Understanding
Air Barriers
By Robert Davenport
P.E., RGD Consulting Engineers, Jupiter, FL
The
envelope of an air conditioned space inside a building is required to have an
air barrier. Without an effective air barrier, the space may and probably will
experience moisture related damage in time. An air barrier is required by
section 502.3.5 Building Cavities, the 2010
Florida Building Code – Energy Conservation. The code further specifically excludes batt insulation and lay
in type drop ceilings as effective air barriers. Reference the term “Air
Barrier” in Section 2 Definitions. This requirement is frequently overlooked in
building design and construction.
The air
barrier’s primary purpose is to prevent the commingling of conditioned and
unconditioned outside air masses. For most of the year in South
Florida , the outside air has high humidity. The temperature at
which the water molecules begin to condense to the liquid state for much of the
year is 73 degrees F. This is called the Dew Point. Since most air conditioned
spaces operate at or near this temperature, it is easy to see that when the two
air masses come into contact with each other, there is a high probability that
liquid water can form. Water droplets condense on the nearest building material
which may be the mineral fibers of batt insulation, the lay in ceiling or the
roof structure. The potential for damage when this happens is well documented.
To be an
effective air barrier, it must prevent air movement between conditioned and
unconditioned spaces. The code specifically identifies gypsum wall board with
taped joints to be an effective air barrier. Other systems may also be
acceptable but are not identified in the code. It is essential that the barrier be able to
prevent air movement across itself when subjected to a pressure differential. Buildings
can experience positive and negative pressure differentials for a variety of
reasons. Properly designed air conditioning systems will typically maintain a
positive pressure differential on the space. However both conditions have the
potential to create serious negative effects on the building if the space is
not supplied with an effective air barrier.
The
condition observed most frequently in the authors experience is a commercial
building with a wood truss roof structure. (Figure 1-1) The batt insulation is
installed on the bottom chord of the truss and the lay in ceiling installed
below that. The condition is exacerbated by code required attic ventilation
above the batt insulation provided by soffit vents and or ridge vents. Conditioned
spaces that have this condition usually operate with elevated humidity levels
and with moisture related damage occurring at or above the suspended ceiling.
The most obvious characteristic of spaces with this condition are sagging
ceiling tiles with mold growing on the back side in many cases. Reference
figure 1-2.
To insure
optimum building performance and longevity, the air conditioned envelope must
be provided with an effective air barrier. The building code identifies drywall
with taped and mudded joints as one method of providing the air barrier. Figure
1-3 illustrates the same configuration with a gypsum layer added to the bottom
chord of the truss.
Alternatively,
the code allows the insulation to be applied to the top chord and the attic
space to be sealed. This method is becoming popular as research indicates that
sealed attic spaces are preferable in 
One system
that is gaining popularity is Icynene
spray insulation. This material is
sprayed on the roof decking and top chord of the trusses. It can be applied in
a range of R values, usually R-30 on roof structures and is accepted as an
approved air barrier by most building officials.
For more information about our
services, visit
RGD Consulting Engineers corporate website - www.rgdengineers.com.
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