replacing duct work

[bighead51]

I have flex duct in my home and I am going to replace with round metal duct. I was wondering since I have the flex laying around can I just slide the flex over the metal for the insulation after sealing all seams? like a 12" flex over the 10"metal?

 

[PWC Repair]

Depending upon the brand of flex you might be able to pull out the spiral liner which would leave you with an insulation "sleeve".