It’s early March 1988 400 miles north of the Arctic Circle, a small team of special operations guys are loading into the torpedo tubes of a diesel submarine to lock out and conduct a mission sinking a small craft. Climbing into the tube taking care not to hit our dive rigs and gear on the lip or rails, we work our way deep into the dark tunnel. Situated in the tube, cold steel closing in all around us the loading door is sealed shut just before the tube fills with arctic water. Pitch black, water fills the tube and pressure equalizes with the outside depth. The exit door opens, and we escape to the open water. The water is cold like an ice cream headache, we can feel it thru our dry suits but there is something else… wetness slowly expanding around my right knee. As the dive continues, I get wetter and wetter…. soon my whole clothing system is soaked with seawater.

This may not be your normal occurrence for the hiker or climber but getting wet and needing to maintain body heat is. This is where a little discussed thermal value comes to play called “Wet CLO”. Let’s segment that a little, starting with CLO. The CLO Value is a measurement of warmth and can be used to characterize apparel items including garments, gloves, headwear and footwear.  1 CLO is the amount of insulation that allows a person at rest to maintain thermal equilibrium in an environment at 21 degrees Celsius or 70 degrees Fahrenheit.  This is said to be equivalent wearing a three-piece business suit with undergarments at that temperature.   When we discuss Wet CLO, we talk about the insulation value degrading because of the water content. That water can come from sweat, rain, condensation, etc.

Let’s look at the insulation spectrum starting with Down which can have a super high Dry CLO value of 1.68, but a super low Wet CLO value, near zero! Please note, that the Dry CLO value is dependent on garment design, and grade and thickness of the Down.  The high differential in thermal capability is problematic to the novice adventurer and most military folks, as they do not get to pick a blue bird day to execute a mission.

Air is one of the best insulators for apparel systems. Most insulated apparel systems work by trapping air next to the wearers’ body. When an insulation becomes wet, the trapped air within the insulation is replaced by liquid water. This can lead to huge decreases in in warmth or CLO value, as air is 24x more insulative than water.  As you can see from the graph below, even a little bit of water pick-up can lead to a huge decrease in CLO value.

If the water in the insulation later freezes, the impact is even worse, as air is 90x more insulative than ice.   On top of decreasing the CLO value of the insulation, water inside insulation can evaporate, causing cooling. This is similar to how our bodies cool ourselves by sweating on a hot day. But in a cold environment, this can be dangerous. In some wet situations, having wet insulation can be worse than having no insulation at all!

Wool has been a baseline measure for years; we have all heard the ‘warm when wet’ moniker. Today’s insulation world has been inundated with synthetics from Primaloft to Thinsulate, Climashield to Gore Thermium. These insulations have been developed to deliver the highest CLO value per gram, be hydrophobic and feel comfortable inside the garment.  Ultimately, the easiest way to reduce the impact of water on the CLO value is to minimize the amount of water that the insulation can pick up in the first place!  This can either be done by changing the properties of the insulation itself (ex., making it hydrophobic), or by protecting the insulation from water exposure (ex. by utilizing a GORE-TEX barrier to prevent rain ingress).  Some insulations, like Gore Thermium, do not pick up any moisture, therefore the CLO value does not change when exposed to water. Thus, minimizing the water content within the insulation is key to effectively closing the gap between wet CLO and dry CLO.

Today, there is no standardized test method for measuring Wet CLO across the outdoor industry, but engineers and scientists are working to characterize this phenomenon as it an important issue in protective apparel. Primaloft has worked on this issue since the early days of PCU and developed Primaloft Gold, their best performing insulation is 97-98% clo value when wet.

Our goal in the military and those that ‘GO’ when duty calls is to build clothing and sleep systems with the narrowest CLO differential dry or wet possible. With a narrow CLO differential, the user can select an insulation for the appropriate temperature range and the moisture content of the clothing system has little to no effect on warmth. USSOCOM made a deliberate decision to build their clothing and sleep systems to complement each other, remain unaffected by moisture and be ‘continuously drying’.

Back to my story, upon exiting the water in soaking wet clothing we had to make every attempt to walk for the next twelve hours to dry the clothing system out as it had little insulation value wet, was hard to dry and made for a challenging night above the Arctic Circle.  No teammates were injured during this event.

How do you know you’re getting wet after 2-3 days in the field? Your sleeping bag just doesn’t fit the same as when you left the house, your jacket is a little heavier, all signs that moisture is in the system, you just don’t feel it next to skin. That moisture degrades your comfort and warmth. So the next time you’re out for a trip weigh your kit before and as soon as you return… check your water weight!

Scott Williams, NSW (ret), Former OIC Naval Special Warfare Center, Det Kodiak, USSOCOM Cold Weather Equipment Project Director. Currently at the Wing Group leading Defense efforts.