ASHRAE Journal - January 2023 - 44

TECHNICAL FEATURE
FIGURE 2 Using a closed-cell insulation reduces the potential for moisture penetration into an insulated system.
(Source: ASTM Standards/Manufacturer Data Sheets)
Insulation Water Vapor Permeability
Non Closed-Cell
Phenolic Foam
Polyisocyanurate
Flexible Elastomeric Foam
Cellular Glass
0.1
Perm Inch
previously discussed, is the reason
condensation remains a cause for
concern for insulation failures on
chilled water lines.
Insulation System Design,
Permeability, Vapor Barriers
When designing an insulation
system, one element to consider is
the permeability of the insulation
to be used. Permeability relates to
how willing a material is to allow
liquids or gases to pass through it.
When using permeable insulation,
an external vapor barrier is needed
to prevent water from entering the
system. If the barrier is damaged,
it can allow moisture and water
vapor to penetrate the insulation.
Activities that can impede vapor
barrier function include incorrect
installation practices, physical damage,
rough maintenance practices
and daily traffic. If the barrier is
damaged, permeable insulation also
can absorb and retain the moisture
vapor, which would compromise
the thermal performance, or thermal
conductivity, of the insulation.
Different studies have found that
increasing the moisture level within
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insulation by 1% can reduce thermal
effectiveness by 7.5% or up to 23%.1,10
This shift can lower the external
surface temperature of the insulation,
attracting more moisture,
and allowing condensation to start
collecting.
However, not all insulation is
permeable (Figure 2). Closed-cell
materials tend to have much lower
permeability than materials that
are open-cell or granular in nature.
A low or zero permeability insulation
will better protect against water
vapor traveling into the insulation,
which will help maintain its thermal
performance.
Surface Temperature
Implication, Calculation
When designing an insulation
system for chilled water lines, one
important step involves calculating
the surface temperature of the outermost
layer of the insulation system.
Insulation systems of sufficient
thickness and surface emittance will
have an outer surface temperature
that is above the surrounding air's
dew point, mitigating the risk of surface
condensation taking place.
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Note that the dew-point and calculated
surface temperature should
account for worst-case scenarios,
not just average conditions. This will
ensure that the system remains protected
against condensation control
even during warmer humid days.
The temperature of air (Tair) and
the relative humidity (RH) can be
used to establish the dew point.
To prevent surface condensation
from taking place, the outer surface
temperature of the insulation system
should be kept above the dew
point. For example, if the Tair is 80°F
(26.7°C) and the RH is 75%, then the
dew point would be 71.3°F (21.8°C),
which could then be considered the
lower temperature limit for the surface
of the insulation-it should be
kept above this temperature. If the
outer surface of an insulation system
in this environment is found to be
74°F (23.3°C), surface condensation
will not take place along the system
(Figure 3).
However, because the pipe is well
below the ambient temperature, a
point will exist within the insulation
where the temperature drops
below the dew point. As long as an
impermeable insulation is used, or
as long as the vapor barrier remains
effective, moisture will not be able to
reach this location and condense.
Jacket Emittance
When determining the surface
temperature of an insulation system,
one element to consider is the
surface emittance of the jacketing
material used. Emittance describes
how effectively a material emits
and absorbs heat by radiation and
is most relevant to the outermost
surface of an insulation system.
The emittance of a given material is
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