An automated guarded-hot-plate apparatus was designed and built to meet the requirements of ASTM standard test method C 177 for measuring the thermal transmission properties of thermal insulation. The apparatus is controlled by a scientific desktop computer. Measurements with this apparatus can be performed at temperatures from 300 to 750 K in environments of different gases at pressures ranging from atmospheric pressure to roughing-pump vacuum. A novel design for a thermocouple device is used; this device more accurately senses the average temperature over the surface of each heater plate. An improved algorithm for the control sequence provides more stable heater powers and specimen temperatures. Initially the algorithm brings the system rapidly to a temperature setpoint with minimal overshoot. It also permits highly sensitive control of the plate temperatures in later phases of the measurement sequence when thermal stability of the specimen boundaries is very important in measuring the thermal conductivity with high precision. Overall uncertainties of thermal conductivities at atmospheric pressure are 2% at 300 K and 3% at 750 K.