Description First of all our telescopic underfloor vent extension are designed for use with the telescopic underfloor ventilator. Each extension sleeve allows the vent to reach a further two brick courses vertically. The sleeve can also be fitted to the base of the telescopic vent to provide horizontal extension through larger cavities. 6,600mm² airflow. Allows a 3-5 course stagger 2 courses per unit (a further 150mm vertically or 185mm horizontally). Regulations – BRE Digest 17, BRE Digest 262, Building Regulations C4, NHBC Standards 5.2 and BS 5250:2011 Control of Condensation in Building. Made from polypropylene.   Data Sheet