ASHRAE Journal - April 2020 - 80

COLUMN BUILDING SCIENCES

FIGURE 11 Two-Stage Joint. Continuous inner seal and a drained and vented cavity

FIGURE 12 Two-Stage Joint Section. Section through a two-stage joint that uses

between the continuous inner seal and a weeped outer seal.

sealant to direct water to the exterior.
Sealant Backer Rod
Inner Seal

Inner, Protected Seal

Outer, Exposed Seal

Drain and Vent Opening

Wind Pressurizes Chamber
Between Inner and Outer Seal

Outer Seal
Vent Tube

Sealant Backer Rod

and ultraviolet radiation)-but the assembly still works
because of the interior protected seal. Figure 12 shows
a section through a two-stage joint that uses sealant
to direct water to the exterior. Figure 13 shows a threedimensional detail of the drainage paths.
This approach of inner and outer seals is also common in window and door design and window and
door installation (connection to the rough opening).
The air seal around the entire interior perimeter of
the window or door provides for reduced air pressure
differences across the unsealed bottom flange of the
window or door. The exterior seal around the exterior
of windows and doors is weeped at the bottom or is
unsealed at the bottom to permit drainage to the exterior (Figure 14).
Now, over the years I have learned that it is important
to be specific with language...and I have more than once
gotten things wrong. In the hope that I have not gotten it
wrong I am going to define four terms that I think are a
big deal. Here goes:

FIGURE 13 Three Dimensional Detail of Two-Stage Joint. Drainage paths are

shown. The outer seal is exposed to the elements-ultraviolet light, heat and
water-but the assembly still works because of the interior protected seal.
Drainage Space
Backer Rod
Sealant
Sealant
Backer Rod

Exterior Face of
Cladding
Drainage Opening

Drained Assemblies
A rainwater control strategy that accepts some water
will penetrate the exterior cladding. Drained assemblies have multiple lines of defense for rainwater
entry. In addition to the exterior cladding, drained
assemblies layer building materials shingle fashion
inward of the exterior cladding to direct rainwater
that penetrates the cladding back to the exterior. The
inward layer is called the water control layer. A space
or air gap between the water control layer and other
materials promotes drainage, ventilation and moisture
redistribution.
80

ASHRAE JOURNAL

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APRI L 2020

Barrier Assemblies
A rainwater and groundwater control strategy that
typically relies on the exterior cladding or exterior
surface of the foundation to act as the water control
layer. Above grade "barriers" can be constructed from
materials that are defined as waterproof such as glass,
steel, dense concrete or roof membranes. Below grade
"barriers" can be constructed from sheet waterproofing. Examples of barrier assemblies are precast panel
walls, glass curtain walls, insulated metal panel

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ASHRAE Journal - April 2020

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