The ideal R-value for rectangular flexible ducts depends on factors such as climate, local energy efficiency standards, and specific application requirements. Generally, higher R-values indicate better insulation properties, leading to increased energy efficiency and cost savings over time. Typical R-values for flexible duct range from R-1 to R-8, with R-6 to R-8 being recommended for most residential and commercial applications in colder climates. A certified R-Value of R6, offering up to about 200 degrees, is considered appropriate for many instances but opting for a higher R-Value rating might provide better insulation properties, though it could be more costly. Always refer to local building codes and consult with an HVAC professional to determine the appropriate R-value for your specific needs.