caulk

Caulking fills the gaps between building materials, or between a material and a structure, to prevent water, air, and dust infiltration. It is applied as a semi-solid material, usually from a cartridge, into joints that are too wide or irregular for simple weatherstripping. The caulk remains flexible after curing, which allows it to accommodate slight movement from thermal expansion, settling, or structural flex without cracking.

Common caulking materials include silicone, acrylic latex, polyurethane, and butyl rubber. Silicone cures slowly but remains flexible indefinitely and resists moisture and temperature extremes; it is favored around bathtubs and sinks. Acrylic latex is water-based, paintable, and easy to clean up, but less durable outdoors. Polyurethane is rigid when cured and bonds well to masonry and wood, making it useful for exterior joints. The choice depends on the joint location, expected movement, and whether the caulk needs to be painted.

Proper caulking requires that the joint be clean, dry, and free of loose debris. The bead must be continuous and pressed firmly into the joint so that it contacts both surfaces; an air gap behind the caulk renders it ineffective as a seal. Joint width affects application: gaps wider than about 0.5 inches may require backer rod, a foam cord inserted first to support the caulk and ensure the proper thickness of the seal. Underfilled joints or voids behind the caulk commonly allow water penetration and mold growth.

Caulking fails when the material separates from one or both substrate surfaces, a process called delamination. This happens if the surface was dirty or wet at application, or if the wrong caulk type was chosen for the substrate and expected movement. Excessive movement in the joint also causes the caulk to tear. Interior caulk may crack if exposed to uncontrolled humidity or temperature swings over time.

The term caulk is distinct from caulking compound (the material) and from caulking (the process or the noun for the material itself). In shipbuilding, caulking refers to the act of driving oakum into seams between hull planks and sealing them with pitch, an older meaning that survives in the verb form. In modern construction, caulking and caulk are used interchangeably for the contemporary process of sealing joints with flexible compounds.