Could anyone tell me if it is acceptable to pin a breathable type membrane to the rafters in the loft. The problem i have is the entry of dust through the tiles as i have an old house that is closeboarded btween the tiles not felt, hence the entry of dust making going up the loft like entering a coal mine. It just seems to me to be the most practical solution as the roof is sound.
Yes no problem at all just don't take it all the way down as you must allow for air circulation within the roof - breathable membrane or not. Leave a six to nine inch gap at the bottom.
Well in my opinion it is necessary to leave a small gap in order to be certain not to interfere with eaves to eaves ventilation that should be present to conform to the requirements of Part F of the Building regulations.
Reliance on gaps between the tiles, as SB seems to be suggesting, to provide ventilation seems to me to be rather inadequate.