Moisture Dynamics in Building Construction
by Bob Higgins
Uncontrolled moisture is the most prevalent cause of deterioration in buildings. It
leads ultimately to the destruction of materials, finishes, and eventually structural
components. Ever present in our environment, moisture can be controlled to provide the
differing levels of moisture necessary for human comfort as well as the longevity of
building materials and furnishings. The challenge to building owners and building
professionals alike is to understand the patterns of moisture movement in order to manage
it, not to try to eliminate it. There is never a single answer to a moisture problem.
Diagnosis and treatment will always differ depending on where the building is located,
climatic and soil conditions, ground water effects, and local traditions in building
construction.
Remedial advice about controlling the sources of unwanted moisture is provided which,
regardless of who does the work, are the principles that should guide treatment decisions:
- Avoid remedial treatments without prior careful diagnosis.
- Undertake treatments that protect the flooring material(s).
- Address issues of ground-related moisture and rain run-off thoroughly.
- Manage existing moisture conditions before introducing humidified/dehumidified
mechanical systems.
- Implement a program of ongoing monitoring and maintenance once moisture is controlled or
managed.
- Be aware of significant landscape and archeological resources in areas to be excavated.
- Finally, mitigating the effects of catastrophic moisture, such as floods, requires a
different approach and will not be addressed in this article.
How and Where to Look for Moisture
Finding, treating, and managing the sources of damaging moisture requires a systematic
approach that takes time, patience, and a thorough examination of all aspects of the
problem-including a series of variable conditions. Moisture problems may be a direct
result of one of these factors or may be attributable to a combination of interdependent
variables.
Factors Contributing to Moisture Problems
A variety of simultaneously existing conditions contributes to moisture problems in
buildings. For recurring moisture problems, it may be necessary for the owner or
professional to address many, if not all, of the following variables:
- Types of building materials and construction;
- Type and condition of roof and site drainage systems and their rates of discharge;
- Type of soil, moisture content, and surface /subsurface water flow adjacent to building;
- Building usage and moisture generated by occupancy;
- Condition and absorption rates of materials;
- Type, operation, and condition of heating, ventilating, cooling,
humidification/dehumidification, and plumbing systems;
- Daily and seasonal changes in sun, prevailing winds, rain, temperature, and relative
humidity (inside and outside), as well as seasonal or tidal variations in groundwater
levels;
- Unusual site conditions or irregularities of construction;
- Conditions in affected wall cavities, temperature and relative humidity, and dewpoint;
- Amount of air infiltration present in a building;
- Adjacent landscape and planting materials;
Diagnosing and treating the cause of moisture problems requires looking at both the
localized damage and understanding the performance of the entire building and site.
Moisture is notorious for traveling far from the source, and moisture movement within
concealed areas of the building construction makes accurate diagnosis of the source and
path difficult. Obvious deficiencies, such as broken pipes, clogged gutters, or cracked
surfaces that contribute to moisture damage, should always be corrected promptly. For more
complicated problems, it may take several months of evaluation to complete a full
diagnosis. Rushing to a solution without adequate documentation can often result in the
unnecessary removal of flooring materials and worse, the creation of long-term problems
associated with an increase, rather than a decrease, in the unwanted moisture.
Looking for Signs
Identifying the type of moisture damage and discovering its source or sources usually
involves the human senses of sight, smell, hearing, touch, and taste combined with
intuition. Some of the more common signs of visible as well as hidden moisture damage
include:
- Presence of standing water, mold, fungus, or mildew;
- Wet stains, eroding surfaces, or efflorescence (salt deposits) on interior and exterior
surfaces;
- Flaking paint and plaster, peeling wallpaper, or moisture blisters in flooring finishes
- Dank, musty smells in areas of high humidity or poorly ventilated spaces;
- Rust and corrosion stains on metal elements, such as anchorage systems (e.g. tack
strips)
- Cupped, warped, cracked or rotted flooring materials
- Condensation on window and wall surfaces
Uncovering and Analyzing Moisture Problems
Moisture comes from a variety of external sources. Most problems begin as a result of the
weather in the form of rain or snow, from high ambient relative humidity, or from high
water tables. But some of the most troublesome moisture damage in existing buildings may
be from internal sources, such as leaking plumbing pipes, components of heating, cooling,
and climate control systems, as well as sources related to use or occupancy of the
building. In some cases, moisture damage may be the result of poorly designed original
details.
