Hi Kos, there are a few very good American books about timber frame by Larry Haun, published by Taunton Books. I've just got my local library to buy "Energy efficient Building" by Taunton Press - very good stuff about ventilation and insulation. It all had me pretty confused for a while but now I see the sense in it as follows:
(this refers to a timber frame house with timber cladding on the outside and no cavity) Behind the cladding on the outside you have a breathable membrane (housewrap) which stops rain and damp getting in but allows moist air and condensation to escape to the outside and allows the inside to dry out on dry and windy days. However, on the inside, you must ensure that internal house-generated condensation (cooking, bathing etc..) does not get into your walls as it could condense there and wet the timbers. So you fit an impermeable barrier on the inside surface of the wall, behind your plasterboard, to stop any moisture in the house getting into the walls. So, you say, how does the house breathe? Answer: a ventilation system. Does'nt have to be complicated - could be just opening windows, trickle vents built into windows, extractor fans over cookers and in bathrooms etc.. So, you let the house breathe but not into the walls - it breathes through windows, doors, vents fans etc... It appears that it's essential that the the waterproof impermeable barrier on the inside of your timber walls is very well fitted with no tears or breaks (well sealed at openings, window edges, sockets, pipes etc..) - the moist air will go straight for the gaps and holes if it is the easiest way to escape. Good luck in your endeavours.
Atmop
ps: Please, everyone, ask/annoy/badger your local library to throw out all those rubbish 1950s 'building' books (including horrible 1970s bungalow plans books) and buy the latest on design, building, heating tech etc...