The objective of this study is to investigate the influence of convection on the thermal performance of loose-fill attic insulation under realistic climatic conditions. Measurements have been performed in a large-scale attic test model to determine heat transfer through the attic floor due to convection. Natural convection occurs in low-density materials only, leading to an increase in heat flow through the insulation and, thus, to a decrease in the thermal resistance of the insulation. The measurements show that the critical modified Rayleigh number for the low-density attic insulation in the studied setup is 22 to 25. At a modified Rayleigh number of 34, the thermal resistance of the insulation in the attic test model is only 75% of the highest measured thermal resistance.