PRACTICAL
GUIDE
The following article was published in ASHRAE Journal, April 1999. © Copyright 1999 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper
form without permission of ASHRAE.
UNDERSTANDING WHAT HUMIDITY
DOES AND WHY
By Kenneth M. Elovitz, P.E.
Member ASHRAE
P
eople sometimes attribute ef fects to
“humidity” without understanding the
underlying physics. For example, we have all
experienced “hot, humid” summer weather. Yet the
outdoor air relative humidity on a “hot, humid” summer
day (95°F db/78°F wb [35°C db/26°C wb]) is less than
50%. By contrast, the outdoor air relative humidity on a
“cold, dry” winter day is typically around 80%. This article
examines the difference between relative humidity,
specific humidity, and vapor pressure. It goes on to
explore how those measures influence phenomena
loosely attributed to “humidity.”
Measures of Humidity
Different measures of humidity quantify different physical
properties of the mixture of water vapor (moisture) and air. Understanding how moist air behaves requires understanding those
measures of humidity.
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor air can hold at that
temperature. At 100% relative humidity, the dry bulb, wet bulb,
and dew point temperatures are equal. At 100% relative humidity, the air is saturated, which means it cannot hold any more
moisture. Raising the temperature without changing the amount
of moisture in the air reduces the relative humidity. The relative
humidity goes down because warmer air can hold more moisture
than colder air. For example, a comfort cooling system might be
designed to maintain 75°F (24°C)/55% RH at design load using
56°F (13°C) coil leaving air temperature. The system might
have enough sensible capacity to cool the room to 70°F (21°C)
at less than design load, or the system might be oversized. The
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coil leaving air temperature does not change, so the available
dehumidification capacity does not change. The resulting room
relative humidity at 70°F (21°C) will be 65%, possibly generating complaints that the relative humidity is too high.
While room conditions should be analyzed in accordance with
ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy to evaluate comfort, if relative
humidity itself is the problem, one practical solution might be to
operate the system at the design temperature setpoint of 75°F
(24°C). Achieving moderately low humidity at low room temperatures may require using a reheat system. Achieving low relative humidity at low temperatures usually requires specialized
systems like desiccant dehumidification.
Specific humidity is the amount of moisture in the air per
unit mass of air. It is usually expressed as grains of water per
pound of dry air (gr/lb) or pounds of water per pound of
dry air (lbw/lbda, kgw/kgda). Specific humidity is proportional
to the enthalpy or total energy content of the moist air mixture.
Specific humidity changes when moisture is added or removed.
Changing temperature does not change specific humidity unless
the air is cooled below the dew point.
Dew point is the temperature where moisture begins to condense out of the air. When air is cooled to its dew point, it reaches
100% relative humidity or saturation. Cooling the air any further
causes water vapor in the air to change to the liquid phase. Liquid
water molecules accumulate, droplets form, and moisture condenses
out of the air. At the new conditions, the air contains less moisture,
has lower specific humidity, and has a lower dew point temperature,
but it is still at 100% relative humidity. Raising the temperature of
air at its dew point reduces its relative humidity but does not change
its water vapor content (specific humidity) so does not change its
dew point.
Vapor pressure is the pressure exerted by free molecules at
the surface of a solid or liquid. Consider water in a closed vessel
at 75°F (24°C). Water will evaporate until the partial pressure
of the water in the vessel reaches 0.44 in. Hg (1.49 kPa), which
April 1999
Practical Guide
Figure 1a: Temperature and moisture gradient in a wall
(condensation).
Figure 1b: Temperature and moisture gradient in a wall (no
condensation).
is the vapor pressure of water at 75°F (24°C).
For a given substance, vapor pressure is a function of temperature. As temperature increases, vapor pressure increases.
When the vapor pressure reaches atmospheric pressure (29.92
in. Hg [100 kPa]), the liquid boils. For water at sea level, this
condition occurs at 212°F (100°C). At 5,000 ft (1524 m) above
sea level, atmospheric pressure is only 24.89 in. Hg (84 kPa).
