wayno wrote:you're going to get wet from sweat when its raining regardless how good your DWR is... when its raining, the moisture stops moving through the waterproof membrane because the outside air is saturated with moisture and it won't accept any more, the humidity has to be below 100% . the lower the better. high humidity then the waterproof membranes won't shift the amount of moisture advertised, that's from perfect conditions in a factory...
manufacturers don't tell you this because its not good for their marketing...
Whilst it is correct that in the rain waterproof membranes don't work as well at shifting water vapour, as in the dry, it isn't true that water vapour stops being able to get through the membrane. For example, people wearing goretex dry suits still experience water vapour transfer from the inside into the dense cold 100% water outside the membrane when diving.
What happens when hiking is that, when the DWR inevitably fails, it leads to a temperature drop on the surface of the jacket; as water sits there in a slick. As a result water vapour condenses inside the jacket - not from not being able to leave but from dropping out of the water vapour into liquid - that liquid cannot leave the jacket frnm the inside unless it turns to vapour again - we all know that that is never going to happen.
Some water vapour still leaves but it is true that the water slick on the outside jacket surface also inhibits water vapour egress. This is as what was explained to me by an independant textile expert.
The big problem with this, as we all know, is that the condensation inside the jacket conducts heat from the body to the cold membrane, which can conduct heat really well to the water slick on the outside of the jacket. The external membrane jackets (that don't rely on impermanent DWR) still get overwhelmed by a bushwalker working hard on a cold wet day but they don't suffer the same condensation/conductive heat loss problem - except fo rthe back of the jacket where sweat accumulates under the pack (but then you get the wetsuit effect).
I agree with Wayno when he states that if you are walking in wet weather you will get damp from outside or inside and I also agree that the solution is ventilation/ managing sweat and temperature as if the jacket is a wetsuit. But the mechanism is a lot more complex and chemical DWR -less systems (external membrane - such as Gore Shakedry and Columbia Outdry) is likely to be the best option if you want a traditional jacket: other than an umbrella.
Another way of solving this problem is a silnylon garment which doesn't breathe (transfer water vapour) at all - but also doesn't maintain a water slick on the surface, so it is less conductive. The downside to this is that you need good ventilation or a slower work rate otherwise water vapour accululates really heavily inside the jacket - hence they work best as ponchos/pack covers.