Convection phenomena in loose-fill attics

Convection phenomenon inside the insulation materials that are representing porous media may be responsible for important increase of the heat flow density through the building element, when the climatic conditions are very cold. A numerical study based on CFD software simulations is performed in order to find out the conditions for which the convection onsets and furthermore to quantify its influence on the U-value of the building element – that is a loose-fill attic. The geometry of the grid is very close to a 2D cut in a real attic. The most significant heat transfer phenomena present in the attic – conduction in solid elements, convection in porous medium, convection in air spaces are taken into account. This is done exclusively by using the mass, momentum and energy conservation equations. The insulation materials analyzed are highly permeable mineral wools. The results are expressed in terms of critical Ra * numbers and variations Nu * against Ra*. A critical Ra * value of 28 is found. Three products, (2 –in glass wool, 1 – in stone wool) are studied – all showed absence of convection for outdoor temperatures higher than –40°C. The stone wool product is the most exposed to convection onset. 1