|
Water Vapor Migration & Condensation Control in Buildings
by William G. Acker
Share This Article
The
basics of psychrometric analysis of moisture conditions, including evaluation of vapor barriers
and other construction features, and internal and external moisture sources. Examples help
guide the discussion of this complex topic.
Water vapor is the gaseous
form of water and is an invisible
source of many
problems in today's buildings.
This article will provide a basic
understanding of how water vapor
transport occurs, when it condenses,
and when it can cause
damage and health problems.
Water vapor can travel through
building structures by:
• Diffusion caused by vapor
pressure differentials
• Air flow created by temperature
differentials
• Air flow created by mechanical
systems
• Rain penetration through intake
louvers
|
Articles
|
To begin with, psychrometric
conditions on the inside of a building
are usually different than the
outside conditions. The difference
in psychrometric properties results
in a vapor pressure differential
from inside to outside, which
sets up the driving force for water
vapor diffusion. The direction of
water vapor flow is from high vapor
pressure or high humidity to
low vapor pressure or low humidity.
In colder climates, such as the
northern U.S. and Canada, the
amount of water vapor in the outside
air is very low (low vapor
pressure) and is less than the
building inside air (high vapor pressure). Thus the water vapor
diffusion is from inside to outside.
In warmer climates with short
heating seasons, the water vapor
drive is from outside to inside due
to the drying effect of indoor air
conditioning. If the water vapor
condenses in the wall or roof section
and does not have the opportunity
to dry out, then over a period
of time, there will develop an
accumulation of water that could
cause damage and/or mold
and mildew. Keep in mind
that this is a dynamic situation
where condensation
may occur during part of
the day and evaporation
during another part. The
engineer's job is to design
the cavity so that condensation
does not occur, does
not occur frequently, or
only occurs in a safe region,
such as an air space with
drainage.
The terms and equations for water vapor transmission
or diffusion are listed
in the accompanying sidebar.
Equations 6 and 7 are
the vapor diffusion equations
used to calculate the amount of
water vapor that passes through
the wall or ceiling cavity. The
overall coefficient of vapor transmission,
M, is determined by
adding the resistances to vapor
transmittence for the construction
materials (and air films) and
then taking the inverse of that
summation. This procedure is illustrated
in Equations 4 and 5.
The inside and outside vapor
pressures can be determined from
test data or by using typical psychrometric
data for that region. If
there is concern over the amount
of water vapor diffusion or concern
over condensation in the cavity,
the engineer may need to install
a vapor retarder.
The ASTM definition of a vapor
retarder is a material with a vapor
performance (PERM) of 1.0 or
less. The resistance to vapor
transmittance is illustrated in
Equation 1. The resistance to vapor
transmittance is the inverse of
the PERM value; therefore, the
lower the PERM value, the
greater the resistance to water vapor
diffusion. Some state codes require
a PERM rating of less than
1.0 to qualify as a vapor retarder.
Water Vapor Diffusion
Water vapor diffuses through
many building materials other
than metals when a vapor pressure
difference exists across the
construction. One consensus that
seems to have been reached is that
vapor retarders are needed in cavity
walls and ceilings. In cold and
moderate climates, the vapor retarder
is placed on the indoor side
of the cavity. This is because the
water vapor flow is from inside to
outside (the inside is the higher vapor
pressure).
In some warm climates
that have a short heating
season, the vapor retarder is
placed on the outside of the cavity
because air conditioning of the indoor
air dries the air, thus lowering
the indoor vapor pressure below
the outdoor vapor pressure. In
some warm climates, vapor retarders
are installed on both theindoor and outdoor sides of the cavities.
Some typical vapor retarder
materials are listed in Table 1.
To calculate the amount of water
vapor diffusion, the engineer
must define the design indoor
and outdoor psychrometrics to
obtain the needed vapor pressures. If the analysis involves an
existing facility, the engineer can
test the actual conditions. The
properties that must be determined
are barometric pressure,
gauge pressure, dry bulb temperature,
and something to identify
moisture in the air, such as wet
bulb temperature, dew point
temperature, or relative humidity.
