R8 Flexible ducting is the Construction Standard and required in most areas due to the new Title 24 heat loss standards. Also note that any fittings should be wrapped with insulation for additional heat loss requirements.
We are using R-8 Ducts for most of our projects. With Energy Upgrade CA program, we are required to install the R-8 flex ducts in order to qualify our clients for their rebates.
You should install Flex insulated air ducts