Fluke 971 Application Note 2412973

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Evaluating relative
humidity: Key factors
and measurements
Application Note

Understanding the relationship between dry bulb
temperature, wet bulb temperature, relative humidity,
and dew point temperature is essential in all facets of
air conditioning. These psychrometric processes play
an especially important role in building and materials
integrity, occupant health and comfort, and overall
indoor air quality.

The good news, strangely
enough, is that poor humidity and
temperature levels are likely to
cause occupant discomfort.
Occupant complaints open a window of opportunity for the HVAC
contractor to proactively discover
related undesirable psychrometric
effects on materials integrity and
indoor air quality, including
microbial propagation.
To evaluate relative humidity,
wet bulb temperature, and dew
point, HVAC technicians traditionally used a sling psychrometer
and psychrometric chart.
Now-days they use “humidity”
meters that are accurate, more
convenient, and usable in
confined locations unsuitable for
sling psychrometers.

Standards adoption
Many states have adopted ANSI/
ASHRAE Standards 55-2004 on
humidity and 62-2004 on IAQ
into their building codes. Since
both standards have been newly
updated, the following descriptions may help inspectors and
contractors update practices to
meet new requirements.

Relative comfort
ANSI/ASHRAE standard 55-2004,
Thermal Environmental
Conditions for Human Occupancy,
sets an upper limit to absolute
humidity levels (0.012 humidity
ratio, or 0.012 x 7000 = 84 grains
moisture/lb dry air, also equivalent to a dew point (DP) of 62 °F),
above which most occupants
become uncomfortable.
Since all occupants won’t be
satisfied by the same thermal
conditions, especially all at the
same time, the standard attempts
to identify a norm based on a
PMV (Predictive Mean Vote) of
80 % satisfaction. From that, a
PPD (Predicted Percentage
Dissatisfied) of 10 % is calculated
for general thermal comfort
dissatisfaction and 10 % PPD
from local (“my ankles are cold”)
comfort dissatisfaction.
The standard lists six primary
factors that affect thermal comfort: metabolic rate, clothing
insulation, air temperature, radiant temperature, air speed,
humidity.
Understanding the combined
affects of these factors can help
technicians configure building
systems appropriately.

The Fluke 971 Temperature Humidity Meter measures temperature from -20 °C to 60 °C (-4 °F to 140 °F), dewpoint, wetbulb,
and relative humidity from 5 % to 95 %.

Humidity levels
ANSI/ASHRAE Standard 62-2001,
Ventilation for Acceptable Indoor
Air Quality, specifies that
“Relative humidity in habitable
spaces preferably should be
maintained between 30 % and
60 % relative humidity to minimize growth of allergenic or
pathogenic organisms.”
The updated ANSI/ASHRAE
Standard 62.1-2004, Ventilation
for Acceptable Indoor Air Quality,
is more specific. Now, relative
humidity upper limits are based
on peak values. “Occupied space
relative humidity shall be

From the Fluke Digital Library @ www.fluke.com/library

Psychrometrics has a language all its own. To better understand
how the various parameters interact to support thermal comfort, here
are some of the more common terms described in this document:
Wet bulb temperature: Represents the
cooling effect of evaporating water, the
temperature air will cool to when water
evaporates into unsaturated air.
Dewpoint temperature: The temperature under which water will condense
out of the air.
Dry bulb temperature: Air temperature
determined by an ordinary thermometer.
Relative humidity: Ratio of water
vapor pressure (amount currently in the
air) to the saturation vapor pressure (the
amount the air can hold) at a given air
temperature.
Met(abolic) rate: The rate by which
the body transforms chemical energy
into heat and work through activity. In
ASHRAE 55, this rate is measured in
“met units” (18.4 Btu/h*ft2).
Sensible cooling: Factors such as people, appliances, solar radiation, and
infiltration create heat gain, each
adding a sensible load to the environment within a house, office, etc. This

