Air filter with spiral-wrapped frame

A framed air filter, comprising:

• an air filter media comprising a generally rectangular perimeter with four major edges;
• and,
• a frame comprising four frame portions, each frame portion being mounted on one of the four major edges of the air filter media and with neighboring frame portions meeting to form corners of the frame,
• wherein each frame portion is provided by single frame piece that comprises multiple segments that are foldably connected by fold lines, which single frame piece is spiral-wrapped in a generally rectangular configuration with two segments of the single frame piece in overlapping relation with each other with an edge portion of the filter media sandwiched therebetween so as to provide a multilayer front wall of the frame portion;
• and, wherein a terminal segment of the single frame piece extends from an outward-front vertex of the frame portion and terminates at an inward-rear vertex of the frame portion and provides a reinforcing strut that extends at least generally diagonally across an at least generally rectangular interior space defined by inward, outward, front and rear walls of the frame portion.