A comprehensive numerical study of the equivalent thermal conductivity of a multi-holed clay brick with the size of 240 x 115 x 90 (in mm) has been conducted, and 50 kinds of combination of holes and arrangements are examined. The indoor-outdoor temperature difference varies from 50 degrees C to 20 degrees C. The effects of following factors are studied in details: the hole surface radiation, the width-wise and length-wise hole numbers, and the indoor-outdoor temperature difference. The major findings are as follows: (1) the radiation between hole surfaces has some effect on the equivalent thermal conductivity, thus it should be taken into account; (2) the hole number and arrangement affect the thermal conductivity in a rather complicated manner. Analysis shows that depending on the relative importance of natural convection, surface radiation and heat conduction through the clay solid, the thermal conductivity may decrease with the hole number or increase with the hole number and (3) among the 50 kinds of combination, the optimum configuration is found which has five length-wise holes, four width-wise holes and all the holes are from bottom to top in the depth direction of a brick. Its equivalent thermal conductivity is 0.419 W/(m K), which is only 53.1% of solid clay of which it is made, showing great energy-saving possibility if it is adopted in the construction of building wall. Detailed discussion of the simulated results is conducted and flow field and temperature distributions are also provided for some typical configurations. (c) 2008 Published by Elsevier Ltd.