designed to be limited to 65 % or
less at either of the two following
design conditions:
1. at the peak outdoor dew-point
design conditions and at the
peak indoor design latent load
or
2. at the lowest space sensible
heat ratio expected to occur
and the concurrent (simultaneous) outdoor condition.”
Good HVAC equipment selection
practices generally recommend:
• 68 °F to 70 °F and 30 % RH
(relative humidity) winter
design,
and
• 74 °F to 76 °F and 50 % to
60 % RH summer design
at
• outdoor conditions of 97.5 %
winter and 2.5 % summer dry
bulb (DB).
This means that on average,
2.5 % of the extreme seasonal
temperatures will be beyond
equipment capacity. The equipment will be effectively
undersized during these times.
This is critically important in
equipment selection, since only
30 % of the operating hours of
comfort cooling equipment occur
within 5 % of outdoor design dry
2 Fluke Corporation

sensible load raises the dry-bulb temperature. The process by which the
sensible, or dry bulb, temperature is
reduced without changing the moisture
content of the air is referred to as a
sensible cooling process.
Latent cooling: An amount of moisture
is added to the inside air by plants,
people, cooking, and other sources. A
latent cooling process involves the condensation of moisture out of the air,
reducing the wet bulb, dewpoint, and
humidity levels, but leaving the dry
bulb temperature untouched.
S/T ratio: Sensible to total heat ratio,
or sensible heat factor. Of the total
capacity of a cooling system, there is a
sensible capacity and a latent capacity.
The sensible capacity cools the air by
absorbing heat to lower the dry bulb
temperature. The latent capacity
absorbs the latent heat of vaporization
to remove moisture from the air without
changing the actual dry bulb temperature. The S/T ratio, when used with a
psychrometric chart, will provide the

bulb temperature. Summer latent
load control is more difficult to
control at part load conditions,
although most commercial equipment is staged or has some form
of capacity control.
If comfort cooling equipment is
oversized, moisture related complaints and problems will
increase. Residential heat pumps
should be selected according to
the cooling requirements, not the
heating requirements, especially
in geographic areas where “dirty
socks syndrome” is prevalent and
air handling equipment is located
in a crawlspace.

temperature at which a cooling coil
must operate in order to support both
sensible and latent heat removal.
“Dirty socks syndrome”: A common
condition during the cooling season that
describes
an odor generated by burning airborne
contaminants on an indoor coil, typically during the heat pump defrost
cycle.
clo: A unit of measurement to express
the amount of thermal insulation provided by clothing and other garments.
• Example: An ensemble including
briefs, t-shirt, calf-length socks,
shoes, straight trousers,
long-sleeve dress shirt,
double-breasted (thick) jacket totals
1.14 clo*
• Example: An ensemble including
briefs, short-sleeve knit sport shirt,
walking shorts,
sandals totals .31 clo*
*clo values per ASHRAE 55-2004

Relative humidity above 60 %
can support fungal growth on
hygroscopic (sorbent) surfaces
and hygroscopic surfaces at 80 %
RH are likely to promote fungal
growth. Nearly all surfaces are, or
can become, sorbent and include
painted surfaces, gypsum dry
wall, carpets, wall coverings, and
masonry products. Even glass
with a dirt film and dust on it can
support fungal growth.
Masonry products such as
brick, cinder block and concrete
are excellent sorbents and can
adsorb vast quantities of moisture
and become an inviting breeding
environment for molds. The vapor
Fungus
pressure within the pores of
manufactured masonry can be
With enough knowledge and
measurement, HVAC systems can less than the vapor pressure of
be set at the appropriate summer the ambient air which moves
and winter psychrometric condi- moisture from the air into the
masonry pores. As the pores
tions to discourage fungal
become wetted, capillary action
growth. Conditions for fungal
growth include spores settling on takes over and fills the pores,
thus providing an ideal breeding
a surface, a micro-environment
ground for fungal proliferation.
ensuring oxygen, optimal temThis explains why some surfaces
peratures, nutrients, and
moisture. Four of these conditions above dew point can become
wetted.
are found in nearly every environment. The most controllable
variant is moisture.