That is why water boils at 202°F (94°C) in Denver.
Vapor pressure is a measure of the affinity of a substance
for itself. If a substance has low affinity for itself, it evaporates readily even at low temperature. The substance will have
a high vapor pressure. For most HVAC processes, the vapor
pressure of interest is for water in contact with itself. However, water in contact with other substances (e.g., wood, paper, salt) also has a vapor pressure. The vapor pressure of
water in contact with those other substances may be different
from the vapor pressure of water in contact with itself.
cific humidity, or vapor pressure. Engineers must identify the
operative parameter before they can design HVAC/R systems
that avoid or mitigate the effects of moisture in the air.
Effects of Humidity
Understanding how moisture affects materials and processes
requires understanding whether those effects are a function of
relative humidity, specific humidity, or vapor pressure. Much of
the literature on effects of humidity covers a narrow temperature
range. Those studies likely used relative humidity because it is
easy to measure. At constant temperature, relative humidity varies directly with moisture content — the lower the moisture content, the lower the relative humidity. Since the studies were conducted over a narrow temperature range, the data lend little insight into whether the operative factor is relative humidity, spe76
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Condensation
Condensation is strictly a function of relative humidity. When
air cools to a temperature below its dew point, moisture condenses out of the air. It is not necessary to cool the entire air
mass to get condensation. Condensation occurs on the coldest
surface in a room. A cold window might cool nearby air below
its dew point and cause condensation while the rest of the room
remains at normal temperature.
Condensation causes a variety of problems. Condensation is a
housekeeping problem if moisture puddles on the floor or if droplets
stain the materials they contact. Condensation can damage wood,
paper, and fabric, and it accelerates rusting of steel. It can also hurt
products like frozen foods in a supermarket. No one wants to buy
the package of ice cream coated with frost. Moreover, for water
vapor in the air to form frost on the package of ice cream, it must
give up its heat of vaporization (approximately 1000 Btu/lb [2326
kJ/kg]) and its heat of fusion (approximately 144 Btu/lb [335 kJ/
kg]). It gives up some of that heat to the air and some of it to the ice
cream. The ice cream warms up a bit and can even begin to soften
or melt if the freezer is not cold enough.
HVAC/R designs generally try to avoid condensation in the
conditioned space. For cooling applications, they accomplish that
goal with dehumidifying coils that remove moisture from the supply air before it enters the conditioned space. Most comfort cooling systems are designed to control temperature, so they control
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INDOOR TEMPERATURE
INDOOR MOISTURE
70°F (21.1°C)
22.4 GR/LB
OUTDOOR TEMPERATURE
OUTDOOR MOISTURE
R-VALUE
CUMULATIVE
R-VALUE
0.68
0.68
0.091
62.7 (17.1)
nil
nil
0.68
0.091
62.7 (17.1)
5.0
½ inch Wallboard
0.45
1.13
0.151
57.9 (14.4)
3½ inch Air space
1.01
2.14
0.286
¾ inch Polystyrene
3.75
5.89
0.786
½ inch Plywood
0.62
6.51
0.869
Clapboards
0.81
7.32
0.977
Outside air film
0.17
7.49
1.000
ITEM
Inside air film
Paint
Notes: (1) Perms are grains/hr per sq ft per in. Hg Pressure difference
(2) Reps are 1/perms
FRACTION OF SFCE. TEMP.
TEMP. DIFF.
°F (°C)
–10°F (–23.3°C)
1.3 GR/LB
CUMULATIVE
REPS
MOISTURE
DIFFERENCE
SURFACE
GR/LB
DEW POINT
°F (°C)
0.00
0.000
22.4
28 (–2)
0.20
0.20
0.074
20.8
27 (–3)
37.5
0.03
0.23
0.084
20.6
26 (–3)
47.1 (8.4)
34.3
0.03
0.26
0.094
20.4
26 (–3)
7.1 (–13.8)
1.6
0.63
0.88
0.325
15.5
20 (–6)
0.5 (–17.5)
0.7
1.43
2.31
0.852
4.4
–4 (–20)
–8.2 (–22.3)
2.5
0.40
2.71
1.000
1.3
–27 (–33)
–10.0 (–23.3)
nil
2.71
1.000
1.3
–27 (–33)
PERMS
REPS
(3) SFCE. Temp. is on outside face of surface.
