Attic spaces are one of the most dynamic components in the building envelope. Engineers, designers, architects, and contractors can utilize different attic designs, materials, and mechanical equipment in attic spaces to optimize attic thermal and energy performance and to meet the needs of home owners, codes, and standards. The design of attic spaces becomes extremely important when considering energy usage. Simple changes can often drastically change the energy characteristics and long-term durability performance. Historically, interest has been focused on attic spaces with gable ends and materials with constant thermal properties. However, recent years have seen increasing interest in more geometrically complex roofs, and in novel materials tailored for the building envelop. Modification of the state-of-the-art attic simulation software to include these changes is very difficult, since most of the software developments date back to the 1970s. This paper discusses development and verification of the Fraunhofer attic thermal model (FATM). FATM's numerical predictions are verified against benchmark thermal problems and other energy load calculation software. FATM has several novel features, including (a) a capability for analysis of both convex and non-convex attic geometries, (b) incorporation of temperature-dependent material properties, (c) temperature dynamics of the conditioned space, and (d) a careful software engineering informed approach to build a generic, modular, and flexible numerical framework. The contributions of this paper increase the applicability of the framework to a larger percentage of attic spaces around the world, allow for future changes to the framework to be easily made, and introduce methods to create a modular framework to implement into whole building simulation programmes. With this newly developed approach to determining energy loads in attic spaces, the form of attic spaces and novel material construction systems can be easily explored.