This research quantifies the impact of degradation of thermal insulation on building energy performance over time. Thermal insulation is the ability of a material to resist thermal conduction; the unit of measure is Thermal Resistivity or R-value. High-performance building design guidelines aim at reducing energy consumption by increasing the R-value of the building envlope beyond the minimums specified by building energy codes. Building energy calculations assume that published R-values remain constant for the lifespan of the installed material. There is growing academic and industry evidence that the R-value of thermal insulation degrades over time. High-performance building standards are based on the R-value at the time of installation. The implication of these studies is that the reduced insulative quality of the building envelope currently underestimates long-term energy consumption. In small or medium commercial buildings this degradation in performance may have a substantial impact on energy use. The objective of the study is to estimate the potential error of omitting insulation degradation in small and medium commercial buildings. We will use data generated by Parametric Analysis Tools in energy modeling programs to study the impact of insulation aging over a ten-year period. Based on these outcomes, we will propose a list of corrections to long-term energy performance estimations. The data provided in this study may also be used to suggest modifying energy standards. It may also have a significant impact on current life-cycle cost analysis techniques used to assess the performance of building insulation.