When it comes to building façades, the No. 1 complaint of building owners and managers has been (and continues to be) water leakage. Much of this leakage can be attributed to window systems and their interface with other façade components. Understanding the basic waterproofing principles of window systems, common failure modes, and typical repair strategies is not complicated.Types of Window SystemsThe most common type of modern, non-residential (inoperable) window system is known as a drainage system; also referred to as a rain screen or skin-barrier system. This system has essentially two lines of defense against water leakage: the outer seals and an internal drainage system. The outer seals generally consist of rubber gaskets, preformed pliable tape, or sealants. The internal drainage system utilizes a network of window framing components, internal flashings, and sealant to capture any water that penetrates the outer seals and channel it back to the exterior.A less common type of window system is a barrier system, which relies on the outer seals as the only line of defense against water leakage. These outer seals almost exclusively consist of sealant.Failure ModesPlain and simple: Whether influenced by normal weathering, improper design, or poor installation, sealant failures are the Achilles’ heel for both types of window systems. It is typically the sealant within the internal drainage system or the sealant forming the outer seal of the barrier system that ultimately allows water to penetrate the building façade.For new drainage-type window systems, the outer seals initially prevent significant quantities of water from reaching the internal seals. Since new window systems are typically tested with the outer seals in place, the integrity of the internal drainage system is not known. Years later, when the outer seals begin to weather and deteriorate, any breaches in the internal drainage system could lead to uncontrolled water leakage into the wall cavity.Methods of RepairRemedies for leaking barrier window systems simply consist of repairs to or replacement of the outer seals (i.e. sealants). For leaking drainage-type window systems, there are generally three options: 1) create a barrier system by waterproofing every exterior joint (including gasketed joints) with sealant; 2) disassemble windows and perform invasive repairs to the internal drainage system; or 3) replace the windows. The first option is generally the only economic and practical solution. The second option is very labor intensive, requires extensive quality control, and is more disruptive to normal building operations. Justification for replacing the windows – the most expensive option – generally includes factors beyond water leakage such as increased energy efficiency, reduced sound transmission, and aesthetics.Since the most probable repair methods for both types of window systems involve sealants, it is critical that proper sealant design and installation techniques be followed. The five essential elements of sealant joint design and installation include proper sealant dimensions for movement capability, adequate bonding surface preparation, appropriate joint backing material selection, proper tooling of wet sealant, and verification of sealant compatibility with bonding surfaces and adjacent materials. Some difficult-to-seal applications may also require the use of preformed sealant components.Mark K. Schmidt, unit manager at Wiss, Janney, Elstner Associates Inc. (www.wje.com), headquartered in Northbrook, IL, specializes in the assessment, preservation, and repair of building façades.