Moisture Control Contents:
Moisture control within the roof assembly includes
such factors as moisture migration, dew point, condensation, ventilation,
and vapor retarders. Some migration of interior moisture is allowed due
to the natural downward drying cycle that occurs in most climates; and
therefore, the criteria for determining the need for vapor retarders shall
take this into consideration. The term vapor retarder refers to a Fields
waterproof membrane installed prior to the BUR membrane so as to surpress
the flow of moisture vapors from the interior of a building into the roof
assembly. Moisture from the interior of a building generally comes in
the form of water vapor from the following four sources:
||Construction processes which include
the drying of interior concrete, cementitious roof fills, plaster
compounds, water-based finishes, and fuel burning space heaters.
||Operational sources such as swimming
pools, greenhouses, laundries, textile manufacturing, food processing,
and paper plants.
||Occupancy sources which include kitchens,
shower facilities, exercise rooms, and even breathing from high-density
||Cool climate regions where moisture
vapor generally flows upward through the roof system from a heated,
more humid interior toward a colder, drier exterior during the winter
months. Such a climate condition causes vapor to migrate into an insulated
roof system where it can cool and condense into water.
Buildings with high interior relative humidity require
vapor retarders, as well as buildings located in the Central and Northern
regions of the United States. For such buildings, the designer shall calculate
the roof assembly dew point so as to determine the correct location to
install the vapor retarder within the roof system. Included in the calculations
shall be the R-value of the roof assembly components, anticipated interior
and exterior temperatures, regional temperatures, the building's planned
relative humidity, and any other pertinent data.
The Three Primary Moisture Control
||Ventilation is the exchanging
of air in a given space. Passive or mechanical ventilation is designed
to control moisture and reduce heat buildup. Exchanging the air in
a given space is accomplished by venting moisture-laden air to the
outside, while inducing fresh outside, or conditioned drier air to
||Insulation provides R-value
and aids in temperature control within a building. Insulating a roof
reduces the amount of energy required to heat and cool a building
and can also reduce the potential for condensation on interior surfaces
or within the roof assembly.
||A vapor retarder is a Fields
two or more sheet waterproof membrane adhered and coated with Fields
asphalt. The vapor retarder is generally installed somewhere between
the deck and the insulation. Vapor retarders are installed to inhibit
the flow of moisture vapor into the roof assembly from the interior
of a building.