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Vapor Retarders |
5.1 Vapor Retarders
5.1.1 Roof structures have recently
come under greater scrutiny in controlling the flow of energy into and out of buildings,
because the roof area is a large portion of the exterior building surface. The
preservation of the thermal efficiency of the roof insulation is also becoming more
important and therefore designers should give careful consideration to the use and proper
design of vapor retarders in the roofing system.
5.1.2 The need for a vapor retarder should
be determined by the architect and/or engineer in accordance with current engineering
practices and vapor theory, based on data provided by the building owner. The owner must
provide the designer with information as to the present and future planned use of the
structure. A JM Technical Service Specialist can assist the designer, architect or
engineer by providing advice and technical guidance on vapor retarders.
5.1.3 In general, any time that
condensation can occur, the designer should consider designing a vapor retarder into the
roof system. Because the potential for condensation is related to a number of factors, it
is very difficult to give hard and fast rules when and where to install a vapor retarder.
There are two guidelines or "rules of thumb" that are used to help determine if
the designer should investigate, with actual calculations, if a vapor retarder should be
used.
5.1.4 The older and more established
"rule of thumb" says that if any of the following conditions exist, a vapor
retarder should be strongly considered.
A. Structures in areas where the January mean temperature
is 40°F (4°C) or less.
B. Structures which are humidified or where operations
generate a considerable amount of moisture and humidity (in excess of 45% R.H.), such as
paper and textile mills, laundries, bakeries, locker rooms, etc.
C. Structures that are roofed, enclosed and heated, when
subsequent interior construction activities generate large quantities of moisture.
D. Other situations that require a vapor retarder which the
designer should consider.
5.1.5 A new and more analytical "rule
of thumb" has been developed by Mr. Wayne Tobiasson of the U.S. Army Cold Regions
Research and Engineering Laboratory (CRREL). He has developed a map of indoor relative
humidities at 68°F (20°C). Buildings which have an indoor relative humidity above these
figures should be considered for the inclusion of a vapor retarder. His article
"Vents and Vapor Retarders for Roofs" (available as Miscellaneous Paper MP-2246
from CRREL, Hanover, NH) contains more details and charts to convert to indoor
temperatures other than 68°F (20°C). When estimating an indoor relative humidity during
the design phase, it should be noted that human occupied spaces very often have relative
humidities near 50%.
5.1.6 As with any "rule of
thumb," the information above should only be used as a guideline. Specific building
requirements should be confirmed with an actual analysis. There are many other issues that
must be considered, such as the presence of a swimming pool or other
high-humidity-producing operations. These would include cooking, washing, or unusually
high humidity during the first months due to construction moisture. Construction moisture
within a building that is being finished late in the season can be a problem. Operations
such as the pouring of concrete will add large amounts of moisture to the air in a
building, which can cause condensation in a roof system that does not have a vapor
retarder. This condensation can drip back into the building and give the appearance of a
roof leak. These are called "phantom leaks," because they do not correspond to a
rain nor are they in the same place all the time.
5.1.7 Information is given below on the
mechanism and causes of condensation, as well as how to design an effective vapor
retarder.
5.1.8 The air inside an occupied building
is a mixture of dry air and water vapor. The water vapor can be thought of as steam at
very low pressure and it behaves like a gas. Its pressure forces it to move to areas of
lower vapor pressures. This gas cannot be seen except when saturated conditions occur and
then it appears as fog. The quantity of water vapor in the air depends on the rate that
moisture is released in the building and the relative resistance to moisture vapor
transmission of the building shell.
5.1.9 If the building is relatively
"sealed" so vapor cannot escape to the outside, the water vapor content of the
inside air will increase and may finally reach a point where condensation will occur on
cool surfaces such as on outside walls, windows, the underside of the roof deck or within
the roof system. Condensation of liquid water from water vapor will occur whenever the
temperature of a surface is at or below the dew point of the air-water vapor mixture in
the building.
5.1.10 The water vapor tends to equalize
by moving from warmer, more humid space to cooler, less humid space. In most regions of
the United States, water vapor within a building typically moves upward during the winter
months, from the heated, more humid interior up through the roofing system, toward the
colder, drier exterior. As the water vapor moves through the roofing system, it will
condense and change to water if it reaches its dew point (the point at which it reaches a
relative humidity of 100%). This movement is reversed in an air-conditioned building in
humid summer conditions.
