A façade system includes a mullion having exterior and interior portions and defining a glazing pocket between the exterior and interior portions, a thermal break arranged within the glazing pocket and extending between the exterior and interior portions, the thermal break dividing the glazing pocket into a shallow pocket and a deep pocket larger than the shallow pocket, and a collapsible element arranged within the deep pocket and extending between the thermal break and a lateral side of a panel introduced into the deep pocket. The collapsible element is movable between a collapsed state and an expanded state. The collapsible element divides the deep pocket into two or more thermal chambers when in the expanded state to reduce heat transfer by convection through the glazing pocket.