This paper presents an investigation on the wall layer interactions in a lower-half heated, upper-half cooled rectangular enclosure in both steady and unsteady laminar flow regimes. Results show that the flows have four wall layers and center core regions. At low Rayleigh numbers, both the wall layers and cores are steady. Each of the streams in the core region is formed by the combination of two wall layer segments from the same half of the enclosure. Fluid exchange between the two halves is very efficient. At high Rayleigh numbers, the wall layers at the same side of the enclosure collide head-on and generate unsteady streams. The interaction between the streams establishes a strong mixing region at the median height, which homogenizes the fluid temperature in the core regions. The overall flow structure and the path of the heat flow in the enclosure are determined by the momentum strength of the wall layers and their interactions.