This research provides an understanding of how the weight of Nomex® and Kevlar® fiber blend compositions determine performance of a selected group of single-layer aramid protective fabrics in static and dynamic TPP tests. It demonstrates that material factors controlling fabric barrier and insulative performance are fundamentally different: aramid fabric weight determines the comparative protective performance of these materials in a static TPP test that uses an air gap between the fabric specimen and heat sensor to measure barrier protection. In this test, greater fabric weight slows the rate of fabric heating, prolonging the time required for the fabric to become a significant reradiator of thermal energy. By comparison, heat transfer in a dynamic TPP test used in this study occurs, not only by backside radiation, but by conductive transmission occurring whenever the fabric contacts the instrumented thermal sensor. Thermal transfer can also take place as a result of catastrophic break-open of the test specimen inducted by the mechanical restraints and flexing action of the dynamic tester. In either case, the thermal stability of the aramid fabric material, determined by its strength retention in flames or resistance to heat shrinkage, emerges as a crucial factor determining protective insulation. This research advances a quantitative model for predicting the heat shrinkage of aramid fabrics based on the blend ratio of Kevlar® and Nomex® fibers. This model effectively explains the performance of aramid fabrics in a dynamic TPP test. It explains the role of Kevlar® fibers in stabilizing a Nomex® blend against heat shrinkage and suggests a minimum concentration of Kevlar® to optimize fabric performance in dynamic TPP exposures.