(4) SFCE. Temp. = Indoor Temp. – (Frac. of Temp. Diff. x Total Demp Diff.)
Table 1: Stud wall dew point analysis.
relative humidity and the risk of condensation only indirectly.
However, matching both the sensible (temperature) and latent
(dehumidification) capacities to the cooling loads is part of a
successful design.
Excessive winter humidification risks condensation on cold
window and wall surfaces. Excess humidification is humidity
above what the building envelope was designed to accommodate. Besides condensation, excess humidification can cause problems like peeling paint, either inside or outside.
Moisture in the Building Structure
Condensation problems are not limited to the occupied space.
Condensation inside walls can be a serious problem. Any conditioned building has a temperature gradient between indoors
and outdoors. The temperature difference across each element
of the wall structure is proportional to the insulating value of
that element. Buildings also have a moisture gradient between
indoors and outdoors. The moisture difference across each element of the structure is proportional to the vapor diffusion resistance of the element.
Figure 1 illustrates the temperature and vapor pressure gradients in a wood stud wall and shows how insulation placement
affects performance.1 While this example is for a modern house,
the analysis applies to any structure, including historic buildings. The house had urea formaldehyde foam insulation that
had shrunk away from the studs, leaving large areas effectively
uninsulated. Moisture from the humidified house condensed on
the back side of the sheathing, ruining it. The owner wanted to
install insulated sheathing for energy conservation and to avoid
another condensation problem.
Table 1 is a dew point calculation for Figure 1a. Like the
temperature gradient, the moisture gradient is proportional to
the resistance of each element in the wall. Where the temperature gradient is expressed in degrees, the moisture gradient is
expressed in vapor pressure (in. Hg or kPa) or specific humidity
(grains/lb, lbw/lbda, or kgw/kgda). Since vapor permeance data
are commonly tabulated in grains in the I-P system of units, it is
April 1999
easier to work in grains/lb than lb/lb.
Although the units are different, the principle is similar to the
more familiar temperature gradient calculation:
Quantity
R-Value
Btu/h
Permeance (perms) gr/h
Area
ft2 (m2)
ft2 (m2)
Driving Force
°F (°C)
in. Hg (kPa)
In Figure 1a, with the insulation inside the exterior sheathing, the surface of the sheathing falls below the dewpoint and
damaging condensation can occur. The following calculation
shows the basis for that conclusion:
R-value of all components up to plywood: 5.89
Total R-value of assembly: 7.49
Temperature on inside surface of plywood:
Inside R-value
Temp. Ratio
Temperature
Difference
70°F – 5.89 × [70°F – (–10°F)] = 7.1°F (–13.8°C)
7.49
Vapor diffusion resistance of components up to plywood: 0.88
Total vapor diffusion resistance of assembly: 2.71
Dew point calculation for surface of plywood
Inside
Moisture
Rep
Ratio
Moisture
Difference
22.4 gr/lb – 0.88 × (22.4 – 1.3) gr/lb
2.71
= 15.5 gr/lb dew point = 20°F (–6°C)
Since the temperature on the plywood is lower than the dew
point, moisture can condense.
Figure 1b shows that installing the insulation outside the sheathing keeps the sheathing above the local dewpoint, avoiding conASHRAE
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Practical Guide
densation. Note that these conditions result in part from the fact
that plywood sheathing is a moderately effective vapor retarder.2
In hot, humid climates, the indoor temperature and dewpoint
are below the outdoor temperature and dewpoint much of the
year. In those situations, the vapor retarder is usually installed
outside the insulation.3
If a wall is not designed for the anticipated indoor/outdoor
moisture gradient, or if the indoor humidity is higher than the
building design contemplated, moisture can condense inside the
wall. That moisture can eventually cause structural damage. New
construction can include vapor retarders to accommodate indoor
humidification. Depending on their construction, it might not
be feasible to humidify existing buildings without risk of condensation and damage to the building structure.
