Abstract: A fresh air ventilator system for a building or the like having a conditioned air plenum and a return air aperture communicating interiorly of the building, respectively, for circulation of conditioned air within the building, characterized by a ventilator unit disposed in a weather-proof cabinet on a sealing roof curb on the roof of the building. The ventilator unit includes a fresh air inlet adapted to admit fresh air and exclude precipitation and having a fresh air damper for controlling the quantity of fresh air from zero to the maximum; an air circulating blower suitably connected to effect circulation of air through the building; an exhaust means for at least periodically exhausting a portion of the return air; a proportioning damper disposed at the return air aperture for proportioning the amount of return air, from zero to the maximum, fed to the circulating blower; and air conditioning means for conditioning the air to be circulated.