The G961 extension sleeve provides 7,000mm2 airflow. The G961 Vertical Extension Sleeve can be used to extend the G960 Telescopic Under Floor Void Ventilator vertically and horizontally. Each extension sleeve enables a further two course brick adjustment vertically by 150mm . Alternatively the sleeve can also be clipped to the base of the G960 to provide horizontal extension of 185mm through larger cavities or wider internal block work. The G961 Vertical Extension Sleeve should be used in conjunction with other products from the Manthorpe range to provide a complete through wall ventilation system.