Q. We are renovating a 1980 home. The hvac sub says that since older fiberglass duct board disintegrates from the inside out, all of the ducts should be replaced. Could this be true?

A.Jeri Donadee, vice president of H.B. McClure, a heating and cooling contractor in Harrisburg, Pa., responds: When duct board in a crawlspace or attic is not sealed with UL 181 duct tape, condensed moisture can accumulate near unsealed duct joints, leading to duct deterioration. If such deterioration is severe, replacement of the ducts may be necessary. But it is unusual for properly installed duct board to "wear out."

Your subcontractor’s diagnosis can be easily confirmed by a visual inspection. Simply cut open a section of duct and look at the interior surface to see if there is any flaking. If the duct is sound, be sure to reseal the duct using appropriate UL 181 mastic or tape.