The formula for the outside surface Temp of a duct is:

T(surface)= OAT- (OAT-IAT) * OSF/( ISF+DI+OSF)

T(surface) Is the outside surface temperature of the duct
OAT is the Outside Air Temperature, that is the air temperature the
outside of the duct is exposed to
IAT is the Inside Air Temperature, that is the supply air temperature
inside the duct
OSF is the Outside Surface Film (R-factor), that is the insulation
factor for a nearly still air film at the outside surface of the duct,
(usually about1.67 for shiny new duct)
ISF is the Inside Surface Film (R-factor, that is the insulation factor
for moving air at the inside surface of the duct (About R-0.2)
DI is the duct insulation R-factor, (about R-8 for your ducts), if the
contractor is being truthful

Run the formula to find the surface temperature of the duct. Note that
if the air is trapped in a pocket formed around the duct by contact
with other ducts or structure, yhe OAT can approach supply air
temperatures very closely. That is usually where the problems start.

The dew point of the attic air will be the same as for the outdoor air,
which you can get from the weather man or a Sling Psychrometer.



Stretch