An apparatus for determining the density of insulation in a cavity of a structure includes a sensor that is held in a substantially fixed position relative to the insulation for sensing the force of the insulation against the sensor. The force is used to determine the density of the insulation, which, in turn, is used to determine the thermal resistance or R-value of the insulation. The apparatus may include a fixture for supporting the sensor and holding the sensor in the substantially fixed position. A method for determining the density of loose-fill, blown-in-place insulation comprises the step of providing a structure with a cavity having a known depth. The cavity is filled with insulation. A sensor is held in a substantially fixed position relative to the insulation to measure force exerted on the sensor by the insulation. The measured force is used to determine the density of the insulation. The thermal resistance of the insulation is determined from the known cavity depth and insulation density.