ASHRAE Journal - June 2020 - 51

COLUMN HVAC APPLICATIONS

probability of negative health
effects:
* "Moldy, musty, earthy" odors;
* Visible mold;
* Current water damage; and
* History of visible mold or water damage.
The human nose turns out to be
a useful instrument for investigating health-relevant dampness.
Musty odors are the indicator most
strongly associated with buildings
or HVAC systems that have become
damp enough to increase the probability of negative health effects
for some occupants.
The report also suggests four
metrics with quantitative thresholds that indicate abnormal conditions that, if persistent, serve
as early warnings of excessive
dampness. Readers are welcome
to download the report for a full
explanation of the early warning
metrics, the epidemiological studies that support the conclusions
and 16 pages of speciﬁc suggestions for building professionals.
One of the report's four early
warning metrics deserves further discussion here, to alert
ventilation system designers to a
signiﬁcant change to the widely
applied ASHRAE/ANSI Standard
62.1, Ventilation for Acceptable Indoor
Air Quality (Figure 2). As of 2019,
the standard calls for limiting the
absolute humidity to less than a
dew-point temperature of 60°F
(15°C), during both occupied and
unoccupied hours, whenever the
dew point outdoors is above that
level.
Sound logic exists for placing
ASHRAE's upper limit for absolute humidity in the ventilation
design standard. Ventilation air

represents the largest annual
humidity load for nearly all commercial and institutional buildings, in nearly all climates.4
Removing the humidity load
imposed by ventilation air is
important in any building that
is mechanically cooled. Cold
HVAC equipment and distribution systems often condense and
drip moisture, and cooled building surfaces are more likely than
warm surfaces to absorb and
retain moisture. So because ventilation design imposes the largest
humidity load, placing the dewpoint limit in Standard 62.1-2019
is helpful. It alerts the designer
to the issue of humidity control at
an early and therefore inﬂuential
stage of design. Later, it may be
more difﬁcult to adjust the system
conﬁgurations and equipment
selections to ensure humidity
control at levels low enough to
limit indoor dampness and health
consequences.
The revised limit is a prudent
upper boundary for absolute
humidity to avoid building dampness, rather than a desirable target for comfort. Cooling systems
are typically expected to keep
indoor air at 75°F (24°C) db, 50%
RH, which is a 55°F (12.8°C) dewpoint temperature.4,5 So designing to limit absolute humidity to
less than a 60°F (15°C) dew-point
temperature does not change typical design practice, except in one
important respect: system operation during the unoccupied mode.
This new guidance to keep
buildings dry even when not fully
occupied reflects the fact that
indoor dampness that occurs long
enough for mold to grow is often

FIGURE 1 The new ASHRAE publication is available in
PDF (free) and print formats. Use the QR code below
to download it.

Figure 2 The revised ASHRAE/ANSI Std 62.12019 describes a prudent upper limit for absolute
humidity that is consistent with the findings of the
MTG: Damp Buildings. Use the QR code for more
information.

caused by high humidity when
buildings remain partly active,
but largely unoccupied. Examples
that often appear in the news
include K-12 schools and university dormitories after vacations,

J U N E 2020

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ASHRAE Journal - June 2020

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