Quantcast Section 4: Vapor Control

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Psychrometrics.  The design of roofing must take into
account the principles of moisture in air to prevent condensation
of moisture in roof insulation and under roof surfaces.
Psychrometrics deals with determination of the properties of
moist air and utilization of these properties in analysis of
conditions and processes involving moist air.  A detailed
discussion and tables of moist air properties are included in
ASHRAE l993 Handbook - Fundamentals, Chapter 6.
Vapor Pressures and Flow.  In order to determine the
magnitude and direction of water vapor flow, examine the range of
indoor and outdoor conditions that will exist over the course of
a year.  Moisture vapor flows from regions of higher vapor
pressure to regions of lower vapor pressure.
Indoor.  Indoor water vapor pressure is increased by
the liberation of moisture from construction materials and
building occupants, as well as moisture generated by plants,
building processes and humidification.  This tends to make the
indoor vapor pressure higher than outdoor, causing the potential
for flow of water vapor up through the roof in cold weather.
When indoor space is air conditioned, including dehumidification,
it is possible for the indoor vapor pressure to be lower than
Outdoor.  Outdoor vapor pressure is continuously
varying throughout the year in any given location.  When outdoor
pressure is higher than indoor vapor pressure, there will be a
tendency for vapor to flow outdoors to indoors through the roof.
Design Criteria.  ASHRAE 1993 Handbook - Fundamentals
shows outdoor design conditions for most locations throughout the
world.  Once indoor design conditions are established, the vapor
pressure differential between indoor and outdoor can be readily
determined.  In order to establish the magnitude and direction of
potential vapor flow under all conditions of operation, determine
the variations in indoor and outdoor vapor pressures throughout a
typical year of building operation.
Dewpoint.  An analysis should be conducted to determine
the potential for condensation to take place throughout all
conditions of operation, both within the roof itself and on the
surfaces of the roofing assembly.  Such an analysis will
determine the need to consider increasing the thermal resistance


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