The five most common sources of unwanted moisture include:
- above grade exterior moisture entering the building,
- below-grade ground moisture entering the building,
- leaking plumbing pipes and mechanical equipment
- Interior moisture from household use and climate control systems, water used in
maintenance and construction materials.
Above grade exterior moisture generally results from weather related moisture entering
through deteriorating materials as a result of deferred maintenance, structural settlement
cracks, or damage from high winds or storms. Such sources as faulty roofs, cracks in
walls, and open joints around window and door openings can be corrected through either
repair or limited replacement.
Vines or other vegetative materials allowed to grow directly on building materials without
trellis or other framework can cause damage from roots eroding mortar joints and
foundations as well as dampness being held against surfaces. In most cases, keeping
vegetation off buildings, repairing damaged materials, replacing flashings, rehanging
gutters, repairing downspouts, repointing mortar, caulking perimeter joints around windows
and doors, and repainting surfaces can alleviate most sources of unwanted exterior
moisture from entering a building above grade. Below grade ground moisture is a major
source of unwanted moisture for buildings. Proper handling of surface rain run-off is one
of the most important measures of controlling unwanted ground moisture. Rainwater is often
referred to as "bulk moisture" in areas that receive significant annual
rainfalls or infrequent, but heavy, precipitation. For example, a heavy rain of 2"
per hour can produce 200 gallons of water from downspout discharge alone for a house
during a one-hour period. When soil is saturated at the base of the building, the moisture
will wet footings and crawl spaces or find its way through cracks in foundation walls and
enter into basements. Moisture in saturated basement or foundation walls-also exacerbated
by high water tables-will generally rise up within a wall and eventually cause
deterioration of the masonry and adjacent wooden structural elements. Builders will
provide a working area, known as a "French drain," around the exterior of a
foundation wall. These drains have been known to increase moisture problems if the pitch
of the drainage is insufficient, since the rubble backfill may act as a reservoir holding
water against the walls and foundation. Broken subsurface pipes or downspout drainage can
leak into the drain and dampen walls some distance from the source. Any subsurface
penetration of the foundation wall for sewer, water, or other piping also can act as a
direct conduit of ground moisture unless these holes are well sealed. A frequently
unsuspected, but serious, modern source of ground moisture is a landscape irrigation
system set too close to the building. Incorrect placement of sprinkler heads can add a
tremendous amount of moisture at the foundation level and on wall surfaces.
The ground, and subsequently the building, will stay much drier by;
- re-directing rain water away from the foundation through sloping grades;
- capturing and disposing downspout water well away from the building;
- developing a controlled ground gutter or effective drainage for buildings historically
without gutters and downspouts, and
- reducing splash-back of moisture onto foundation walls.
The excavation of foundations and the use of dampproof coatings and footing drains should
only be used after the measures of reducing ground moisture listed above have been
implemented. Leaking plumbing pipes and mechanical equipment can cause immediate or
long-term damage to building interiors. Routine maintenance, repair, or, if necessary,
replacement of older plumbing and mechanical equipment are common solutions. Interior
moisture from building use and modern humidified heating and cooling systems can create
serious problems. In northern U.S. climates, heated buildings will have wintertime
relative humidity levels ranging from 10%-35% Relative Humidity (RH). A house with four
occupants generates between 10 and 16 pounds of water a day (approximately 1 ½- 2
gallons) from human residents.
Moisture from food preparation, showering, or laundry use will produce condensation on
windows in winter climates. When one area or floor of a building is air-conditioned and
another area is not, there is the chance for condensation to occur between the two areas.
