The NA-FD1 fan duct kit enables significant improvement in the performance of Noctua NH-L9i and NH-L9a CPU fans in Small Form Factor (SFF) environments where there is a 5 mm or more gap (up to 45 mm) between the heatsink and the perforated top or side panel. By bridging this gap, the duct prevents the cooling system from taking in hot air from the interior and allows it to draw in clean air from the exterior through the perforated frame, which can, depending on the setting, improve CPU temperatures by up to 5° C or more.