Construction details are presented for an improved 6-in.-diameter flat-plate calorimeter used in measuring thermal conductivity of evacuated or nonevacuated solid, foamed, fibrous, or particulate insulation materials. The apparatus incorporates the important features of the designs of an earlier 6-in. apparatus and the 12-in. double-guarded calorimeter and results in a compact self-contained unit which can be used with a standard 4-in. high vacuum pumping system. The measuring vessel of the apparatus is 3 in. in diameter; the guard ring vessel is 1 1/2 in. wide. The space between the warm and cold plates can be adjusted from outside the apparatus both before and during the test. Specimens up to 1 in. thick can be tested under a mechanical compression of up to 20 psi. The apparatus is designed to operate between 77 and 450 K. A thermostatically controlled oil bath is provided for controlling the temperature of the warm plate between 300 and 400 K. Techniques for control of warm-plate temperatures below 300 K are described, and the results of several performance tests using evacuated specimens are presented.