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Vapor Retarders |
5.1.11 An important distinction
should be made in regard to condensation as there are two different types of condensation
with which we must be concerned when designing a roof structure.
5.1.12 Surface condensation on interior
surfaces of a building occurs if the interior ceiling or bottom of a roof deck is cold
enough to cause the interior air to reach its dew point. This type of condensation cannot
be prevented by use of a vapor retarder. This condensation can only be avoided by use of
an adequate amount of thermal insulation or by proper dehumidification of the interior of
the building.
5.1.13 The condensation which occurs
within the roofing system, however, is a more serious concern. Nearly every structural
roof deck is porous to some degree, or has joints open to water vapor migration, and it is
possible for condensation to occur in the roof insulation layer or on the bottom of the
roofing membrane.
5.1.14 There is a thermal gradient within
the roof system, graduating from warm on the inside to cool on the outside in winter and
the reverse in air-conditioned buildings in the summer.
5.1.15 This condensation may occur as
liquid water or frost, depending on the outside temperature. In either case, it will
eventually saturate the insulation and reduce its thermal efficiency, and possibly its
structural integrity. It may also flow back into the building and appear as a roof leak.
5.1.16 The only way to prevent the
condition just described is to stop the water vapor from reaching the dew point. If a good
vapor retarder is placed on the warm side of sufficient insulation R-value, the
possibility of surface or internal condensation is minimized.
5.1.17 The effectiveness of a vapor
retarder is measured by its "perm" rating. The porosity of a material to the
passage of water vapor is measured in perms. This is defined as the number of grains of
water vapor that will pass through one square foot of the material in an hour when the
vapor pressure differential between the two sides is equal to 1 inch of mercury (0.49
psi).
5.1.18 To be classified as a vapor
retarder, a material should have a permeance of less than 0.5 perms.
5.1.19 Vapor retarders protect roof
insulation and membranes against moisture attack that is a result of condensation.
However, vapor retarders will not protect roof systems from entrapped moisture due to the
installation of damp or wet insulation, or due to leaks in the roof membrane or flashings.
Any condition which allows entry of surface water contributes to loss of thermal
efficiency and possible premature failure of the roof insulation and roof membrane.
5.1.20 Vapor retarders must always be
placed on the warm side of the insulation. JM FP-10 One-Way Roof Vents, compatible with
the membrane used, may be helpful in venting a roof system whether or not a vapor retarder
is used. The primary function of these vents is to relieve vapor pressure within the
roofing system. It should be noted, however, that the installation of roof vents is not a
practical method of drying out wet roof insulation.
5.1.21 These vents can also be used to
relieve the moisture pressure in roof systems applied over non-cellular lightweight fills
or old wet decks, reducing the possibility of moisture problems within the system.
5.1.22 Another system of venting the roof
construction is to incorporate a venting base sheet with an insulation overlay. JM
Ventsulation Felt is a coated felt with the bottom embossed or patterned in such a way as
to provide channels for moisture vapor to travel to the outer edges of the building or to
roof vents.
5.1.23 The addition of a vapor retarder to
a bituminous roof system assembly on a steel deck typically requires double layer
insulation with the vapor retarder sandwiched in between the two layers. The first layer
is a minimum thickness base sufficient to span the flutes of the deck and to receive the
vapor retarder membrane. The second layer provides the primary insulation for the system.
5.1.24 The trend toward high thermal roof
insulation has accentuated the need to protect the insulation with an adequate vapor
retarder.
