ Hydration is super important, we normally think of it in warm weather, but it's equally important in cold weather, without going into the numerous benefits of good hydration for divers lets just look at it from a heat point of view. Water is required for a healthy and relative easy blood flow, de-hydration diminishes this and the body will retract blood from the upper epidermis (skin) and causes us to feel colder, as guess what, that's where the majority of our temperature sensing nerve endings are.

Tip 3: Eat Right, Quick Fuel. Proteins and fat are hard to burn, great for weight loss, but not for quick heat. Don't start your dive on an empty stomach, go for quick and dirty carbs and sugars, fast cheap fuel for quick heat. Yeah Baby....

Tip 4: Stay Warm and Dry on the Surface. Starting a dive cold, means staying cold. It takes much less effort to stay warm than to get warm. If you are cold on the surface you will definitely be cold in the water, layer up prior if required and move around. If the surface is warmer than the water, take it easy, limit movement and only get fully suited prior to the dive, not doing so causes pre-dive transpiration, and will result in a colder dive. Finally, make sure your undersuit has been properly dried between dive days.

Tip 5: Correct Suit & Layering. Make sure you pick the right suit for the season. I own; Flex80, Flex190, Primo, BZ400x, Flex 2.0 heated, various different base layers, a heated vest, a x-core vest, and I use all of them, in different climates and dive requirements. Read the above sections to get clued up on the right layering and suit selection. Don't believe the hype, everyone who buys a new suit will say it's warmer than their old one........older suits loose insulation ;)

Tip 6: Undersuit Washing. Owww I love the smell of fabric softener. NOOOOOOO
Fabric softeners make fabrics soft by damaging them. If you have to wash your undersuit because it smells, use something like the Fourth Element gear wash, put it on a low spin, and no temps higher than 40. Wash the suit separate from your other washing. Once it's washed, wash it again, but without detergent, the detergent (smells nice) can bond together fibers diminishing the suits insulatory properties. When Drying, DON'T hang your suit up, it can damage or move the inner layers, lay it down or tumble at low speed, low heat, turn inside out halfway. To be certain check the manufacturers specific instructions.

Tip 7: Hood & Hats. If you have the same stylish buzzcut like me you'll know that having a cold head ruins your day and your dive. Keep that noggin warm on the surface and warm in the water, don't forget you have some big blood vessels in your neck, keep that stuff warm and protected. You loose a lot of heat through you head and you would be surprised if you changed your hood how much warmer the rest of your body will be. Personally, I change hoods every 6months to ensure tightness and perfect fit, and like the Santi 7 and 11 hoods, the Fourth Element 7mm hood and the XR 7mm hood. 

Tip 8: Dry Gloves. Play with the right outer and inner layer combo's to keep optimum heat and dexterity. Super thick outers and thin inners might allow you to move, but they will give you tingly fingers. Super thick inners and thin outers might keep you warm, but you better stay away from any rusty sharp objects or you'll have cold water influx. For winter we like the no-gravity winter gloves, and a medium thickness outer, like the Santi outer Dry glove.

Tip 9: Correct Footwear. Let's rock your socks off! DON'T WEAR COTTON.....did I mention that before? Please wear something decent, a good dense wool sock will work, personally I use the SANTI Primaloft Sock, and have used the No Gravity Sock as well at the Fourth Element Hot Foot. All these are great options, but I prefer the Santi. Bare in mind, thicker sock means more space needed in your boot/wetsock. When ordering your new drysuit go for neoprene socks, and get 2 pairs of over boots, smaller ones for summer, and bigger ones for winter. There is no point having so much compression on your foot that you reduce blood circulation, as you'll still get cold, and it will hurt your feet.

Tip 10: Air bubbles. I hate air bubbles creeping into my hood, passed my cheek into my hood, cooling me with their frosty touch.....yes this problem doesn't exist on the Rebreather, and yes the gas is nice and warm, but if you look forwards more that down, most of the bubbles will go passed your mask and less into the hood. If you put your hood over your mask skirting, as you were shown in your course, you create an easier entry into the hood, so be careful with pulling it over your mask skirting too much. Most importantly don't wear a hood that's too big, and just go and book a Rebreather course with me so you can enjoy the warm non-dry air