a roof for the cab of vehicle is constructed with a hollow plastic body that is molded in one piece with a blower and a heat exchanger fitted to the hollow body . the hollow body defines a cavity that forms an air duct through which air for ventilating the cab is ducted . shuttered external and internal air intake vents allow the selection between fresh air from externally of the cab and recirculated air from within the cab . a well open through the roof provides an emergency exit from the vehicle cab .