The heavy-duty spun glass is a continuous filament spun glass. The frame is made with 1 continuous piece of chipboard to guard against corner separation. A tough metal media retainer is adhered to the frame on the upstream side for extra filter strength (one-sided metal reinforcement, on air exit side). Maximum air flow. Alleviates strain on equipment. For peak equipment protection and energy efficiency, these filters should be changed monthly in periods of high use. UL Class II. Type: Flat Panel Non-Electrostatic Material: Fiberglass Size: 20 In. x 30 In. x 1 In. Actual Size: 19-7/8 In. x 29-7/8 In. x 3/4 In. Rating: MERV 4 Life of Product: 1 mo