The 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 5°C or more. 

This video guides you through the installation process of the NA-FD1, using the NH-L9i CPU cooler as an example. For a full list of compatible coolers, or further information on this product, please see the Noctua website.