An unguarded longitudinal heat flow apparatus for measuring the apparent thermal conductivity (Λ_α) of insulations was tested with mean specimen temperatures from 300 to 330°K on samples up to 0.91 m wide, 1.52 m long, and 0.15 m thick. Heat flow is provided by a horizontal electrically heated Nichrome screen that is sandwiched between test samples that are bounded by temperature controlled copper plates and 9 cm of mineral fiber insulation. A determinate error analysis shows Λ_α measurement uncertainty to be less than ± 1.7% for insulating materials as thin as 3 cm. Three-dimensional thermal modeling indicates negligible error in Λ_α due to edge loss for insulations up to 7.62 cm thick when the temperature difference across the sample is measured at the screen center. System repeatability and reproducibility were determined to be ± 0.2%.Differences of Λ_α results from the screen tester and results from the National Bureau of Standards were 0.1% for a 10-kg/m³ Calibration Transfer Standard and 0.9% for 127-kg/m³ fibrous glass board (SRM 1450b). Measurements on fiberglass and rock wool batt insulations showed the dependence of Λ_α on density, temperature, temperature difference, plate emittance, and heat flow direction. Results obtained for Λ_α as a function of density at 24°C differed by less than 2% from values obtained with a guarded hot plate.These results demonstrate that this simple technique has the accuracy and sensitivity needed for useful Λ_α measurements on thermal insulating materials.