Abstract: A ventilator adapted for wall mounting use in ventilating buildings such as agricultural buildings and the like used for housing livestock, and having inlet and exhaust duct means, located one above the other, and adapted to be mounted in the wall of the barn so as to provide for horizontal transfer of air into and out of the barn, inlet and exhaust fan means, the inlet fan means being located in the inlet duct and the exhaust fan means being located in the exhaust duct, operable simultaneously to blow air in their respective directions in and out of the barn, opening means extending between said inlet and exhaust ducts for transferring air from the exhaust duct back into the inlet duct, and movable air transfer duct means mounted between said inlet and exhaust ducts, and movable in said opening means between a fresh air intake position corresponding to substantially no transfer of air between the two said ducts, and an exhaust air recycle position corresponding to substantially total transfer of exhaust air