Page 53 - Green Builder Nov-Dec 2021 Issue

Figure 2: Common air barrier details
COURTESY: STEVEN BACZEK / JOURNAL OF LIGHT CONSTRUCTION
Skylight shaft
Plumbing and electrical penetrations

Insulation baffle Exterior walls at tubs and fireplaces
Band Joist at garage
Dropped ceilings
Exterior walls at garage Attic hatches

Knee walls Intersecting walls

Windows

Band joists Exterior walls at
unconditioned spaces

Floor above garage Material change
Soffits and Exterior
Cantilevers Doors Corners

barrier. In addition, the drywall needs to be sealed to the top and
bottom plates adjoining unconditioned attics or basements, and Figure 3: Air barrier missing
at all penetrations (outlets, switches, and duct boots). at dropped ceiling
The dropped framing detail adjoining a vented attic and exte-
rior wall shown in Figure 1 demonstrates how the absence of a
complete air barrier contributes to thermal bypass and moisture
risk. As shown here, the air barrier below the attic insulation
and at the exterior wall (e.g., drywall) is often missing in older
construction because it is difficult to install once framed.
In the winter condition shown, the exterior and attic are at
the same sub-freezing temperature and the house is heated to
70 degrees Fahrenheit. The space in the dropped ceiling would

quickly assume the 70 degrees Fahrenheit room temperature

(not shown in the diagram) since the drywall covering the sof-
fit provides little thermal resistance. Where there is a pressure
difference between the attic and home (e.g., windy day), there
will be driving forces from more to less air pressure, and more
to less heat.
Since fibrous insulation resists thermal flow but not air flow,
the driving forces would result in a thermal flow between the
dropped ceiling and attic until some balance point is reached. This
is shown in the diagram as 45 degrees Fahrenheit, the mid-point
of the temperature difference across the dropped ceiling and attic.
This results in a cold surface temperature at the dropped ceiling
drywall. Also note that if fibrous insulation is used in the walls,
the same driving forces could result in air flow through the wall
insulation to the cold exterior sheathing where condensation and
moisture damage can occur.
In a home missing air barriers at dropped ceilings, a wintertime

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