A portable gauge and method are used to determine an as installed property of a loose-fill insulation in a building cavity wherein the cavity has a given depth, an open front, a rear surface, and sidewalls defined by spaced apart framing members. The gauge includes a frame for extending between and overlapping front surfaces of the framing members to position the gauge for determining an as installed property of the insulation; a plunger assembly mounted on the frame for compressing a portion of the insulation in the cavity to a compressed density sufficiently dense to prevent plunger creep and to a compressed thickness less than the as installed thickness of the insulation; and a readout assembly calibrated to indicate an as installed property of the insulation based on the compressed thickness of the insulation.