A rainscreen wall is built from the outside in, but it is checked from the inside out. By the time the cladding is on, the barrier and the cavity behind it are invisible for the life of the building, so the only chance to look at them is the afternoon before they are covered.
Before the barrier goes on
The sheathing is dry and the openings are framed. A barrier laid over wet sheathing traps the moisture it is supposed to keep out, and a barrier laid over an unfinished opening has to be cut again later.
Before the cavity is closed
The barrier’s laps run the right way, the flashing at every opening is in and turned up at its back edge, and the base is open so the cavity can drain. This is the one inspection that decides whether the wall works.
Before the cladding goes on
The cavity is clear — no offcuts, no mortar, no packaging left behind by another trade — and the fixings are set out so nothing bridges the gap the wall needs.
None of it is complicated. All of it is invisible afterwards, which is why it is worth writing down what was checked and when.