The apparent thermal conductivity and thermal resistance of several building insulations were determined at the National Institute of Standards and Technology (NIST) using three apparatus. Reference values were determined with NIST's 1-metre Line-Heat-Source Guarded Hot Plate (GHP). The other two apparatus were heat-flow-meter (HFM) apparatus calibrated with a Standard Reference Material of fibrous-glass board (138 kg/m3). The apparatus -- NIST's 1-metre GHP; the 610 mm HFM; and the 305 mm HFM -- were operated at a mean temperature of 24°C and temperature difference of 22°C across the thickness of the specimen. Measurements of thermal conductivity were determined for individual specimens nominally 25 mm thick, covering a range of thermal conductivity from 0.02 to 0.16 W/(m•K). The thermal resistance of thick specimens (1 to 7 m2•K/W) of cellular plastic, fibrous and loose-fill insulation was also determined. Deviations from reference values of the GHP were calculated for the HFM apparatus. For the 610 mm HFM, the majority of deviations were within ±3% of the GHP. Substantial improvement was achieved by calibrating the 305 mm HFM as a function of thickness; however, deviations ranged from +1.5 to +9.2% from the GHP. The apparatus, test procedure, and analysis of the deviations are described in the paper.