Page 202 - Construction Waterproofing Handbook

ABOVE-GRADE WATERPROOFING 3.91
utmost care in detailing and installing termination and transition details, not only between
various rooftop components but transitions between roofing and other building envelope
components of the envelope. These transition details are covered fully in Chap. 10.

VAPOR BARRIERS

Vapor barriers are used in above-grade construction to prevent moisture vapor transmis-
sion between interior and exterior areas. In winter conditions, warm, moist interior air is
drawn outward to the drier outside air by the difference in vapor pressures (negative vapor
drive). In summer, moisture vapor travels from moist and warm outside air to cool and dry
interior areas (positive vapor drive).
Vapor barriers or retarders are not waterproofing materials but are used as part of wall
assemblies to prevent vapor transmission and allow this vapor to condense into liquid
form. Vapor barriers are most useful in hot tropical areas where vapor transmission into
air-conditioned areas can be so severe that mold and mildew frequently form on exterior
walls. This problem is often mistaken for water leakage or infiltration when it is not.
Attempts to repair, including applying breathable coatings to an envelope (e.g., elastomeric
coatings), will not solve this problem.
Should a nonbreathable coating be applied to an envelope under these conditions, how-
ever, coating blistering and disbonding will occur when negative vapor drives occur. This
requires that vapor retarders or barriers be applied to the interior or warm side of insulated
areas. In tropical areas, the barrier is placed on exterior sides to prevent condensation or
vapor from wetting insulation caused by positive drive. In most areas, barriers are placed
on interior sides of insulation due to the predominance of negative vapor drive.
Vapor barriers are commonly available in polyethylene sheets or aluminum foil sheets
on laminated reinforced paper. Sheets must be applied with seams lapped and sealed to
prevent breaks in the barrier.
A vapor barrier’s performance is measured in perms (permeability). This is the measure
of vapor transmitting through a particular envelope material or component. Materials such
as masonry block have high permeability, whereas polyethylene materials have very low
permeability. Glass is an example of a barrier. Moisture collects and condenses on glass
because it cannot pass through the glass.
Although vapor barriers are not used as waterproofing systems, they can affect the
selection and use of waterproofing materials for use on an envelope. If negative vapor drive
is possible (winter conditions), it is necessary for permeable waterproof materials to be
used to allow this moisture to pass without damaging the waterproofing material by blis-
tering or delaminating.

INTERIOR WATERPROOFING APPLICATIONS

Most of the waterproofing systems described in the previous sections and Chap. 2 are
applicable for interior spaces including showers, kitchens and specialty areas such as
steam or locker rooms, laboratories, and mechanical rooms. Liquid-applied membranes