UKH

I have had a couple of Mountain Equipment sleeping bags with DriLite outers. Unfortunately I often end up with condensation on the inside of the DriLite and it makes the down wet.

Can anyone isolate the cause and/or offer a possible solution. 

No waterproof breathable membrane is actually very breathable, they only allow something like 20% of the water vapour through even in the most ideal lab based situations and in the real world they dont even allow that much through. Unfortunately if it's colder outside the bag than inside moisture will always form. You could try a vapour barrier (basically sleeping in a plastic bag to keep the moisture away from the down) or moving the dew point away from the down by putting a blanket or something over the top you. Condensation is a fact of life when camping and even bags that don't use membranes can suffer with it. I generally just try and dry them out as best I can either at home if the trip is only 1 or 2 nights or in any sun if the trip is longer. 

Thanks for that. Yes I understand VB. I wondered if adding and insulting bivy bag on the outside might move the dew point outside the bag and any condensation would form in that rather than on the down.

The only time it worked well was without any shelter in the Alps in winter. So maybe that's where it works best, but that throws up all sorts of other problems , not least if which is getting into your bag covered in snow!

It was absolutely useless in a snow hole. I spent 3 days drying my bag out after that!