Manthorpe Telescopic Underfloor Vents with Extensions. Designed to provide a clear airflow passage to the void underneath a suspended floor. Provides 6,600mm² of airflow. The telescopic feature enables it to be adjusted between 3 and 7 brick courses. Meets NHBC requirements for the control of condensation in buildings. This product allows ventilation to an underfloor void and can be adjusted for a 3-7 course stagger. Additional depth and cavity span are available with an extension sleeve. It complies with BRE Digest 17, BRE Digest 262, Building Regulations C4, NHBC Standards 5.2, and BS 5250:2011 Control of Condensation in Building.