Abstract: A poultry house having ventilation windows with curtains movable vertically relative to the windows to open the windows incrementally has a control cable associated with each curtain for pulling the curtain upwardly by curtain cords connecting the control cable to the curtain to close the ventilation through the window or to release the pull on the curtain to permit the curtain to drop incrementally and open the window. Each control cable is trained about a control pulley mounted on a power driven rod and is connected at one end to a manually rotatable winch. Other pulleys guide the cable to the control pulley and from the control pulley to the winch. In normal power operation the power driven rod pulls or releases the cable by retraction or extension, but in the event of a power failure the manually operable winch may be utilized to raise and lower the curtains.