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preference for vapor retarder location is close to the indoor
Application.  Pay particular attention to the manner of
installing and sealing the vapor retarder material.  The design
should also consider damage of the vapor retarder both during
construction and operation of the building.
Ventilation.  Since it is possible that some moisture
will accumulate in the insulation, allow escape of the moisture
by providing means for ventilation.  In the case of wood frame
residential type construction with attics, a variety of methods
are commonly available to allow moisture which passes through the
ceiling insulation to escape to the atmosphere.  With low slope
roof construction, other methods for ventilation are required.
This approach can be called a self-drying system.
Below Insulation.  Using boards or sheets with
perforations or slots underneath low slope roof deck insulation
will allow moisture to escape from the roofing system to the
outdoors or indoors.
Above Insulation.  Ventilation above the insulation in
low slope roof decks can be accomplished either by using edge
vents or top vents.  Top vents penetrate the membrane and will
increase the potential for leaks.  Top vents, if used, should be
spread uniformly throughout the roof area.  Use caution when
applying ventilation above or below insulation, because the same
paths that allow moisture to escape will also allow moisture to
enter if the vapor pressure differential is reversed.  Top
venting should not be used over insulation having limited
moisture movement capability, such as plastic foams.


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