These properties and the
equations to calculate the vapor
pressure are illustrated in the accompanying
sidebar.
Relative humidity, RH, is a very
misunderstood term. It describes
the amount of moisture the air
holds relative to the maximum
it can hold at that temperature.
If, for example, the air temperature
is 70 F and the
relative humidity is, say 50
percent, the air at that temperature
contains only 50
percent of the moisture it is
capable of holding. If the
temperature then drops
from 70 to 52 F, the relative
humidity increases to 94.8
percent even though the
amount of moisture in theair
remained unchanged. The reason
is that cold air cannot hold as
much moisture as warm air. In
both cases, however, the absolute
humidity, W (lb water vapor/lb
dry air), is the same.
To summarize, a change in the
air dry bulb temperature will
cause a shift in the relative humidity
even though the amount
of moisture in the air remains unchanged.
A psychrometric computer
analysis of these two conditions
are illustrated in Tables 2
and 3. You will also note that the
dew point temperature is the
same in both cases, which supports
the theory that the moisture
in the air did not change.
Also, you will note that the vapor
pressure is the same in both
cases. The vapor pressure, Pw
(partial pressure of water vapor
in the mixture), of the outdoor
and indoor air can be calculated
by using the equations in the
sidebar and by using the steam
tables to obtain the pressure of
saturated pure water, Pws. Using
the 70 F/50 percent RH condition (Table 2), we will calculate the
vapor pressure of air.
Given:
• TDB = 70 F
• Pbar = 29.921 in. Hg
• Pgauge = 0 in. Hg
• Pabs = 29.921 in. Hg
Calculations:
• Pws = 0.7369 in. Hg (from
steam tables)
• RH = 50 percent = Pw/Pws
• Pw = 0.3684 in. Hg
The following example will illustrate
how to calculate the
amount of water vapor diffusion
through the wall section and will
be used to illustrate the importance
of a vapor barrier in this
case. The diagram in Fig. 1 is a
typical 2 by 6 in. wall section. The
materials of construction and air
films are shown with their PERM and REP values. You will note
that this wall does not have an indoor
vapor retarder. Psychrometric
conditions for this analysis are
shown in Table 4. Using this information
and the summation of
the REP values in Fig. 1, we can
calculate the amount of water vapor
flow:
W = (1/4.32024) 3 (0.23563 –
0.03313) = 0.04687
The diffusion of water vapor
through this wall is 0.04687
grains of water vapor per hr per
sq ft of wall surface. Now let's determine
what happens if we add a
4 mil vapor barrier between the
gypsum board and glass fiber insulation.
A 4 mil polyethylene vapor
barrier has a PERM of 0.08 or
a REP of 12.5. This increases the
REP summation to 16.82024. Now let's recalculate the amount of water
vapor diffusion through the
cavity:
W = (1/16.82024 3 (0.23563 –
0.03313) = 0.012039
You can see that without the
vapor barrier, the amount of water
vapor diffusion through the
cavity is 3.89 times higher or, in
other words, 289 percent higher
than the wall with the vapor retarder. This is a significant
change, but is it needed?
To answer this question, we put
the parameters into a computer
program that calculates the surface
temperatures of all the materials
and also calculates the surface
dew points to check for
condensation. If condensation occurs, the program prints out stars
next to the calculated surface dew
point temperatures. Table 5
shows the results of the computer
analysis for the wall without the
vapor barrier, and Table 6 is with
a vapor barrier. Without the vapor
barrier, you can see that condensation
does exist. Since water
vapor is moving from inside to
outside, you can see that it starts
at the glass fiber insulation. The
condensation may stop at the
glass fiber insulation due to the
reduction of vapor flow through
the remaining materials. If, however,
the condensation is frequent,
it could cause a loss of insulating
value (R-value), which
could cause the condensation to
move into the adjacent materials.