Most periodic condensation does not create a long-term problem.
Humidified climate control systems are generally a major problem in buildings. They
produce between 35%-55% RH on average which, as a vapor, will seek to dissipate and
equalize with adjacent spaces. Moisture can form on single-glazed windows in winter with
exterior temperatures below 30°F and interior temperatures at 70°F with as little as 35%
RH. Frequent condensation on interior window surfaces is an indication that moisture is
migrating into exterior walls and floors. Moist interior conditions in hot and humid
climates will generate mold and fungal growth. Unvented mechanical equipment, such as gas
stoves and dryers generate large quantities of moisture. It is important to provide
adequate ventilation and find a balance between interior temperature, relative humidity,
and airflow to avoid interior moisture that can damage buildings.
Moisture from maintenance and construction materials can cause damage to floors. Careless
use of liquids to wash floors can lead to water seepage through cracks and seams,
dislodging adhesives or cup and curl materials. New concrete in a building may hold
moisture for months. Concrete should be fully cured before any flooring is installed. The
use of materials in projects that have been damaged by moisture prior to installation or
have too high a moisture content may cause concealed damage.
Moisture Transport and Movement
Knowing the five most common sources of moisture that cause damage to building materials
is the first step in diagnosing moisture problems. It is equally important to understand
the basic mechanisms that affect moisture movement in buildings. Moisture transport, or
movement, occurs in two states: liquid and vapor. It is directly related to pressure
differentials. For example, water in a gaseous or vapor state, as warm moist air, will
move from its high-pressure area to a lower pressure area where the air is cooler and
drier. Liquid water will move as a result of differences in hydrostatic pressure or wind
pressure. It is the pressure differentials that drive the rate of moisture migration in
either state. Because the building materials themselves resist this moisture movement, the
rate of movement will depend on two factors: the permeability of the materials when
affected by vapor and the absorption rates of materials in contact with liquid.
The mechanics, or physics, of moisture movement is complex, but if the driving force is
difference in pressure, then an approach to reducing moisture movement and its damage is
to reduce the difference in pressure, not to increase it. That is why the treatments
discussed in this article will look at managing moisture by draining bulk moisture and
ventilating vapor moisture before setting up new barriers with impermeable coatings or
over-pressurized new climate control systems that threaten building materials and
construction systems.
Two forms of moisture transport are particularly important in understanding infiltration,
capillary action, and vapor diffusion. Remember that the subject is infinitely complex
and, thus, one of continuing scientific study. Buildings were traditionally designed to
deal with the movement of air. For example, cupolas and roof lanterns allowed hot air to
rise and provided a natural draft to pull air through buildings. Cavity walls in both
frame and masonry buildings were constructed to allow moisture to dissipate in the air
space between external and internal walls. Radiators were placed in front of windows to
keep cold surfaces warm, thereby reducing condensation on these surfaces. Many of these
features, however, have been eliminated or altered over time in an effort to modernize
appearances, improve energy efficiency, or accommodate changes in use. The change in use
will also affect moisture movement, particularly in commercial and industrial buildings
with modern mechanical systems.
The way a building handles air and moisture today may be different from that intended by
the original builder or architect, and poorly conceived changes may be partially
responsible for chronic moisture conditions. Moisture moves into and through materials as
both a visible liquid (capillary action) and as a gaseous vapor (infiltration and vapor
diffusion). Moisture from leaks, saturation, rising damp, and condensation can lead to the
deterioration of materials and cause an unhealthy environment. The amount of moisture and
how it deteriorates materials is dependent upon complex forces and variables that must be
considered for each situation.
Determining the way moisture is handled by the building is further complicated because
each building and site is unique. Water damage from blocked gutters and downspouts can
saturate materials on the outside, and high levels of interior moisture can saturate
interior materials. Difficult cases may call for technical evaluation by consultants
specializing in moisture monitoring and diagnostic evaluation. In other words, it may take
a team to effectively evaluate a situation and determine a proper approach to controlling
moisture damage in old buildings.