Mold and Fungus Growth
Mold and fungus spores are difficult to eliminate from a building. The spores themselves are not much of a problem until they
grow. To grow, mold spores need moisture and a food source.4
Neither moisture nor food necessarily comes from the air. Rather,
they both more often come from the substrate where the spores
land and germinate.5
Mold can grow inside air-handling units. In cooling systems,
cooling coil condensate may be available as a moisture source.
Although the relative humidity can be 95% or higher for months
at a time, mold does not always grow in air-handling units. Mold
will not grow even in high humidity environments unless it has
food. When mold grows in air-handling units, the food source is
accumulated dust and dirt. Keeping systems clean is the key to
avoiding mold growth in air-handling units and ducts.
Maintaining relative humidity below the oft-cited 60% level
does not guarantee against mold growth. Mold can not only
obtain food from a substrate, it can also obtain moisture from a
substrate. Some substrates allow mold to germinate with fairly
low moisture levels. Dirty surfaces and accumulated salts tend to
deliquesce moisture out of the air. That moisture in the material
promotes mold growth. Where moisture is unavoidable, as in a
cooling system, the key to avoiding mold growth is to eliminate
food sources.
Materials that hold moisture can be sites for mold growth
even in a room where the relative humidity is low. Like desiccants, some materials absorb moisture from the air even at low
humidity. Other materials are slow to release moisture once they
get wet. The literature suggests materials absorb moisture faster
than they release it.6 If these materials are organic, they are ideal
substrates for mold growth.
Maintaining relative humidity below 60% at temperatures in
the normal human comfort range may reduce mold growth.
However, low relative humidity is no guarantee. Selecting materials and treating surfaces so they do not absorb or hold moisture
appears to be a more effective strategy against mold growth.
Desiccants
Desiccants are materials that absorb moisture. Commercial
desiccants generally absorb several times their own weight in
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water. While desiccants are usually noted for their ability to
absorb moisture, they also desorb moisture if the water vapor
pressure of the ambient air is less than the vapor pressure of
water in the desiccant. In that respect, desiccants can be a
form of seasonal storage for latent cooling.
Desiccants can be liquid or solid. Liquid desiccants ABsorb
water vapor. Solid desiccants ADsorb water vapor. The difference is that the ABsorbed water goes into solution with the liquid desiccant. ADsorbed water attaches to the surface of solid
desiccants. Solid desiccants have irregular surfaces with numerous pores that provide sites for water vapor molecules to attach.
Liquid desiccants absorb water because they have a stronger
attraction for water molecules than does water itself. Expressed
scientifically, the vapor pressure of water in the desiccant is less
than the vapor pressure of water in the air. The vapor pressure
difference drives water molecules into the desiccant solution. The
vapor pressure of water in the desiccant solution increases as the
solution absorbs water and becomes more dilute. When the vapor pressure of water in the desiccant equals the vapor pressure
of the ambient air, the desiccant stops absorbing water.7
Solid desiccants have numerous small passages or capillaries
that attract water. Water is attracted to the surface of the desiccant, collects into droplets, and condenses in the capillaries. As
with liquid desiccants, water sitting on the surface of the desiccant
has a lower vapor pressure than water in the ambient air.8 Stated
another way, the force attracting water vapor to the desiccant surface is greater than the force attracting water vapor into the air.
Desiccants can achieve much lower specific humidity than
mechanical refrigeration without over cooling the space or
requiring a defrost cycle. As a practical matter, desiccant systems
tend to be economical when the desired dew point is below about
40°F (4°C).
Static Electricity
Static electricity results when charges accumulate on a body.
