A. J. NEWMAN Building Research Establishment.

Four separate stages are identified in the mechanism of rain penetration. Workmanship is particularly important in construction of the outer leaf, so as to reduce the quantity of water passing into the cavity, and in keeping the cavity clear of obstructions and other faults which can lead water across. If cavity insulation is to be used, good workmanship is important also for installation of the insulant, either during construction (for built in board or batt materials) or after construction (for blown-in or injected fills).