Infiltration is created by wind, temperature gradients (hot air rising), ventilation fan
action, and the stack or chimney effect that draws air up into tall vertical spaces.
Infiltration as a dynamic force does not actually move liquid water, but is the vehicle by
which dampness, as a component of air, finds its way into building materials. Older
buildings have a natural air exchange, generally from 1 to 4 changes per hour, which, in
turn, may help control moisture by diluting moisture within a building. The tighter the
building construction, however, the lower will be the infiltration rate and the natural
circulation of air. In the process of infiltration, however, moisture that has entered the
building and saturated materials can be drawn in and out of materials, thereby adding to
the dampness in the air. Inadequate air circulation where there is excessive moisture
(i.e., in a damp basement) accelerates the deterioration of historic materials. To reduce
the unwanted moisture that accompanies infiltration, it is best to incorporate maintenance
and repair treatments to close joints and weather-strip windows, while providing
controlled air exchanges elsewhere. The worst approach is to seal the building so
completely, while limiting fresh air intake, so the building cannot breathe.
Capillary action occurs when moisture in saturated porous building materials, wicks up or
travels vertically as it evaporates to the surface. In capillary attraction, liquid in the
material is attracted to the solid surface of the pore structure causing it to rise
vertically; thus, it is often called "rising damp," particularly when found in
conjunction with ground moisture. It should not, however, be confused with moisture that
laterally penetrates a foundation wall through cracks and settles in the basement. Not
easily controlled, most rising damp comes from high water tables or a constant source
under the footing. In cases of damp concrete foundations with capillary action, there is
usually a whitish stain or horizontal tide mark of efflorescence that seasonally
fluctuates about 1- 3 feet above grade where the excess moisture evaporates from the wall.
This tide mark is full of salt crystals, which have been drawn from the ground and
building materials along with the water, making the concrete even more sensitive to
additional moisture absorption from the surrounding air. Capillary migration of moisture
may occur in any material with a pore structure where there is a constant or recurring
source of moisture. The best approach for dealing with capillary rise in building
materials is to reduce the amount of water in contact with the surface. Moisture should
not be sealed into the wall with a waterproof coating, such as cement parging or vinyl
wall coverings, applied to the inside of damp walls. This will only increase the pressure
differential as a vertical barrier and force the capillary action, and its destruction of
materials, higher up the wall.
Vapor diffusion is the natural movement of pressurized moisture vapor through porous
materials. It is most readily apparent as humidified interior air moves out through walls
to a cooler exterior. In a hot and humid climate, the reverse will happen as moist hot air
moves into cooler, dryer, air-conditioned, interiors.
The movement of the moisture vapor is not a serious problem until the dewpoint temperature
is reached and the vapor changes into liquid moisture known as condensation. This can
occur within a wall, floor or on interior surfaces. Condensation as a result of vapor
migration usually takes place on a surface or film, such as the underside of a flooring
material, where there is a change in permeability.
Long-term and undetected condensation or high moisture content can cause serious
structural damage as well as an unhealthy environment, heavy with mold and mildew spores.
Reducing the interior/exterior pressure differential and the difference between interior
and exterior temperature and relative humidity helps control unwanted vapor diffusion.
This can sometimes be achieved by reducing interior relative humidity. In some instances,
using vapor barriers, such as heavy plastic sheeting laid over damp crawl spaces, can have
remarkable success in stopping vapor diffusion from damp ground into buildings. Yet,
knowledgeable experts in the field differ regarding the appropriateness of vapor barriers
and when and where to use them, as well as the best way to handle natural diffusion in
insulated walls.
Adding insulation to buildings, particularly in walls of wooden frame structures, has been
a standard modern weatherization treatment, but it can have disastrous consequences. While
insulation has the benefit of increasing the efficiency of heating and cooling by
containing temperature controlled air, it does not eliminate surfaces on which damaging
moisture can condense. For insulated residential frame structures, the most obvious sign
of a moisture diffusion problem is peeling paint on wooden siding, even after careful
surface preparation and repainting. Vapor impermeable barriers such as plastic sheeting,
or more accurately, vapor retarders, in cold and moderate climates generally help slow
vapor diffusion where it is not wanted.
