As a result of increased concern with energy consumption in the industrial world, it is only natural to look towards the building sector to seek significant improvements to meet expectations of the society. After all, the building sector consumes more energy than the transportation sector. Yet, the procedures that are used to define the thermal performance of, for example a wall, are typically based on the tests performed on dry materials, without consideration of air and moisture movements. In other words, these tests represent arbitrary rating conditions because we know that the energy performance of materials and building assemblies are affected by moisture and air flows. It is believed that to improve their energy performance one must have more precise means of evaluation of their field performance that would also include the consideration of air and moisture transfer conditions. In the first part of this article a background for the evaluation of thermal performance by traditional testing with calibrated boxes shows that use of these tests is limited. The average heat flow that they measure is sufficient to rate the wall assemblies but insufficient to calculate its thermal performance under field conditions. To include the effect of climate on thermal performance one must use computer models that are capable of simultaneous calculations of heat, air, and moisture transfer. Effectively, to characterize energy performance of the building enclosure one must simultaneously use assembly testing and modeling, i.e., an integrated methodology. In the second part of the article, this integrated testing and modeling methodology is applied to a few selected residential and commercial walls to highlight the magnitude of air flow effects on the steady-state thermal resistance. The integrated methodology proposed by Syracuse University includes several other aspects of hygrothermal performance evaluations. Those aspects will be addressed in later parts of this article series.