75mm. Add that to the outer profile of every single primary duct run you size, right at the schematic stage. Nobody warns you in studio that sheet metal dimensions are inside clear-air measurements. Up here where every supply run needs heavy foil-faced external wrap so the drop ceiling doesn't sweat, that neat 14x20 trunk you tucked under the glulams is actually pushing 17x23. The sheet metal crew won't redesign their route on site—they’ll just hang it lower, and suddenly your clean nine-foot ceiling drops four inches to clear a beam transition. Model the insulation thickness into your clearances on day one, not when shop drawings hit your desk. @johnlura