Surveying and Diagnosing Moisture Damage: It is vital that the building be surveyed first,
with evidence and location of suspected moisture damage systematically recorded before
undertaking any major correction work. This will give a baseline from which relative
changes in condition can be noted. When flooring materials become wet or damaged, there
are specific physical changes that can be detected and noted in a record book or on survey
sheets. Every time there is a heavy rain, snow storm, water in the basement, or mechanical
systems failure, the owner or consultant should note and record the way moisture is
moving, its appearance, and what variables might contribute to the cause. Standing outside
to observe a building in the rain may answer many questions and help trace the movement of
water into the building. Evidence of deteriorating materials that cover more serious
moisture damage should also be noted, even if it is not immediately clear what is causing
the damage.
Don't jump to conclusions. Use a systematic approach to help establish an educated theory,
or hypothesis, of what is causing the moisture problem or what areas need further
investigation.
Surveying moisture damage must be systematic so that relative changes can be noted. Tools
for investigating can be as simple as a notebook, sketch plans, camera, aluminum foil,
moisture meters, calcium chloride tests, pH tests, smoke pencils. The systematic approach
involves looking at buildings from the top down and from the outside to the inside.
Photographs, floor plans, site plan, and exterior elevations-even roughly sketched-should
be used to indicate all evidence of damp or damaged materials, with notations for musty or
poorly ventilated areas. Information might be needed on the absorption and permeability
characteristics of the building materials and soils. Exterior drainage patterns should be
noted and these base plans referred to on a regular basis in different seasons and in
differing types of weather. It is best to start with one method of periodic documentation
and to use this same method each time. Because moisture is affected by gravity, many
surveys start with above-grade influences and work down from there.
Any obvious areas of water penetration, damaged surfaces, or staining should be noted. Any
recurring damp or stain patterns, both exterior and interior, should also be noted with a
commentary on the temperature, weather, and any other facts that may be relevant (driving
rains, saturated soil, high interior humidity, recent washing of the building, presence of
a lawn watering system, etc.). The interior should be recorded as well, beginning with the
attic and working down to the basement and crawl space. It may be necessary to remove
damaged materials selectively in order to trace the path of moisture or to pinpoint a
source, such as a leaky window flashing. The use of a basic resistance moisture meter,
available in many hardware stores, can identify moisture contents of materials and show,
over time, if wall surfaces are drying or becoming damper. A smoke pencil can chart air
infiltration around windows or draft patterns in interior spaces.
Comparing current conditions with previous conditions, historic drawings, photographs, or
known alterations may also assist in the final diagnosis. A chronological record, showing
improvement or deterioration, should be backed up with photographs or notations as to the
changing size, condition, or features of the deterioration and how these changes have been
affected by variables of temperature and rainfall. If a condition can be related in time
to a particular event, such as efflorescence developing on a concrete surface after the
building is no longer heated, it may be possible to isolate a cause, develop a hypothesis,
and then test the hypothesis (by adding some temporary heat), before applying a remedial
treatment. If the consultant has access to moisture survey and monitoring equipment such
as resistance moisture meters, dewpoint indicators, salt detectors, infrared thermography
systems, psychrometer, fiber-optic boroscopes, GPR (ground penetrating radar) and
miniaturized video cameras. Additional quantified data can be incorporated into the
survey. If it is necessary to track the wetting and drying of floors over a period of
time, deep probes set into concrete and in the soil with connector cables to computerized
data loggers or the use of long-term recording of hygrothermographs usually requires a
trained specialist. Miniaturized fiber-optic video cameras can record the condition of
subsurface drain lines without excavation. It should be noted, however, that
instrumentation, while extremely useful, couldn't take the place of careful personal
observation and analysis. Relying on instrumentation alone rarely will give the owner the
information needed to fully diagnose a moisture problem.
