In reply to Clive G:
> My memory may be at fault but I seem to recall 5cl of water requires some 850 litres of dry air to evaporate into.
Without looking up a psychrometric chart, or crunching the numbers, I couldn't say for sure. But the general principle certainly holds; a waterproof breathable fabric will only allow a certain 'vapour flux', and, if you sweat more than this, water will accumulate in your clothing system.
Which is why we generally wear a wicking base layer to absorb the sweat, acting as a reservoir or buffer, and removing the sweat from the skin, making us feel more comfortable. Some base layers will increase the rate of evaporation, due to their complex surface structure which increases evaporative surface area.
The downside of the reservoir effect is that, if the reservoir is large enough, it can cause chilling if the fabric is saturated when high output activity stops (especially if it's a sudden stop). Ideally, we won't run for long periods at high levels of activity, but instead will ramp up and ramp down the level of activity, allowing the buffer to fill and empty without saturating, and without causing excessive chilling.
The downside of wicking is that we remain comfortable so that we may not be aware that we're sweating and overheating, and we delay removing insulation until it's too late, and the fabric is saturated...