Convection in Loose-fill Attic Insulation
Doctoral thesis, 2001
The objective of this study is to investigate the influence of convection on the thermal performance of loose-fill attic insulation under realistic climate conditions. In order to achieve this, a large-scale attic test model has been constructed in a climate chamber. Heat flows through the attic floor have been determined with a metering box and temperatures on the upper and lower surface of the attic floor have been measured to detect air movements and to determine the thermal resistance of the attic floor. The performance of the attic test model has been assured by gradually increasing the complexity of the studied attic floor. Measurements have been made both with and without joists, with and without attic ventilation and on rockwool boards, rockwool loose-fill insulation and low-density, glasswool loose-fill insulation. The upper surface of the attic floor has been subjected to conditions representing Swedish winter and the lower surface to indoor conditions. When natural convection occurs, it leads to an increase in heat loss through the insulation and, thus, to a decrease in the thermal resistance of the insulation. A number of factors influence natural convection, such as temperature difference over the attic floor, the thickness of the insulation and permeability of the insulation material. Measurements of natural convection showed that the critical modified Rayleigh number for attic insulation, in the studied set-up, was approximately 22. At a modified Rayleigh number of 34, the thermal resistance of the insulation in the attic test model was only 75% of that obtained at the onset of convection. This means that there was a substantial increase in heat loss through the insulation due to convection. The measurements of natural convection have been compared to simulations and similar tendencies were obtained. There was no indication of a decrease in the total thermal resistance of the whole attic floor due to attic ventilation.