Although contemporary and complex building energy models such as EnergyPlus are highly advanced and predictive, a simplified building energy model has many advantages and applications. Chief among the advantages are adaptability to the measured response of a building and conciseness in computer implementation. Important applications will include automated fault detection. One of the most important aspects of a successful building energy model is the ability to accurately account for the heat capacity of the structure, especially the envelope. As part of the development of a simplified physics-based building energy model, a variety of roof and wall models were considered. Three different generic models were considered for opaque walls and roofs (1) an analytical model with closed-form solution, (2) a time-series solution using the ASHRAE transfer function method, and (3) a lumped capacity model with various numbers of capacitive nodes. This paper will describe an investigation of various simplified models for the transient conduction through building walls. Specifically, how different kinds of wall constructions and materials can change when the conduction load passes through the wall and when it impacts the HVAC system. Understanding how the physics of dynamic envelope loading can change how much energy a building uses for heating or cooling and when room conditioning loads occur is vital for energy savings and proper modeling.