Figure 18 shows the installed R-values of fibrous glass duct liner required to prevent moisture condensation on the outer duct surface under varying conditions of ambient temperature and relative humidity. Curves are based on internal air temperature of 55˚F (13˚C) and no air movement on the external surface.

NOTE: R-values as required by ASHRAE 90.1-1989 may not be sufficient for condensation control, as may be seen in the figure below.