Stretch wrote:

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...
IAT is the Inside Air Temperature...
OSF is the Outside Surface Film (R-factor)... about 1.67 for shiny new duct
ISF is the Inside Surface Film (R-factor... About R-0.2
DI is the duct insulation R-factor, (about R-8 for your ducts)

OAT = 90 F and IAT = 40 F makes

T(surface) = 90-(90-40)*1.67/(0.2+8+1.67) = 81.54 F...

Like this, viewed in a fixed font:

T(surface)
|
ISF DI | OSF
IAT ---www---www------www--- OAT

---------------------
I

Heatflow I = (OAT-IAT)/(ISF+DI+OSF) = (90-40)/(0.2+8+1.67) = 5.066 Btu/h,
and T(surface) = OAT-I*OSF = 90-5.066x1.67 = 81.54 F.

Wrapping it all with R19 fiberglass insulation and poly film duct might
decrease I to 1.732 and raise T(surface) to 90-0.67x1.963 = 88.87, while
reducing the rate of condensation (if any) and saving energy. Exposing
ducts to more attic air ~~~wastes~~~ energy.

T(surface)
|
0.2 8 1.67 19 | 0.67
40 ---www---www---www---www------www--- 90

---------------------------------

I = (90-40)/(0.2+8+1.67+19+0.67) = 1.693 Btu/h

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.

Or Td = (460+OAT)/(1+(460+OAT)ln(RH/100)/9621)-460. For instance, 90 F air
with RH = 50% has Td = 550/(1+550ln(0.5)/9621)-460 = 69 F.

Nick