AIRLET TL98 Fresh Air Inlet (4&rdquo;-8&rdquo; Sleeve) - Passive, Filtered Fresh Air Inlet for Make Up Air Passive Self-Regulating, Filtered Fresh Air Inlet For Make Up Air Integrated Flow Regulating Damper Provides Draft Free Fresh Air Small 4 1/8" Round Opening Required. Expandable for 2 1/2 to 8" walls. 4" Expansion Sleeve available for walls thicker than 8". See Specification Sheet For Details