The present work examines a system that uses two separated wall cavities filled with air which can be used as a thermal insulator in buildings. It is well known that stationary air has lower thermal conductivity 0.026 W/m.K. This has made it of practical interest in many applications such as in thermal insulation materials (e.g. porous materials) and as thermal insulator (e.g. enclosures, wall cavities). Specifically this work investigates the use of two separated wall cavities that trap air inside each one and placed in a building wall where the heat transfer is expected. And to achieve that, the rate of heat transfer through this wall is first calculated with the assumption of pure conduction (i.e. trapped air in the two wall cavities is assumed to be stationary). Then convection heat transfer rates through the two wall cavities are estimated based on the Nusselt numbers and hence the dominating heat transfer mechanism (conduction and/or convection) is explored. Taking into account all the above, introducing two cavities filled with air to building wall can work effectively as thermal insulator as indicated by lower Nusselt numbers and hence lower convection heat transfer rates through the two wall cavities. Advantage of using two wall cavities over one wall cavity of larger thickness is evidenced by the higher convection heat transfer rates which are associated with one wall cavity of larger thicknesses – that's the wall with two cavities can effectively resist the heat flow than the wall with one cavity of larger thicknes
