3D numerical thermal simulation on conjugate heat transfer in hollow roofs is conducted by self-compiled program based on the finite volume method to improve insulation performance. Eight kinds of hollow roofs with different box fillers are optimized by regularly designing partitions to divide air-filled cavity. In addition, the effect of box filler materials on thermal behavior is considered. The numerical results demonstrate that: horizontal partitions decrease the equivalent thermal conductivity by 13.65%-40.42%, whose effect is more remarkable for higher void roofs, whereas vertical partitions cause few effects. Besides, different box filler materials can bring about slight variation no more than 3%. (C) 2016 Elsevier B.V. All rights reserved.