Four test cells of a field laboratory have been used to determine the effect of natural convection on the thermal performance of insulating materials in various typical flat-ceiling residential attic constructions. Both blanket and loose fill mineral insulating materials are used in both vented and unvented assemblies. Temperature, air movement, heat flux and air humidity are continuously measured in each cell, and outdoor weather conditions are monitored. For the purpose of this study, data from the month of January 1991 are used; during that period the attic temperature remained colder than the 'indoor' temperature. Field measurements of temperature at the top and bottom of the insulation, and heat flux measurements, are used to determine temperature difference across the insulation and instantaneous R-value. The results show that attic ventilation has negligible impact on the thermal performance of blanket insulation. The results also show an effect of elevated wintertime temperature differences upon loose-fill insulating materials.