A combination experimental and theoretical study of the thermal performance of typical residential attics and commercial ceiling spaces was carried out. The response factor and transfer function methods were used in models simulating the attics. It was concluded that the model using response factors represented residential type attics quite well. This model was then used to generate effective thermal resistances and total equivalent temperature differences (TETD) for attics and ceiling spaces for various conditions and to verify the model using the transfer function method. The transfer function method was then used to verify existing cooling load temperature differences (CLTD) and generate values for new cases. It is recommended that the new design data be incorporated in the ASHRAE Handbook.