If you’re thinking of entering the world of drysuit diving, you may have noticed that there’s a lot to consider when it comes to getting the right suit for your diving needs. Choosing an undersuit for your drysuit is one such consideration. Buyers often don’t consider the undersuit until after they’ve purchased a drysuit, but you should devote some time to your choice — it can mean the difference between feeling snug and warm or cold and miserable on a dive.
Most drysuit manufacturers offer undersuits, but there is a wide range on the market: one-piece, two-piece, traditional duvet-like onesies, and skin-tight garments made of synthetic materials. How well they keep you warm is based on a combination of what they’re made of, how well they fit, their thickness, what kind of drysuit you’re wearing, the water temperature, and the duration of your dives. With that in mind, here are some tips on choosing an undersuit for your drysuit.
Once upon a time, wool was the material of choice, as it’s a very good insulator and does a better job than most materials at trapping heat when wet. But wool is bulky and can be itchy. Modern undersuits tend to be made of Thinsulate, but aerogel is the new kid on the block. Nylon-lined polyester fabric is also popular, as is fleece. Each material is designed to trap as much air as possible. Your body warms up the air and the undersuit retains it so that it acts as an insulating layer.
A key advantage of Thinsulate and aerogel undersuits is their ability to wick moisture away from your skin. This prevents any moisture from taking heat away from your body, which it does faster than air. This moisture passes through the material so — in theory at least — sweating doesn’t mean that you’ll end up with a damp undersuit. Some suits utilize different materials on certain parts of the suit, based on which parts of the body lose heat faster. You can also use a neoprene wetsuit as an undersuit. These can keep you a little warmer should your drysuit flood, but they can make you sweaty on land. They can also restrict your movement when you combine them with a drysuit.
The thickness of your undersuit will depend somewhat on the type of drysuit you’re wearing. Membrane drysuits are very thin and offer no thermal protection, relying entirely on the undersuit for warmth. Neoprene drysuits do offer some thermal protection, so you can go a little lighter with your undersuit.
Thinsulate and aerogel fabrics are very efficient at trapping heat, so they can be thinner than other materials yet have the same insulation properties. Polyester undersuits tend to be thicker, and are less efficient at removing moisture. Although a thicker undersuit won’t necessarily keep you warmer, adding additional layers will. But note that wearing more material will reduce your range of motion. This becomes important when you’re technical diving — reduced mobility may mean you struggle to reach the valves on a twinset. In very cold water you might want to wear a base layer with one or more undersuit layers on top. Just remember that each layer will progressively reduce your range of motion.
One-piece or two-piece
Ease of donning and removal, comfort, and range of motion all factor in here. One-piece undersuits are generally very easy to get into and out of. Some one-piece undersuits are skin-tight in order to trap heat effectively. Although they stretch a little to allow more freedom of movement, they can be a workout to get into and out of. They usually lose elasticity over time, which reduces their overall effectiveness.
Two-piece undersuits come in varying thicknesses, and you wear them just as you would your normal clothes. They can also be skin-tight if worn as a base layer. In very cold water you might prefer a skin-tight one-piece, with a thicker two-piece undersuit over it. The ability to add and remove layers depending on your comfort level is a key consideration.
An undersuit must fit you properly to do its job. If it’s too small and tight, it’ll be difficult to take on and take off. It will wear out quicker and it won’t trap heat efficiently. Plus, it may compromise your range of motion. An undersuit that’s too big can keep the drysuit from fitting you properly, which will also reduce your range of motion. It’s best to have an undersuit that has some give, meaning that it’s not too large, but large enough to comfortably allow for additional base layers in colder waters.
Water temperature and dive duration
With temperatures close to zero degrees, and/or long dives, you’ll need either a membrane drysuit with lots of undersuit layers or a neoprene drysuit with fewer layers. Technical dives will usually be long and possibly deep, which may mean you’ll want a heated undersuit or vest. Even temperate water may have chilly thermoclines. No matter the water temperature, the longer the dive, the colder you’ll get.
It’s not just your drysuit that’s buoyant — your undersuit’s thickness also factors into the equation. The thicker it is, the more weight you’ll need, because all that material is less dense than the water surrounding you. If you are correctly weighted and wear fewer layers than normal for a given dive, you’ll need to reduce your weight. Conversely, if you add a layer on top of what you would normally wear — even a thin top — you may need a few extra pounds.
The thickness of your undersuit also influences your ability to vent air from the drysuit outlet valve. The air must travel through the undersuit’s material, and more layers will slow it down. When you want to dump air, there will be more of a delay before it reaches the valve — just something to bear in mind.
Accessory items tend to be very functional, so do consider them. The most common are thumb and leg loops. When you put the drysuit on, the loops prevent the undersuit material from riding up. Leg loops are particularly handy as it’s difficult to reach down into the inside of the drysuit to push the undersuit back down, and it’s frustrating to remove it to solve the problem. Thumb loops can also be useful, but you can also pinch the sleeves with your fingers when climbing into the arms of the drysuit. Then, let go before pushing through the wrist seals and your undersuit won’t ride up the arm. Some people find the loops quite annoying as they sometimes break the through the wrist seal, which you may only discover once in the water. Conversely, if you’re using dry gloves, you might want to push the loops through the seal so that air can migrate into the gloves and prevent a squeeze. Handy, but if the dry-glove seal fails, it will let water into your entire drysuit.
Another important, and often overlooked, feature (at least for men) is whether to choose an undersuit that has a suitable hole for routing a p-valve hose. It must be large enough for the hose to fit easily, and located in an appropriate place to allow good routing. If not, the hose can kink. This can lead to backpressure on the p-valve when you go to the bathroom, which can in turn mean a failure of the condom catheter with predictable results.
Quite simply, undersuit cost varies considerably. Manufacturers are always developing new materials and new undersuit lines. Depending on the brand and the technology, you may pay over $500 for one. But you can also get a decent new undersuit for between $200 and $300. Heated vests are another matter; they are always expensive and require a costly battery pack as well. Choose the battery-pack size carefully based on projected dive duration and temperature. Don’t dismiss the second-hand market either — you might get lucky with sizing and snap up a bargain.
As with all scuba equipment, talk to your buddies. Go in to dive shops and try on the goods. Do your research to find the proper suit for the kind of diving you intend to do for a price you’re willing to pay. A good undersuit will take care of you for a long time if you do the same for it.