Your body gets cold as it loses heat to the surrounding environment through “thermal conductivity”. Thermal conductivity is the heat transfer from your body to your surrounding environment. An environment with a ‘low thermal conductivity’ means heat transfers away from the heat source (your body) at a relatively slow rate. Air has low thermal conductivity. An environment with ‘high thermal conductivity’ means you lose heat faster, and thus get cold faster. Water has ‘high thermal conductivity’. The nerdy engineering term for thermal conductivity is called the “R-value”. For air, its about 0.024 W/ (m*k). This is basically your energy (heat) lost per degree of difference for a given surface area (your body’s surface area).
Some important R-values are:
Umm, so what does this mean? You lose heat over 20 times faster in water than air (R-water is 20 times larger than R-air). This should make sense when you think about how quickly you get cold in board shorts in 65-degree water versus 65-degree air. Now, if your wetsuit absorbed ZERO water, it would be about 11 times warmer than wearing trunks based on these R-values. Of course this is dependent on wetsuit thickness, a 4/3 would is about 33% warmer than a 3/2. But if your wetsuit absorbs 20% water, which is typical for lower-end wetsuits, then you get an effective insulation only about 4 times better than trunks. (20% xR-water + 80% x R-neoprene is 4 times lower than R-water) Does your suit weigh 3lbs more wet than it does dry? It probably fits in this category then.
Japanese Yamamoto rubber is 98% water impermeable. Actually the rubber itself only absorbs 0.3%, but the jersey material on either side also absorbs some water. This means you get insulation close to 10 times higher than with trunks.
How about those fuzzy liners? Yes, they typically have thermal conductivities very close to neoprene; wool and polypropylene are both around 0.05. But, using polypropylene as an example, it still absorbs up to 30% water. This results in additional insulation for sure, but it also makes them a heavier, less efficient alternative to water-impermeable neoprene. You are better off adding 0.1mm of neoprene than 0.5mm of a typical fuzzy liner.
This analysis is obviously simplified and ignores flushing and leakage. Still, it shows that Yamamoto Japanese neoprene can have up to twice the insulation of lower end rubber, and the reason we chose to use it despite its high cost. It's also the reason some manufacturers say you can wear a Yamamoto suit 1mm thinner than you are used to. We don’t want to make this claim, but we do know it’s substantially warmer.
#39 Yamamoto neoprene at 5X magnification
FERAL jersey material at 5X magnification