The Noctua NA-FD1 fan duct kit makes it possible to significantly improve the performance of Noctua NH-L9i and NH-L9a series CPU coolers in Small Form Factor (SFF) environments where there is a gap of 5mm or more (up to 45mm) between the cooler and a perforated top or side panel. Bridging this gap, the duct prevents the cooler from taking in hot air from the interior and enables it to draw in fresh air from the outside through the perforated panel, which can, depending on the setup, improve CPU temperatures by as much as 5c or more. As the duct is assembled from EVA foam spacers of different thickness, it is fully modular and can be adjusted in 1mm increments from 5mm to 45mm height in order to fit various different cases and configurations. In short, the NA-FD1 is a simple yet smart, flexible, and cost-effective way of boosting the efficiency of NH-L9 series coolers!