What R-factor whould your recomend for duct in a Houston, Texas attic?
Sorry, don't have my code book at home with me, or I'd tell you what the minimum code requirement is for your area.
Your contractor should know.
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How many times must one fix something before it is fixed?
R-6 should be Standard.
I suspect that R-8 duct would not be in common usage.
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Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
R8 or more! the biggest $$ in the labor -- savings from now on --
be sure all joints & seams have mastic first!
The highest you can find, period.
Lowes has foil-wrapped r-8 off the shelf.