SYSTEMS/PRECAST
Air Leakage and Water Vapor Control
As mentioned earlier, uncracked precast concrete panels are airtight and have a high resistance to
water vapor transmission. However, they alone do not constitute an effective air barrier system or
an appropriate vapor retarder. A continuous air barrier system must be specifically designed into
the wall, with its location based on rational design principles. Similarly, a vapor retarder should be
included at an appropriate location within the wall based on the climate and the total wall design.
Architectural Precast Concrete (PCI) contains a thorough discussion of condensation control and air
barriers. This manual states that both an air barrier and a vapor retarder are needed, pointing out
that a single system can sometimes perform both functions.
In heating climates condensation problems arise when interior moisture is allowed to reach cold,
outer elements in the building envelope. Such condensation can cause discoloration and damage
to the precast panels, corrode metal panel supports and wet and degrade thermal insulation. In
cooling climates, moisture from outside will condense on cold elements within the wall, causing
similar problems and potentially damaging interior finish materials. A vapor retarder will slow the
transport of this water vapor due to diffusion, but a much larger amount of water vapor can move
due to air leakage. An air barrier is needed to prevent this means of air and water vapor transport.
The performance requirements and design issues regarding vapor retarders and air barriers are
discussed in the sections Principles/Air Barriers and Principles /Vapor Retarders. Of particular
importance is the relative positioning of these elements and the thermal insulation within the
envelope. In general, it is important to keeps both elements on the warm side of the insulation.
With regard to precast panel walls, there are several options for controlling air and water vapor
transport. The face seal approach was discussed above in the section on water leakage. In this
approach the air barrier is in the facade of the building, placing the air seal material in a relatively
harsh environment. In heating climates, one needs to control the transport of water vapor from the
building interior to this cold air seal in order to reduce condensation problems. A vapor retarder on
the interior side of the insulation will help, but it will not prevent water vapor transport due to airflow.
Because the precast panels and the air seals constitute an effective vapor retarder, installing a
vapor retarder on the inside of the wall results in a wall with two vapor retarders, an undesirable
situation.
A two-stage joint design moves the air seal to a location within the wall, protecting it somewhat from
the elements, but not from cold temperatures. In a heating situation, the air seal is still on the cold
side of the insulation, and moist interior air that reaches the back of the air seal will condense.
Again, an interior vapor retarder can reduce the transport of water vapor by diffusion, but not by
convection. And an interior vapor retarder will result in a wall with two vapor retarders.
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