To avoid jumping to a quick, potentially erroneous conclusion, a series of questions
should be asked first. This will help establish a theory or hypothesis that can be tested
to increase the chances that a remedial treatment will control or manage moisture.
- How is water draining around building and site?
- What is the effectiveness of gutters and downspouts?
- Are the slopes or grading around foundations adequate?
- What are the locations of subsurface features such as wells, cisterns, or drainage
fields?
- Are there subsurface drainage pipes (or drainage boots) attached to the downspouts and
are they in good working condition?
- Does the soil retain moisture or allow it to drain freely?
- Where is the water table?
- What is the flow rate of area drains around the site (can be tested with a hose for
several minutes)?
- Is the storm piping out to the street sufficient for heavy rains, or does water
chronically back up on the site?
- Has adjacent new construction affected site drainage or water table levels?
- How does water/moisture appear to be entering the building?
- Have all five primary sources of moisture been evaluated?
- What is the condition of flooring and construction materials and are there any obvious
areas of deterioration?
- Did this building have a French-drain around the foundation that could be holding water
against the exterior foundation?
- Are the interior bearing walls as well as the exterior walls showing evidence of rising
damp?
- Is damage localized, on one side of the building only, or over a large area?
- What are the principal moisture dynamics?
- Is the moisture condition from liquid or vapor sources?
- Is the exterior wall moisture from rising damp with a tide mark or are there uneven
spots of dampness from foundation splash back, or other ground moisture conditions?
- Is there adequate air exchange in the building, particularly in damp areas, such as the
basement?
- Has the height of the water table been established by inserting a long pipe into the
ground in order to record the water levels?
- How is the interior climate handling moisture?
- Are there areas in the building that do not appear to be ventilating well and where mold
is growing?
- Are there historic features that once helped the building control air and moisture that
can be reactivated, such as operable skylights or windows?
- Could dewpoint condensation be occurring behind surfaces, since there is often
condensation on the windows?
- Does the building feel unusually damp or smell in an unusual way that suggest the need
for further study?
- Is there evidence of termites, carpenter ants, or other pests attracted to moist
conditions?
- Is a dehumidifier keeping the air dry or is it, in fact, creating a cycle where it is
actually drawing moisture through the foundation?
- Does the moisture problem appear to be intermittent, chronic, or tied to specific
events?
- Are damp conditions occurring within two hours of a heavy rain or is there a delayed
reaction?
- What are the wet patterns that appear on a building wall during and after a rainstorm?
- Is it localized or in large areas?
- Can these rain patterns be tied to gutter overflows, faulty flashing, or saturation of
absorbent materials?
- Is a repaired area holding up well over time or is there evidence that moisture is
returning?
- Do moisture meter readings of floors indicate they are wet, suggesting leaks or
condensation?
Once a hypothesis of the source or sources of the moisture has been developed from
observation and recording of data, it is often useful to prove or disprove this hypothesis
with interim treatments, and, if necessary, the additional use of instrumentation to
verify conditions. New data will lead to a new hypothesis that should be tested and
verified. The process of elimination can be frustrating, but is required if a systematic
method of diagnosis is to be successful.
Conclusion
Moisture in buildings, though difficult to evaluate, can be systematically studied and the
appropriate protective measures taken.
Much of the documentation and evaluation is based on common sense combined with an
understanding of historically used building materials, construction technology, and the
basics of moisture and air movement. Variables can be evaluated step by step and
situations creating direct or secondary moisture damage can generally be corrected. The
majority of moisture problems can be mitigated with maintenance, repair, control moisture,
and improved ventilation.
Moisture problems can be intimidating to a professional who has diligently tried to
control them. Keeping a record of evidence of moisture damage, results of diagnostic
tests, and remedial treatments, is beneficial to a building's long-term care. The more
complete a survey and evaluation, the greater the success in controlling unwanted moisture
now and in the future. The flooring professional, working with properly trained
contractors and consultants can monitor, select, and implement treatments to manage
moisture.
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