An Intello membrane has been wrapped around the sole plate (support for the new dormer facade) which will be taped to the plaster below and to the wall / fenestration above which makes this awkward detail airtight for minimal effort and cost.  Keen eyes might notice that the floor joists are not treated to avoid unnecessary chemicals.  The tin roof is proving be be a great investment, allowing protected & dry working conditions and saving £££s every day by reducing temporary works.