The wall with the vapor barrier
(Table 6) has no condensation.
Example Cases
Freezer and cold storage facilities
require a lot of attention to design
details. The indoor vapor pressures
can be as low as 0.011 in. Hg, resulting
in vapor pressure differentials
as high as 0.78 in. Hg, which is
three to four times higher than the
differentials experienced in residential
and commercial buildings.
Vapor retarders are critical to reduce
the moisture drive into the
freezer and to prevent condensation.
One client had major condensation
problems from the underside
of a supermarket floor. The supermarket
was built with parking at
ground level; the cold freezer facilities
and supermarket were on the
second floor above the parking area.
The parking area is totally open to
the outdoor environment. One area
that had condensation problems
was the underside of a walk-in
cooler floor. An elevation sketch of the construction is shown in Fig. 2.
The problem was condensation
in the false ceiling air cavity.
The glass fiber insulation
shown on top of the false ceiling
would get so wet with moisture
it would collapse the false ceiling
into the parking area. Surface
temperatures based on heat
loss calculations are also illustrated
in Fig. 2.
From these temperatures, you
can see that during the summer,
the glass fiber insulation was preventing
heat gain into the air space. This is a problem because
the surface temperatures are well
below the outside air dew point
temperature. With no vapor barrier
and the poor air tightness of
the false ceiling, the air cavity will
have a dew point very close to the
outside air dew point. This, in
turn, results in condensation because the vapor can touch a surface
that is below its dew point.
One solution might be to remove
the glass fiber insulation.
In this case, however, it is not
possible because the air cavity
must be heated in winter to prevent
freezing of the pipes in the
air space. This is why the diagram
shows radiant heat pipes.
The solution used was to run the
radiant heat system in the summer,
which raises the surface
temperatures above the dew
point temperature. To save energy,
one could install sensors to
monitor the metal deck temperature
and the dew point temperature.
A controller could then turn
on the radiant heat to maintain
the surface at 5 F higher than the
dew point temperature.
The computer condensation
analysis program revealed condensation problems inside the
false ceiling.
One industry that has many
problems with water vapor diffusion
and condensation is the paper
industry. The wet end section of a
paper machine takes the water
and fiber and forms it into a sheet.
The stock temperature is around
100 to 120 F; therefore, the evaporation
rate into the building is
high. The dry end section, unlike
the wet end section, has a hood to
capture the water evaporated and
discharges the water vapor outside.
Some of these hoods, however,
have leaks that can raise the
humidity excessively inside the
building. The molecular weight of
water vapor is less than the molecular
weight of dry air. Therefore,
the water vapor rises to the underside
of the paper machine building
roof, thus raising the dew point at
the underside higher than the rest
of the building.
At one mill location, the air
properties taken at the underside
of the roof were as follows:
Pbar = 29.8220 in. Hg
TDB = 111 F
TWB = 103.33 F
TDP = 102.05 F
RH = 77 percent
Pw = 2.0553 in. Hg
This particular machine room
had little dryer hood leakage.
Another survey for a machine
room housing five paper machines
showed a lot of condensation, corrosion,
and spalling of concrete
from the roof deck. A mass air and
water vapor survey of the building
revealed a water vapor flow into
the building of 123,175 lb per hr
or 248 gpm. As you can imagine,
this raised the building humidity
to extreme levels. The worst area
tested—at the underside of the
roof—is illustrated below:
Pbar = 29.8220 in. Hg
TDB = 159 F
TWB = 136.51 F
TDP = 135 F
RH = 54.81 percent
Pw = 5.1651 in. Hg
The vapor pressure differential
across the roof system in this case is over 4.0 in. Hg, which is tremendous
as well as unacceptable.
The theory behind preventing roof
surface condensation is to keep
the inside roof surface temperature
above the dew point temperature
of the vapor rising to the ceiling.
