Figures 20 and 21 show the installed R-values of fibrous glass duct wrap required to prevent moisture condensation on the vapor retarder 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 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 figures below.

Note (Figs. 20, 21) that condensation will occur at higher relative humidity. Condensation on the sheet metal duct surface is controlled by using a sealed vapor retarder of sufficient perm rating.