Evaluating relative humidity: Key factors and measurements

Condensation

equipment). Water pipes can be
insulated. Air handling equipConditions that allow condensament and ductwork must be
tion to form on surfaces are more sealed air tight and insulated
obvious, so action can be taken
with no breaks in the vapor barimmediately. When a surface
rier especially when located
temperature is at or below the
outside of the conditioned envedew point temperature, condenlope. Ductwork in all walls must
sation will form. Likely places for be sealed to reduce unseen moisthis to occur are on basement
ture migration due to air pressure
surfaces, crawlspace surfaces,
differentials.
cold water pipes, on air handling
In cooling systems, relative
equipment and duct work, and
humidity in supply ducts can be
unseen within envelope walls.
95 % or higher, and evaporators
Basements typically require
and condensate pans will be
supplemental dehumidification
wet. So, since moisture control is
equipment, since comfort cooling not feasible, control of airborne
equipment can’t control humidity spores and food (dust and airin basements with minimal heat
borne particles) with good, tight
gain. Crawlspaces are particularly fitting filtration systems in place
difficult and expensive to deal
is essential to control fungus
with, but sealing them with
growth. If evaporator components
vapor barriers up to outside
are resistant to UV radiation, a
ground level, as well as insulatUVC “germicidal” light that can
ing, and incorporating them into
see the entire evaporator surface
the conditioned space and
can kill mold and microbes. UVC
adding additional means of dehu- lights should be selected that do
midification can control many
not emit ozone, which is an irricrawlspace moisture problems,
tant. Oversized equipment will
provided standing water or
experience reduced operating
excessive ground moisture is not
times resulting in less condensate
present (this assumes free crawl- production which may actually
space ventilation air is not
increase the microbial colonizarequired for fossil fuel burning
tion on the fin surfaces.

Temp-humidity meters
From dry bulb temperature and
relative humidity measurements,
temperature-humidity meters such
as the Fluke 971 can calculate wet
bulb temperature and dew point
temperature, psychrometric points
that are essential for HVAC evaluations and diagnostics.
• Wet bulb is very closely related
to enthalpy, or the total heat in
the air (dry bulb and wet bulb).
In a psychrometric chart, the
wet bulb lines are nearly parallel the enthalpy scale values.
Return wet bulb temperature is
mandatory for accurately
charging a cooling system that
incorporates a fixed restrictor
metering device.
• Supply and return wet bulb
temperatures across an evaporator can be used with a
psychrometric chart or enthalpy
table to calculate total cooling
capacity, sensible and latent
capacity, and S/T ratio.
• Total heat may be found by
multiplying cfm x 4.5 x
enthalpy difference across
evaporator (Qt = cfm x 4.5 x
∆h).

Legend
Severe-Cold: A severe-cold climate is defined as a region with approximately
8,000 heating degree days or greater.
Cold: A cold climate is defined as a region with approximately 4,500 heating
degree days or greater and less than approximately 8,000 heating degree days.
Mixed-Humid: A mixed-humid climate is defined as a region that receives more
than 20 inches of annual precipitation, has approximately 4,500 heating degree
days or greater or less and where the monthly average outdoor temperature
drops below 45 °F during the winter months.
Hot-Humid: A hot-humid climate is defined as a region that receives greater
than 20 inches of annual precipitation and where the monthly average outdoor
temperature remains above 45 °F throughout the year*.
Hot-dry/Mixed-Dry: A hot-dry climate is defined as a region that receives less
than 20 inches of annual precipitation and where the monthly average outdoor
temperature remains above 45 °F throughout the year;
A mixed-dry climate is defined as a region that receives less than 20 inches of
annual precipitation, has approximately 4,500 heating degree days or less and
where the monthly average outdoor temperature drops below 45 °F during the
winter months.
*This definition characterized a region that is almost identical to the ASHRAE definition of hot-humid
where one or both of the following occur:
• a 67 °F or higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive
months of the year; or
• a 73 °F or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive
months of the year.