The problem occurs when those charges jump across an air gap
on their way back to their source. People can pick up charges
from walking across carpets. They carry those charges around
with them until they get close to an object that has a conductive
path back to the carpet. If the charges discharge through a computer or other electronic device, the discharge can scramble data
or damage components.
Indoor static electricity discharges are often associated with
dry, winter weather. However, some of the biggest static electricity discharges in human experience occur during humid summer
weather. They are thunderstorms. Even though people associate
static electricity with low indoor humidity, broader observations
show that static electricity discharges are not a function of relative humidity.
The dielectric constant of a substance is a measure of its ability to hold a charge. The dielectric constant of air does not change
very much with humidity. The reduction in static electricity discharges attributed to increasing humidity has little to do with
moisture in the air. Rather, it is the influence of moisture on the
electrical conductivity of materials.9 Static electric charges canApril 1999
Museums &
Renovation
not accumulate on conductive materials. The electrical conductivity of most common materials increases in proportion to their
moisture content. Materials such as plastics, rubber, and machine drive belts that do not readily absorb moisture can accumulate static charges at 100% relative humidity.10
Previous editions of the ASHRAE Handbook implicitly recognize that increasing relative humidity does not necessarily eliminate static electricity. The 1983 and 1988 Handbooks state that
“under some conditions, and with certain materials, maximum
electrostatic charging occurs at relative humidities of 25% to 35%
or higher.”11 That statement disappeared from the same chapters in the 1992 and 1996 editions of the Handbook.
Adding moisture to the air affects static electricity only indirectly. If the materials in the room absorb moisture from the air
and increase their conductivity, the risk of static electricity discharge decreases. However, simply adding moisture is not reliable. NFPA 99-1996, Health Care Facilities, calls for hospital
operating rooms that utilize flammable anesthetics to be humidified to 50% relative humidity. Even with 50% relative humidity,
the same standard calls for additional precautions against electrostatic discharge.12 The need for additional precautions demonstrates that room air relative humidity does not necessarily
have a cause and effect relationship with static electricity discharges. Controlling static electricity discharges seems to depend
on surface conductivity, static dissipating clothing, conductive
flooring, and grounding as opposed to humidifying the air.13
Figure 2: Equilibrium moisture content of wood.
Rust
Atmospheric corrosion (rust) is uncontrolled oxidation of
a metal. In the case of stainless steels, oxidation produces a
thin, protective coating on the metal surface. That oxidation
is part of what makes stainless steel “stainless.” Aluminum
and copper also form protective oxide coatings. On the other
hand, carbon steel forms a loose oxide that readily separates
from the base metal. The loose oxide particles fall off as scale,
exposing new base metal to oxidize. The process continues
until the metal rusts away.
Plain carbon steel reportedly remains uncorroded when exposed to air at a relative humidity less than about 30%.14 The
reference does not indicate whether 30% RH at 85°F (29°C) is
any more aggressive to carbon steel than 30% RH at 25°F
(–4°C). The increase in corrosion with increasing humidity is
attributed to an increase in the electrical conductivity of the environment contacting the metal surface.15 All corrosion is electrolytic in nature, so the increase in conductivity almost certainly
plays a part. However, moisture content does not affect the electrical conductivity of air. Any increase in conductivity associated
with increased moisture can only be due to the interaction of
water vapor with pollutants in the air.
The ASM Metals Handbook describes the influence of surface condition on rust. Rust forms on surfaces with small pores
at lower humidity than on surfaces with large pores. Small pores
draw moisture out of the air by capillary condensation due to
differences in vapor pressure.16
Vapor pressure and capillary condensation make more sense
April 1999
Figure 3: Dimensional change of wood with change in moisture
content.
than relative humidity as a driving force for rusting. If the vapor
pressure of water in the surrounding air is higher than the vapor
pressure of water in small capillaries in the iron/iron oxide surface, the capillaries draw moisture out of the air. Moisture in the
capillaries reacts with contaminants in the air or on the surface,
increasing conductivity and resulting corrosion. Because rust
tends to be irregular, more rust forms more capillaries, fostering
even more rust.
