A 100-150mm cavity closer is an insulated, rigid plastic strip (usually UPVC with EPS core) used to seal gaps around window and door openings in cavity walls, preventing thermal bridging (cold spots), damp, and condensation, while helping meet Building Regulations for thermal performance and acting as a damp-proof barrier at reveals for new builds and refurbishments. These multi-width closers have grooves to easily cut them to size for varying cavity depths between 100mm and 150mm