3 Fluke Corporation

Evaluating relative humidity: Key factors and measurements

•

•

•

Sensible vs. latent cooling
and S/T ratio can be found
by plotting conditions on a
psychrometric chart or from a
psychrometric calculator.
Dew point is critical in both
summer and winter evaluations. Duct surface temperature
must be maintained above dew
point to prevent condensation
whether inside or outside of
the conditioned space.
Winter indoor relative humidity must be kept low enough
to ensure inside wall and
window surface temperatures
do not approach dew point.
If condensation appears on
window or wall surfaces,
condensation hidden within
envelope walls will be likely.

use a Timed-On-Control device,
to provide reduced cfm for the
first 5-10 minutes of cooling
demand, and then switch to the
design cfm to finish the cooling
cycle. A portable dehumidifier
can be located in areas of high
humidity, such as a basement,
reducing humidity, increasing
heat gain, and forcing longer
cooling cycles. Make sure rooms
with intermittent high moisture
gain, such as bathrooms,
kitchens, and laundry areas are
ventilated to the outdoors (not
the attic or crawlspace).

equipment in a crawlspace must
have excellent particulate filtration in place with no return side
air leaks to reduce microbes and
their food sources in the evaporator and supply duct. Humidity
levels in basements must be regulated to less than 60 % RH to
discourage microbial growth.
Painting the surfaces of hydroscopic masonry (cinder blocks,
brick, mortar) will reduce moisture retention, discouraging
microbes.

Addressing dew point
and/or fungus related
complaints

If the complex subjects briefly
treated here pique interest for
further study, additional resources
are available through ASHRAE, at
www.ashrae.org. The ASHRAE
handbooks and monthly journal
are an exceptional vehicle for
discovery. Psychrometric charts
are now available inside software
programs that make easy work of
the calculations. Other HVAC
organizations include:
• ACCA (Air Conditioning
Contractors of America)
(www.acca.org),
• PHCC (Plumbing Heating
Cooling Contractors)
(www.phccweb.org),
• SMACNA (Sheet Metal and Air
Conditioning Contractors’
National Association)
(www.smacna.org), and
• RSES (Refrigeration Service
Engineers Society)
(www.rses.org).

Ducts in unconditioned spaces
carrying cool, humid air must be
sealed airtight using an NFPA
Addressing comfort
approved duct mastic. Any air
related complaints
leaks in a duct will render the
With equipment that does not
insulation useless at that point
have capacity control, or is
and condensation is likely to
staged, most humidity-related
occur. Duct wrap insulation must
comfort complaints occur at part
not be compressed by hangars.
load conditions when run times
Hangars must be placed underbased on thermostat dry bulb
neath duct wrap insulation. Duct
temperatures are shorter. Less
wrap insulation barriers must be
operating time means less moisunbroken and sealed at the
ture removal. Oversized
seams.
equipment will only exacerbate
In unconditioned attics,
this as well as increasing occurincreasing attic temperature may
rences of detrimental coincidental increase heat gain on ceilings
conditions. Changing from a fixed below, but will reduce the occurrestrictor metering device to a
rence of condensation on ducts.
thermal expansion valve will
Attics in homes of newer conensure maximum evaporator
struction techniques may result in
capacity at part load conditions
lower attic temperatures, but this
and utilize more coil surface for
increases the chance of condenmoisture removal.
sation on duct or air handler
Most cooling equipment
surfaces. Sealing attic vents and
can tolerate reduced air volumes
adding humidistat controlled
of about 20 %. If evaporator
flood lights to increase attic temair volumes are reduced from
perature can compensate for this.
400 cfm/ton down to around
Crawlspaces present unique
325 cfm/ton, the evaporator
opportunities. Typical crawlspace
temperature will fall further
vent sizing is inadequate for conbelow dew point and remove
trolling moisture by ventilation.
more moisture from the air. This
100 % ground cover vapor barrier
change will also reduce duct
up the inside wall to a height
surface temperature and register
equal to the outside ground level,
temperature in the direction of
sealing the vents, insulating the
dew point temperature and regis- perimeter walls, and treating it as
ter throw, affecting air patterns in a conditioned space is a preferred
occupied spaces.
method of moisture control, often
A dehumidistat can lower air
requiring additional supplemental
volumes at increased humidity
dehumidification. Air handling
levels. Another alternative is to

Resources

Fluke. Keeping your world
up and running.
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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From other countries +1 (425) 446-5500 or
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Web access: http://www.fluke.com
©2005 Fluke Corporation. All rights reserved.
Printed in U.S.A. 8/2005 2522930 A-EN-N Rev B

4 Fluke Corporation

Evaluating relative humidity: Key factors and measurements



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