You still need the roof timbers to be ventilated to some extent otherwise its asking for dry rot.
There are other ways to ventilate roofs (soffit vents being one) but with a warm roof structure with rooms in the "loft" and the insulation between the roof spars instead of a cold roof structure with an unheated and pretty much useless loft and insulation on the ceiling joists its normal nowadays to just use Tyvek or other breathable membrane as it provides breathabillity but still lessens the loss of heat via a gale blowing through the insulation.
Its all very well having a home constructed from finest woven yogurt but in my opinion if its inefficiant and prone to damp there are some occasions where modern is better, plus it's simpler getting it past building regs (for pretty much the same reasons though).
Sometimes bunging a bit of plastic into a building saves a hell of a lot of bother doing it another way that may well cause more environmental damage anyway
There do seem to be a few people that think building regulations are just there to make it hard for people to do what they want, personally I feel thats misguided as the restrictions are in place to provide better and more energy efficiant housing.
I have a lot of time for building regs blokes but considerably less for planning authority blokes as they seem more concerned with the "look" of a place and in some occasions are a complete waste of skin.
Before too long (if what I read is correct) the building restrictions will take into account retention of rainwater in tanks and membrane lined holes that are then filled with plastic things that look pretty much like milk crates to act as water tanks which will minimise flash flooding because of the water running off the non porous concrete and straight into drains.
Might help reduce the problems associated with stormwater flooding like in Hull at the moment.
In short, I am a builder (well carpenter) and I would have used a breathabe membrane myself, in fact thats excatly what I did when I converted my loft into a bedroom.
