Virtually all residential and commercial windows in the U.S. have some form of window attachment, but few have been designed for energy savings. ISO 15099 presents a simulation framework to determine thermal performance of window attachments, but the model has not been validated for these products. This paper outlines a review and validation of the ISO 15099 centre-of-glass heat transfer correlations for perimeter gaps (top, bottom, and side) in naturally ventilated cavities through measurement and simulation. The thermal transmittance impact due to dimensional variations of these gaps is measured experimentally, simulated using computational fluid dynamics, and simulated utilizing simplified correlations from ISO 15099. Results show that the ISO 15099 correlations produce a mean error between measured and simulated heat flux of 2.5 +/- 7 percent. These tolerances are similar to those obtained from sealed cavity comparisons and are deemed acceptable within the ISO 15099 framework.