Full-scale experimental study on the critical Rayleigh number for the onset of internal natural convection inside a horizontal insulation layer of blown glass wool

Glass wool is a common high-performance insulation material employed in buildings. The effective thermal conductivity of such materials with large open-pore structures can drastically increase if internal natural convection takes place inside the latter. This can occur in the case of a thick insulation layer with high porosity (low density) and subjected to a large temperature gradient. Such conditions can be met for blown glass wool insulation during winter in cold-climate regions. Consequently, the accurate assessment of the criteria determining the onset of internal natural convection in porous insulation is fundamental to optimizing the thermal performance of the latter. However, the numerical simulation of such phenomena is very complex and does not yield reliable results. Experimental investigations in realistic conditions are thus necessary. This article reports the findings of a full-scale experimental replication study on the onset of internal natural convection inside a horizontal insulation layer of blown glass wool with joists. The insulation layer is heated from below with a closed boundary enclosure at the bottom and an open boundary at the top. The thickness of the insulation layer is set to 30 cm or 60 cm, with a glass wool density ranging from 11.9 to 19.7 kg/m 3 , a temperature difference spanning from 5.2 to 59.5 K, and an average temperature ranging from −5.3°C to 26.5°C. The onset of the internal natural convection is identified by the critical Rayleigh number above which the modified Nusselt number (Nu + 1) increases with increasing Rayleigh number. For this configuration, the critical Rayleigh number is found to be situated between 13 and 15. These results are in good agreement with the recommendations from standards and other similar experimental studies on this topic. This study provides additional guidance for insulation design in cold-climate buildings.