The surrounding air dry bulb
temperature heats the roof surface,
but due to heat loss through
the roof, the surface temperature
will always be lower than the dry
bulb temperature. One method of
raising the roof surface temperature
is to add insulation to the
roof. If you look back to the first
paper mill example, you can see
that the dew point is 102 F and
the dry bulb is 111 F. Therefore,
in this case, you can expect the
roof temperature to be less than
111 F but hopefully not at or below
102 F.
To help prevent condensation in
today's machine rooms, one
should supply a roof heating system
for the underside of the building
roof. Roof heating systems are
air handling units that take indoor
building air, heat it to 120 F, and distribute it to the underside
of the building roof. Depending
upon roof insulation levels, this
process will then heat the roof to
around 110 to 115 F. Certainly,
the building does not need any
more heat, but it is required until
the industry puts a hood on the
wet end section. If you look back
at the five paper machine building
example, you will note that the
dew point was at 135 F, which is well above the temperature of the
existing roof heating system. In
this case, the roof heating system
does not prevent condensation.
The cure to the condensation
problem is to find the hood leaks
and patch them.
The problems with water vapor
condensation inside the machine
rooms are complex and require
further review. The water vapor
transmission is tremendous due
to the high vapor pressure differentials.
The next problem is to design
the roof system so that the
vapor can escape without condensation
or to have the vapor condense
in a safe region. Over 45
percent of the roof failures are
caused by poor roof design. Vapor
retarders are a necessity.
An example of a paper machine
building roof is illustrated in Fig.
3. The psychrometric test conditions
at the underside of the building
roof were illustrated in the
first example. This mill was experiencing
condensation and concrete
deck spalling. The design
conditions were put into the condensation analysis program to
check for problems. The results
can be found in Table 7.
The program shows condensation
in the concrete roof deck. This
explains the roof spalling problems.
In this case, there was insufficient
insulation in the roof,
resulting in a low inside roof surface
temperature. The steel reinforcement
in pre-cast concrete
decks will corrode when exposed to condensation and chemicals
such as chlorine. Test samples
taken from concrete decks have
shown acid-soluble chlorine-ion
contents of between 100 and 1100
ppm of concrete. Special precautions
need to be taken to prevent
the corrosion of steel reinforcement.
Poor conditions at the underside
of a paper machine building
roof should not be taken lightly.
High humidities will result in
premature failure of the roof support
steel, concrete deck, and
roofing materials. Some mills
have experienced complete failures
just a few years after installation,
and others have had roof
sections fall into the paper machines.
To give you an idea of the
costs, a mill in the south had to
replace the roof steel, concrete
deck, and roofing materials at a
cost of over $1,000,000 or about
$50 per sq ft of roof. The building
housed a tissue machine, and the
replacement occurred 24 years after
installation.
The last example will illustrate
blower door surveys, infrared surveys,
the use of mass dry air and
water vapor analysis, and adiabatic
mixing to solve condensation
problems in buildings. It was
kept simple so that the analysis
would fit into this article. The
building is a residential home
with severe attic condensation
problems in the winter. The home
has cathedral ceilings of tongueand-
groove maple panels. The
home had condensation in the attic
that dripped through the ceiling.
The contractor told the homeowner
that the problem was the
storage of 22 face cords of firewood
in the basement.
The first test conducted was a
blower door test to determine the
building air change rate. The blower door air flow was
5200 cfm at a differential
pressure of 50 pascals
(0.201 in. WG). The calculated
air change rate in air
changes per hour (ACH) is
as follows:
ACH = (acfm 3 60 min
per hr)/(Building volume,
cu ft/1 air change) = (5200
3 60)/(21,693/1) = 14.4
This is a high air change
rate for a new home.
The next step was to determine
where the air flow
paths were. An infrared-image
heat-loss scan was conducted
with the blower on
and with the blower door off
to help accentuate the areas
with air flow heat loss.
