Manthorpe G961 Extension Sleeve Pack of 1 Extension sleeve for telescopic vents. Part number: G961. Can be used for vertical or rear horizontal extension. Designed to be used with G960 Manthorpe telescopic vents. Each unit extends the under floor ventilator a further 150mm vertically or 185mm horizontally. Further extension can be added. Provides a maximum equivalent area of 6600mm² airflow. Meets NHBC requirements for the control of condensation in buildings. Other ventilation fittings for this product available in our shop.Key Features 6600mm² airflow. Allows ventilation to an underfloor void. Allows 2 courses per extension.Regulations & Standards BRE Digest 17. BRE Digest 262. Building Regulations C4. NHBC Standards 5.2. BS 5250:2011 Control of Condensation in Building.