5.2.8 Recalculate the temperature at the vapor retarder surface:

5.2.9 Therefore, the vapor retarder temperature (64°F [18°C]) is warmer than the condensation temperature (60°F [16°C]) with 1½ " (38 mm) of Fesco.

5.2.10 Psychometric Table

5.2.11 Typical Indoor Humidities in Winter:

5.2.12 To carry this theory further, when a roof system construction is such that the vapor retarder is placed over a minimum layer of insulation, as shown below, the following calculations can be used to determine the thickness of insulation to establish the temperature at the vapor retarder to prevent condensation of vapor at or on the vapor retarder.

The following formula may be used to calculate the minimum "R" needed to prevent condensation.

where:

R_x = Insulation value needed above vapor retarder to prevent condensation.

R_vr = Thermal resistance to vapor retarder, i.e., "R" units below vapor retarder.

T_i = Inside design temperature.

T_o = Outside design temperature

T_vr = Temperature required at vapor retarder., i.e., 1 - 2 degrees above dew point.

R_e = Thermal resistance of existing construction above vapor retarder, not including R_x , (i.e., membrane and air film).