The field test may also be made after the building is completed and in service to determine whether or not reported leakage problems are due to the failure of the installed assemblies to resist water penetration at the specified static air pressure difference. Generally it is possible to conduct tests on window, skylight, and door assemblies without too much difficulty, and to identify sources of leakage. A curtain-wall assembly, on the other hand, may not be accessible from the inside without the removal of interior finished walls and ceilings. Even with removal of interior walls and ceilings, it may not be possible to observe curtain-wall surfaces behind spandrel beams. The feasibility of conducting a meaningful static air pressure difference water penetration test on an in-service building must be carefully evaluated before being specified.