The infrared-image scans
showed air flow leaking
through the tongue-andgroove
ceiling panels. It
was discovered later that
the kraft-back vapor barrier
was not stapled against
the inside face of the cathedral
ceiling rafters. Instead, the
vapor retarder was stapled to the
inner sides of the rafters. If the
vapor retarder is installed against
the inside face of the rafter and
the tongue-and-groove paneling is
nailed against the vapor retarder,
you can achieve air tightness.
This was not done. Without air
tightness, you have a potentially
dangerous situation because air
flow can carry much more moisture
into the attic than diffusion
through the materials. The
amount of water vapor diffusion
through 1364 sq ft of ceiling in
this case is 0.01438 lb of water vapor
per hr.
The blower door air flow of 5200
cfm was converted to air flow at
normal building differential pressure
of 3 pascals (0.012 in. WG),
which is 1270 acfm. If all of this
air flow travels through the
tongue-and-groove ceiling into the
attic, the water vapor it carries
will be 23.70 lb per hr if the home is maintained at 68 F/35 percent
RH. This amount of water vapor
flow is 1648 times higher than the
water vapor diffusion into the attic.
If the house was built with the
proper air tightness, the air flow
at 3 pascals would be around 289
acfm. Therefore, the excess air
flow through this house comes to
981 acfm. If we instead assume
that only 1 percent of the excess
air flow goes into the attic, the
amount of water vapor it would
carry is still 16 times higher than
the diffusion flow. What this illustrates
is that air tight construction
is extremely important.
The next step was to conduct a
mass flow analysis using the
tested air flow at 3 pascals, an indoor
design temperature of 68 F,
and estimates of water vapor generation
inside the home. The outside
winter air is assumed to be 0
F/35 percent RH. To conduct a
mass flow analysis, one must convert
the flows to pounds of dry air and pounds of water vapor. Therefore,
the amount of dry air entering
must be equal to the amount
of dry air leaving. This is not the
case with acfms. The finalized
analysis is illustrated in Fig. 4.
You will note that the storage of
firewood in the home did add a
tremendous vapor load. What is
even more interesting is that the
calculated indoor humidity (air
leaving) is only 12.8 percent, even
with all this water vapor load.
This condition was verified by the
homeowner who said that the humidifier
had to run frequently because
the house was so dry. The
reason for the dry environment is
the excessive outside air flow into
the house, which was drying out
the house. Further investigations
revealed similar homes that were
very hard to heat due to the excess
air flow.
To summarize, the condensation
problem was due to the air
and water vapor flow into the attic. The amount of water vapor entering
the attic was too much for
the attic to handle, which raised
the air dew point, causing the condensation.
This is why state codes
emphasize air tightness.
Conclusion
Due to space limitations, this
article only covered some of the
basics of water vapor analysis and
control. The use of vapor retarders,
for instance, gets professionals
into very heated and complex
debates. Certainly, there are
times when vapor retarders are
needed and times when they are
not. To help engineers with these
decisions, the U.S. Army Cold Regions
Research Laboratory (CRREL)
has developed a procedure,
information about which can be
obtained from the National Roofing
Contractors Association in
Rosemont, Ill. Also, there is a paper
written by the staff of the Oak
Ridge National Laboratory, Oak
Ridge, Tenn., that provides new
information about vapor retarder
selection criteria.
This is a difficult field that could
use additional research work to assist
the engineers' efforts. It is also
a field that is getting a lot of attention
these days due to the concern
of indoor air quality.
[ back to top ]
For engineers who want to
learn more about moisture transport
modeling for buildings, the
1997 ASHRAE Handbook of Fundamentals
recommends the following
but warns that most of the
models are research tools that
may be too complex for users
other than researchers:
• Glasta (Physibel, Belgium)
• EMPTEDD (Trow, Toronto,
Canada)
• Match (Technical University,
Lyngby, Denmark)
• COND (Technical University,
Dresden, Germany)
• MOIST (NIST, Gaithersburg,
Md.) |
|