A flexible cover plate with an airflow opening. A fan carried on a rear side of the cover plate over the airflow opening. A protective screen carried on a front side of the cover plate over the airflow opening. An angle bracket carried in an elongated slot disposed in said cover plate having a first portion extending parallel along the rear side of the cover plate, and a second portion extending transverse to the front side of the cover plate above the airflow opening. A flexible polymer foam sheet carried on the rear side of the cover plate for conforming to a vent opening in a building foundation wall so that an airtight seal is formed against the vent opening. A fan speed controller adjusting a desired speed of the fan and airflow through the airflow opening.