The Truth about Wool as Drysuit Insulation

By Bob Stinton, DUI VP/Engineering

Wool is a retro concept. Before synthetics, insulation garments for drysuits were manufactured using wool. With advances in insulation technology today’s divers have much better choices.

Wool is a unique fabric in the family of natural fibers. Wool fibers do not absorb as much water as other natural fibers. This is because of the oil (lanolin) content of the wool.

However, the more processed the wool the less oil and the higher the water absorption. Lanolin is the natural oil that keeps the sheep’s coat dry in damp weather. It serves a similar function as the oil on the feathers of waterfowl. The oil keeps the feathers dry and maintains their insulation.

In years past fisherman and other individuals working off shore would go so far as to increase the lanolin content of their wool clothing by dipping them in heated liquid lanolin. This made their clothing very water repellant; however, the down side is you smell like a herd of wet sheep.

Wool fibers have little burrs on them much like thorns on a rose stem. These little burrs give wool their scratchy feel. Some of the softer feeling wools have less of these burrs. When wool gets wet and the fiber weight increases the little burrs prevent the fibers from sliding easily along one another, which helps keep the wool garment fully expanded. Because the wool knit does not collapse under its own weight, it maintains more trapped air. More trapped air means more insulation.

Cotton does not have these burrs and will collapse under its own weight when wet. This means cotton will lose most of its insulation when wet. This is why we advise not wearing cotton under your dry suit.

The little wool burrs are why silk is used as a layer under wool garments. The silk garments do not have any great insulating property; its primary purpose is to eliminate the scratchy feeling.

Another downside for divers is the burrs don't help much under a pressure gradient in a drysuit. Wool has the additional property in that as the fibers absorb water they generate a little heat from hydration. The amount of heat is very minimal, however it is kind of like the titanium hype - if the scale is small enough you could measure something, and a marketer could make something big out of it. But again, this is hype.

Wool is an outstanding natural insulator. However, the synthetics polyester and polypropylene are overall better insulators and perform much better when wet.

The test I tell people to try is to feel the garments as they come out of the washer after the spin cycle. I could easily take polyester and polypropylene garments out of the washer and put them on and go outside into the cold. (Note: I would not do this with B series Thinsulate™ – the type of Thinsulate DUI uses - which is an exception to this rule, because at this point it is flooded and the same property that keeps the water out keeps the water in) The spin cycle is the equivalent of warm when wet and which is not the case of flooded insulation, which is the primary concern in a drysuit.

The first people to adopt the synthetics and replace their wool garments were the North Sea fisherman. These fisherman adopted synthetic garments made by Helly Hanson. These synthetics were used offshore for about 15 years before they found their way into the outdoor market where synthetics replaced wool as the fabric of choice.

The Wool industry is always looking to improve the performance of wool and new applications. However, there have been no great technical strides bringing it up to matching the performance of the synthetics. Polyester and polypropylene garments beat wool garments hands down and that is why these garments rule the cold, wet, active environments.

Side note: The main advantage of the type B Thinsulate is the slow rate at which the air spaces flood out when water enters the suit. The trapped air spaces in cottons, polys and wools will flood out instantly.