AC-83-09-2 — Natural Convection within Masonry Block Basement Walls

Internal convection and radiation have long been recognized as important in wall heat transfer and are implicitly factored into the thermal resistance values of hollow masonry block. These values have proved suitable for conventional above-grade walls, as the uniform boundary conditions permit application of simple one-dimensional heat-transfer models. Thermal resistance variations over the height of basement walls promote vertical fluxes that violate the basic assumption of one-dimensional heat transfer normal to the wall. The magnitudes of these thermal bridging effects are determined by comparing the heat losses through empty and sand-filled masonry block basement walls.

The presence of two-dimensional conduction augmented by free convective airflow within the block wall is demonstrated. Due to these thermal bypass mechanisms, the concentric arc heat flow approximation overestimated basement wall thermal resistance and underestimated the effect of eliminating internal convection. The differences, however, are insufficient to warrant modifying present methods for estimating basement wall heat loss.