. 2. 3. This Automatic Shutter is designed to be used with the Gable Mount Powered Attic ventilator. Sufficient air inlay area should be provided at the opposite side of the house thru louvered openings in the gable or screened openings in the eaves. Openings near the gable ventilator should be closed. HVI recommends one square foot of open air inlet per 300 CFM of ventilator capacity. Use 2” x 4" nominal lumber and build a square to the dimensions shown in Fig. 1 inside the gable.