The disadvantages of using R6 ducting include:

1. Higher Initial Cost: Compared to less insulated ductwork, R6 ducting may have a higher initial cost due to the increased insulation material and potential labor associated with its installation.

2. Performance Issues with Improper Installation: Even with its high R-value, R6 ducting can still suffer from problems such as leaks or poor performance if it is not installed correctly. Proper sealing and installation are crucial to ensure the ductwork operates efficiently.

3. Reduced HVAC System Efficiency: If the ducting is not matched with the appropriate HVAC system or if the overall design does not account for the increased insulation, using R6 ducting could potentially result in a less efficient HVAC system. This means that while the ducting itself is more insulated, it might not lead to overall energy savings if other system components are not optimized accordingly.