Designers should review the environmental and operating conditions for the project site and select an appropriate WTP value. Also, specifications should provide a clear definition of water leakage, information designating responsibility for the design, performance criteria, and a definition of the window system to include all components. Current industry discussion has focused on the implications of citing WTPs higher than the AAMA recommendations. Based on field experiences, the higher WTPs do not necessarily guarantee success. Evaluations of in-service leakage patterns indicate that leakage is seldom caused by wind pressures exceeding the WTP but by the window system's failure to perform at marginally lower values.
Many factors influence the behavior of installed window systems to resist water infiltration.
Leakage sources are often related to improper installation techniques, disregard of installation instructions, lack of field quality control, failure to accommodate movement, failure to control water from framing joints within or between windows, and material failures within the window system. Using an appropriate WTP as part of a test program implemented on the mock-ups and installed window units, along with proper detailing and installation procedures, helps protect against water leakage problems in service. Mr. Johnson specializes in the investigation, analysis, and repair of windows and curtain walls. Ms. Slaton specializes in investigation, repair, and preservation of historic and contemporary buildings. The Construction Specifications Institute.