Figures 26 and 27 show the installed R-values of fibrous glass insulation boards required to prevent moisture condensation on the vapor retarder surface under varying conditions of ambient temperature and relative humidity. Curves are based on internal air 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 figures below.