The 1203 Horizontal Front Extension Sleeve is a must-have accessory for the Telescopic Underfloor Ventilator. It fits between the airbrick and upper front opening, allowing for a snug fit on walls with external wall thickness of more than 102.5mm, up to 215mm maximum. This reliable product ensures that your underfloor ventilation system remains sealed, stable and secure, no matter how thick your external walls are.