This analysis suggests that preventing corrosion appears to
have more to do with surface finish and dew point than relative
humidity environment. A smooth, polished surface provides few
capillaries and few sites for capillary condensation. At high temperatures, low dew point results in a low relative humidity. However, as temperature goes down, relative humidity can increase
without necessarily promoting rust if the vapor pressure of moisture in the air is below the vapor pressure required for capillary
condensation.
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Practical Guide
Dimensional Changes
Cellulosic materials like paper and
wood readily take on and give up moisture from the air. Wood holds water in cell
cavities and within its cell walls. Green
wood can start out holding more moisture
than the weight of the wood itself (more
than 100% moisture content). When
dried, wood first gives up water from cell
cavities until the moisture content reaches
about 30%. Further drying removes moisture from the cell walls. As the cell walls
lose water, they shrink. The resulting
stresses cause warping and checking.17
After the water in the cell cavities is
gone, the cell walls give up moisture only
until the wood reaches an equilibrium
moisture content. The equilibrium moisture content depends on species, tempera- Figure 4: Influence of moisture content on dimensions of lithographic papers.
ture, and relative humidity. Relative humidity is the strongest of those three influences. Figure 2 shows how the equilibrium
moisture content for wood varies with temperature and relative humidity. Changing
moisture content makes the wood expand
or shrink. Figure 3 shows the magnitude
of these changes.
Like wood, paper also shrinks and grows
with changes in moisture content. A 1933
study by Weber and Snyder for the National
Bureau of Standards showed the effects of
changing moisture content on the physical
properties of printing papers.19 Figure 4
shows one of the findings from that study.
Although the dimensional changes are small,
they are enough to cause misalignment in
multi-color printing processes.
While the Weber and Snyder study
confirms that relative humidity affects dimensions of wood and paper products, it Figure 5: Moisture isotherm of 194-year-old paper.
is important to put these findings into perspective. First, the analysis relates to equilibrium moisture conFigure 5 shows the results of recent testing by the
tent. Depending on size, thickness and how it is stored, the ar- Smithsonian Center for Materials Research and Education
ticle may take hours or days to reach a new equilibrium moisture on a page from an 1804 law book. The paper was allowed to
content when the ambient temperature and humidity change. reach equilibrium moisture content at various relative humidiFor these materials, temperature and humidity at any one mo- ties at constant room temperature. The dimensional changes
ment or even over short periods are much less important than the were then measured. Figure 4 and Figure 5 taken together
long-term average over time.
relate room relative humidity to equilibrium moisture content
Second, unless a process requires extreme precision, fairly broad for paper. In Figure 4, a 2 percentage point change in moischanges in temperature and relative humidity are required before ture content from 0.5% to 2.5% causes a dimensional change
the dimensional changes become significant. Figure 2 shows that a of 0.18% or a strain of 0.0018. Figure 5 shows that a rather
rather broad room temperature and humidity window of 59°F to extreme relative humidity change of 40 percentage points
87°F (15°C to 30°C) and 25% to 50% relative humidity results in (20% to 60%) to achieve that dimensional change. As a rea 4 percentage point change in equilibrium moisture content of wood. sult, unless extreme precision and dimensional stability are
Figure 3 shows that a 4 percentage point change in moisture required, paper and wood can tolerate fairly broad changes
results in less than 1% change in dimension.
in environmental conditions with minimal impact.
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Summary and Conclusions
see CH2M Hill, Preventing Indoor Air Quality Problems
in Hot, Humid Climates: Design and Construction GuideExcept for avoiding condensation, controlling indoor relative
lines, Orlando, Fla, 1996.
humidity does not necessarily protect materials. Relative humidity at best contributes indirectly to control of static electricity, and 4. See Technical Leaflet “Protecting Books and Paper Against
mold growth. The moisture content of the materials exerts a much
Mold,” Northeast Document Conser vation Center,
greater influence and should be the parameter of interest for preAndover, Mass.
serving books, papers and artwork. Other authors address the
5. See Motylewski, Karen, Insect and Fungus Management
effects of moisture content on materials in greater detail.
