I am remodeling my basement and have a ceiling clearance issue. Local codes require no less than 7'.. I can't meet the 7' mark but the local authorities will give me a variance if I do the following:

Except for the first 7 feet of the supply run, raise the supply run so it is directly up against the floor joists. This would raise the ductwork 1/2" to 1". My question is has anyone heard of putting something such as tarpaper between the top of the duct work and the floor joists? I believe the purpose of this is to avoid any rattling noise that might be caused duct runs during expansion and contraction.

The first 7 feet of the supply run has to be replaced with custom ductwork done to make it wider and 8" deep. There is room to do this. The 7 feet comes from code in this area requiring it to avoid drying out the floor joists and making them more combustable (sp?).