Interstitial space is the gap between the cleanroom ceiling and building structural ceiling, housing mechanical equipment and utilities. Purpose: Contain HVAC equipment, ductwork, FFUs, piping, electrical, and service access without cluttering the cleanroom below. Access: Often walkable space for maintenance access to FFUs, filters, and utilities. Grated walkways over ceiling panels. Height: Typically 3-12 feet depending on equipment needs. Larger fabs may have very tall interstitial spaces. Contents: FFU motors and plenums, ductwork, sprinkler systems, lighting electrical, process piping, utility connections. Maintenance: Technicians access from above to replace filters, service FFUs, repair utilities without entering cleanroom. Cleanliness: Not cleanroom environment but kept reasonably clean to prevent contamination of air supply. Fire suppression: Separate fire detection and suppression for interstitial space. Vibration: Equipment mounted with vibration isolation to prevent transmission to cleanroom below. Design integration: Planned during fab design, structural ceiling must support interstitial equipment loads.