Conference Notes citing Florian, Mary-Lou, “Mold and
Stored materials can take weeks or months to reach their
its life cycles,” http://palimpsest.stanford.edu/bytopic/pest/
equilibrium moisture content. In a humidified environment,
pestnote.html, Nov. 1994.
books and papers do not release moisture
6. Ibid.
during the winter, so they start the mechanical cooling season loaded with mois7. 1997 ASHRAE, Handbook—
ture. If the environment is not humidified,
Fundamentals, Chapter 21.
Using humidity wisely
stored books and papers give up moisture
8. Ibid.
during the winter and go further into the
9. NFPA 921-1988, Guide for Fire
cooling season before they have absorbed
requires understandand Explosion Investigations, section
enough moisture to support mold growth.
14-12.5.1.
Also because hygroscopic materials take
ing the operative
time to absorb and desorb moisture from
10. Ibid.
the air, fairly wide variations in tempera11. ASHRAE Handbook—Equipparameter: relative
ture and relative humidity over the course
ment, p. 5.1.
of a day or even a week most likely do not
12. NFPA 99-1996, Health Care Fahave a significant impact on the stored mahumidity, specific
cilities Annex 2, “Flammable anesterials.
thetizing locations,” section 2-6.3.8:
On the other hand, the risk of condenhumidity, or
Reduction in Electrostatic Hazard.
sation may make the building structure (including historic buildings) more sensitive to
13. K assebaum, J. H. and R. A .
dew point.
the effects of humidity than the stored maKocken, “Controlling static electricterials. In northern climates, winter humidiity in hazardous (classified) locafication adds moisture that can lead to contions,” IEEE Transactions on Indusdensation and increased mold growth. In
try Applications, 33(1):209–215.
hot, humid climates, over cooling can also result in condensation. Attempting to dehumidify without adequate vapor retard- 14. United States Steel. 1971. The Making, Shaping and Treating of Steel, 9th edition, p. 981.
ers will be expensive and ultimately unsuccessful.
Using humidity wisely requires understanding the operative 15. Ibid.
parameter: relative humidity, specific humidity, or dew point. 16. American Society for Metals, Handbook Vol. 13—CorroOver cooling a room in the name of “dehumidification” raises
sion, p. 82.
relative humidity and may be counter productive for some materials. Allowing materials to absorb and desorb moisture slowly 17. Hoadley, R.B., “As dries the air, so shrinks the wood,”
Fine Woodworking, The Taunton Press, 39(2):92–95.
in response to seasonal climate changes may be a successful at
maintaining long term stability in the materials and the build- 18. Weber, C.G. and Snyder, L.W., “Reactions of lithographic
papers to variations in humidity and temperature,” National
ings that house them.
Bureau of Standards Journal of Research, vol. 12, paper
References
no. RP633, January 1934.
1. For a more detailed discussion of this topic, see 1997
ASHRAE, Handbook—Fundamentals, p. 22.19 and
Acker, William G., “Water Vapor Migration and Condensation Control in Buildings,” Heating/Piping/Air Condition- Kenneth M. Elovitz, P.E., Member ASHRAE, is an engiing, 70(6):72–81.
neering consultant and in-house counsel for Energy Economics,
2. The former term vapor “barrier” has fallen out of favor Inc., in Foxboro, Mass. Ken received a bachelor’s degree in metbecause “barrier” can imply an absolute block. Vapor re- allurgy and materials science from Lehigh University. He received
tarders slow water vapor transfer just as thermal insulation a JD from Suffolk University Law School and has been admitslows, but does not eliminate, heat transfer.
ted to practice in state and federal courts. He develops and edits
3. For a thorough discussion of design for hot, humid climates, these special supplements to ASHRAE Journal.
=
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