An unguarded, unidirectional heat flow technique is described for measuring the apparent thermal conductivity (λa) of insulating materials in the range 300 to 330 K. Vertical heat flow is generated by a 0.9 by 1.5 m electrically heated nichrome screen that is instrumented and sandwiched between two specimens. The specimens are bounded by two temperature-controlled copper plates. The system has been used to measure λa of fiber glass batts and obtain data that show the effects of density, temperature, temperature difference, thickness, plate emittance, and heat flow direction on λa. A determinate error analysis indicates a λa measurement uncertainty of ±1.7%. The system repeatability and reproducibility were found to be ±0.2%. The measured performance of the tester agrees with that predicted by a thermal model. Analysis of the transient characteristics of the screen when it is subjected to a step change in power suggests that the apparatus can be used to determine apparent thermal diffusivity.