In conditioning air for an enclosure, a first ambient airstream is cooled by the cooling coil of a refrigerant cooling system to reduce its temperature and humidity content. The thus cooled and dehumidified air is then passed through a segment of a rotating desiccant wheel to reduce moisture content and increase temperature and is then supplied to the enclosure. The desiccant wheel is regenerated by the use of a second ambient airstream which is first heated with the condenser coil of the refrigerant system and then passed through the regeneration segment of the desiccant wheel. A bypass plenum is provided in the apparatus that selectively allows a third ambient airstream to be cooled in the plenum independent of the evaporator coil and desiccant wheel in the first plenum. That airstream is then supplied with the air treated in the first plenum to the enclosure.