the perimeter of the cassette has a wrap of membrane pre-fitted before being craned in. It sits on the lower flap of membrane and then the upstairs wall panels sit on the upper. These flaps of membrane are then ready to be taped to the VCLs. Externally, the upper wall sheathing is draped down over it all and stapled to the lower sheath. In our case it was heavy gauge polythene, you can see it hanging down in this image. All the upstairs studs had a similar strip of poly over the top of them for the ceiling VCL to attach to.