Abstract: Disclosed is a ventilator including a heat exchanger having a fresh air channel and a stale air channel. The respective channels are pneumatically isolated, but, are arranged in contiguous and interwoven fashion such that incoming fresh air may pass in countercurrent heat exchange relation to the outgoing stale air twice. The fresh air channel has an inlet in communication with air external to the ventilated space and an outlet in communication with the ventilated space. The stale air channel has an inlet in communication with the ventilated space and an outlet in communication with the external space. The heat exchanger features heat transfer means in communication with the fresh and stale air channels which is formed by a septum having a first surface in heat exchange relation with the fresh air channel and a second surface in heat exchange relation with the stale air channel. The septum is wrapped in successive spaced turns to form alternating sections of the fresh air and stale air channels.