In this numerical investigation, a detailed comparison of the conjugate and non-conjugate natural convection within a semi-cylindrical cavity has been presented. The cavity is assumed to be filled with a fluid containing uniformly distributed internal heating sources. The bottom circular wall of the cavity is taken to be thick with finite conductive properties, while the top wall is considered to be isothermal. The Navier-Stokes and energy equations are solved numerically by using the SIMPLER algorithm. A Rayleigh number range from 3.2 x 106 to 3.2 x 1011 has been investigated and the effects of solid-to-fluid conductivity ratios of 1.0, 5.0 and 23.0 have been analysed. The present numerical results for a semi-circular cavity with entirely isothermal walls are compared with known results from the open literature. It was found that these results for the non-conjugate problem are in very good agreement. The present results for a conjugate cavity show a remarkable difference from the non-conjugate analysis. The average Nusselt number for the solid-fluid interface shows a decrease while the top wall average Nu number has increased. It has been concluded that these effects increase for a system with a low solid-to-fluid conductivity ratio. It is evident from the present conjugate results that the assumption of isothermal enclosing walls gives somewhat different results when the walls are thick and the solid-to-fluid conductivity ratio is small. © 2001 Elsevier Science Inc. All rights reserved.