Figure 6 shows the R-values of fibrous glass duct board required to prevent moisture condensation on the outer duct surface under varying conditions of ambient temperature and relative humidity. Curves are based on cold duct internal temperatures of 55˚F (13˚C) and no air movement on the exterior 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.