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Vapor
Retarders |
5.2 Design of Vapor Retarders
5.2.1 When a vapor retarder is part of the insulation system, it
is imperative to know how to determine the temperature of the
vapor retarder or any surface within the roof system to prevent
condensation from forming within the system. The temperature
of the vapor retarder is a function of the inside and outside air
temperature, and the thermal resistance of the insulation
system. The following is the formula for obtaining the vapor
retarder temperature.
where:
Tx = Temperature at vapor retarder.
Ti = Design temperature of inside air.
To = ASHRAE winter design temperature of
outside air.
Rx = Sum of all resistances from inside to the
vapor retarder.
Rt = Sum of the total thermal resistances of the
roof/ceiling structure, including air films.
5.2.2 The dew point temperature can be found on the
psychometric table, if the indoor temperature and the relative
humidity of the inside air are known. It is imperative that the
vapor retarder temperature be warmer than the dew point to
prevent condensation at the vapor retarder. If this is not the
case, more insulation must be installed above the vapor
retarder. The additional resistance is added to the Rt in the
formula to obtain a new vapor retarder temperature.
5.2.3 Example: The indoor conditions of a conventional
building are 75°F (24°C) (Ti ) and 60% relative humidity. The
ASHRAE winter design temperature is -20°F (-29°C) (To ) for
outside air. The roof construction and resistances for each
component is as follows:
5.2.4 Determine the temperature at the vapor retarder surface:
5.2.5 The dew point temperature for 75°F (24°C) and 60%
relative humidity is 60°F (16°C) as found on the chart on page
5-5. Therefore, the vapor retarder surface Temperature (Tx) is
colder than the condensation temperature (57°F vs. 60°F [14°C
vs. 16°C]) and moisture will condense on the vapor retarder.
5.2.6 To correct this condition, additional insulation should be
added above the vapor retarder, to make the vapor retarder
warmer than the dew point or condensation temperature.
5.2.7 If 1½ " (38 mm) Fesco Board with an "R" of 4.17 is
substituted for the ¾" (19 mm) layer, the following will result:
