A modular server cooling unit user standard dimension modules to build a variety of components for use in cooling a server or server farm. One module may be the module in which the server(s) are mounted. Another module may be an exhaust plenum, drawing air through the server module and exhausting the air to the outside. A third module may be a cooling module through which outside air is drawn, filtered and optionally cooled, for example, using an adiabatic, or water-wash, cooler. Exhaust air may be selectively mixed with air from the cooling module to provide finer control of server temperature and humidity.