Going back to The Ins and Outs of Caulking, we learn that in contemporary residential construction, windows may not have protruding sills that collect water and shed it to the exterior of the siding. Contemporary metal and vinyl siding systems are designed to allow for drainage of water that penetrates between the siding and fenestration units (windows and doors). These systems incorporate termination and transition accessories that work with concealed flexible flashing materials (installed around the perimeters of fenestration units); they are generally intended to function without caulk joints between siding and fenestration units or fenestration unit trim. Other contemporary cladding systems may, however, rely on caulk joints at cladding system–fenestration interfaces and cladding-to-trim joints.
Caulk joints cannot be expected to remain functional indefinitely; leakage will eventually occur. Sealant joint failure can occur by adhesive failure between the caulk and either of the substrates between which a seal is desired. Cohesive failure or chemical degradation of the caulk can also cause sealant joints to fail. An example of chemical degradation in service is reversion, which sometimes occurs with certain polyurethane sealants. Reversion is the term used when the cured sealant reverts to a tacky state; in extreme cases, reverted sealant can flow from joints under the influence of gravity.
Of these possible types of failure, adhesive failure between caulk and substrate is most common failure mode. ASTM C1193, Standard Guide for use of Joint Sealants, provides extensive guidance for execution of sealant joints. The guide was developed for use in commercial construction, where caulking is a specialized trade. Many of the principles outlined in the guide are, however, also applicable to sealant joints in residential construction.