A Holometrix, Inc., R‐Matic model R‐41 Heat Flow Meter Apparatus (HFMA) is evaluated by measurements on homogeneous materials and anisotropic composite materials. The former class of materials includes: foams, calcium silicate boards, fiberglass batts, and loose‐fill fiberglass. Materials in the latter class are polystyrene with gas‐filled panels and batts with powder‐filled evacuated panels. Thermal models of the anisotropic composite materials allow design of composite test specimens and help in the interpretation of test results by computation of apparent thermal conductivities (k). The HFMA includes a dedicated computer for test control, data acquisition, and data analysis. Thus, the computer helps obtain the conditions needed to conduct accurate and repeatable steady‐state tests to meet ASTM C 518 with minimum operator involvement. Thekvalues from this HFMA and three other HFMAs (model R‐20 and R‐21 units) exhibit imprecisions of 2.2% (two standard deviations) for specimens of foam insulation. Such performance requires proper calibrations of good quality heat flux transducers and use of correct thermocouple thermometry practices. For six lots of fiberglass batt insulation, this HFMA and a HFMA model R‐21 yieldkvalues as a function of density with two standard deviations of 4.5%. Test results from this HFMA for three specimens of loose‐fill fiberglass insulation providekvalues as a function of density with two standard deviations of 5.1%. Because these levels of imprecision (2.2%–5.1%) include material differences, they illustrate the limits of error of the model R‐41 HFMA.