Understanding and
Controlling Condensation
Introduction
All too often, condensation and
its prevention is not given adequate attention in building design and construction.
The following is an attempt to explain condensation, what its effects are, and how it can
be controlled.
Condensation
and Its Terms
Air is composed of many gases.
One of these is water in a gaseous from, called water vapor. The amount of
water vapor that air can hold is a function of temperature. When the air comes in
contact with an object at a temperature where water vapor will change to a liquid, it is
known as the dew point temperature. This process is called condensation.
Condensation is not a property unique to water vapor alone, most other gases behave
in a similar manner. Unlike other gases, water vapor condenses in the temperature
ranges which we find in buildings.
The temperature at which the
air/water vapor mixture no longer retains its water vapor in the gaseous state is called
the dew point temperature of the mixture. The dew point temperature is sometimes
referred to as "dew point." The ratio of the amount of water vapor in
air compared to the maximum amount air can hold at a specific temperature is called the
relative humidity. When air is absolutely dry, its relative humidity is 0 %.
When an air mixture has reached its dew point temperature, its relative humidity is
100%. The higher the relative humidity, the greater water vapor content of the air
at that temperature.
Each component of the air
mixture is driven by the partial pressure of each component. Even though water vapor
can move independently of air, when air is heated or cooled, the water vapor should be
considered as part of the air-vapor mixture. If air is exposed to changing
temperatures, the air will be driven from an area of higher partial pressures to an area
of lower partial pressures. One of the components of air, water vapor, is also
acted upon by partial pressure. It is this natural phenomenon which moves water
vapor though the air or any porous material.
Sources
of Moisture
Moisture is given off in many different processes.
Human beings give off a significant amount of water through respiration and
perspiration. There are other common sources. Gas, oil-fired and propane space
heaters give off a significant amount of moisture through the process of combustion.
The moisture introduced through combustion is sometimes very difficult to detect
because at the point of combustion, the gases are very hot and can hold large quantities
of moisture. When this hot gas is mixed with cooler air, the temperature drops.
As the temperature drops, the amount of moisture that the gas can hold will
decrease and at some point away from the combustion device, the water will condense on
anything that it comes in contact with that is below the dew point of that mixture. Flue
gases should be vented outside to prevent this from happening.
Even before a building is fully completed, there can
be significant amounts of moisture introduced into the building. Excavated earth
contains a significant amount of moisture. As the soil is exposed to surrounding
air, the moisture will be given off. If the building is closed, this moisture will
stay within the building. As soon as the temperature drops below the dew point of
that air/water mixture, condensation will form.
Fresh concrete is another source of large amounts of
moisture. If the building is closed and concrete is poured, a way must be provided
for moisture to be vented to the atmosphere. Ventilation should always be considered
as a preventive measure during the construction schedule.
Effects
of Condensation
Knowing the process by which condensation is formed,
one can look at the effects of condensation on normal building materials. For the
metal building industry, the most commonly used materials are steel sheathing and
fiberglass insulation. Most of the metal in metal building is treated against
corrosion and rust. Rust is a result of an interaction between the metal and salts,
acids or alkalines. Another metal employed is aluminum which does not rust, but does
oxidize. In both instances, the metal itself becomes weaker. In time, both
materials will deteriorate and shorten life expectancy. While many surface
treatments are applied to ferrous and non-ferrous metals to prevent oxidation, the
best protection is to eliminate a principal cause of oxidation--in other words,
eliminating condensation.
The insulating material most commonly used in metal
buildings is fiberglass blanket. Water in its liquid form is a good conductor of
heat. The presence of water vapor or condensed water in fiberglass insulations will
increase its thermal conductivity because of the higher conductivity of water.
However, glass fibers do not absorb water. Only
surface wetting occurs. Once all the moisture is removed from the wet fiberglass
surface, it will revert back to its original insulating value. The same cannot be
said for other materials. Here again, an ounce of prevention is worth a pound of
cure. Eliminating and preventing condensation in the fiberglass will help retain the
insulating values of the fiberglass.
Condensation
Control
One way to control condensation is to exchange the
high moisture content air with air having a lower moisture content. This is commonly
called ventilation. Another method of controlling condensation is through the use of
vapor barriers. Vapor barriers are used to limit the migration of moisture into the
fiberglass and onto the metal sheathing. In the metal building industry, this is
accomplished through the use of different types of facings on the fiberglass insulation.
Vapor barriers do not stop vapor transmission, but they serve to reduce its
movements. Because moisture travels from areas of higher vapor pressures to lower
vapor pressures, the vapor barrier should always be placed at the point of the highest
vapor pressure. This usually means on the inside or the warm side of the structure.
The exception to this is when the interior of the building is a cooler or freezer
and the inside temperature is lower than the outside temperature.
A vapor barrier's effectiveness in controlling
moisture movement is measured by its perm rating. This is a shortened form of the
term "permeance." Permeance is defined as the rate of water vapor
transmission through a material in a given amount of time per unit area. In U.S.
customary units, a permeance of one perm is defined as 1.0 grains of water vapor
transmitted per hour per square foot per 1.0 inch of mercury vapor pressure difference.
The lower the number, the better the vapor barrier. Materials with perm
ratings of 1.0 or greater are usually not classified as vapor barriers. Vapor
barriers come in a wide variety of materials and perm ratings.
A vapor barrier is important because condensation can
appear in two forms. The first is visible condensation, which occurs when
condensation appears on surfaces that are adjacent to the warm side of materials.
One can clearly see this. This type of condensation can form on windowpanes, purlins
or thru-metal fasteners. This type of condensation, while it is a nuisance, is far
less damaging than hidden condensation. This can be somewhat controlled by
additional insulation or by reducing the relative humidity.
The second form of condensation is hidden or
invisible. It can occur inside materials. Some examples of this would be the
moisture that is absorbed by drywall, plywood, ordinary sheathing materials and the like.
Fiberglass insulation will hold moisture either as water or water vapor that has
condensed within the insulation.
Removing the moisture from within the material is a
difficult task. Once the moisture is introduced into the fiberglass blanket,
moisture is trapped between the steel skin and the vapor barrier. The result is that
as the temperature changes inside the blanket, the water will go through a cyclic life of
water vapor and condensed water. The only way that the moisture can be removed is if
the temperature conditions are held long enough for the water to escape the blanket,
either through the vapor barrier or openings in the barriers.
Summary
Because buildings are becoming more thermally
efficient, both through the use of increased insulation thickness and designed to be more
airtight than in the past, the control of condensation should be considered. Quality
vapor barriers and correct insulation should be use to reduce the possibility of hidden
condensation. Buildings should be designed and well constructed to prevent the entry
of exterior water into the structure. If the relative humidities inside the
buildings are high, ventilation should be considered as a method of control. Vapor
barriers will not stop moisture transmission. They will only retard it. One way of
reducing condensation is through ventilation and the reduction of the water vapor in the
air.
Careful planning in the original design and evaluation
of the building's use will save the builder and the owner many problems.
Notes: This Bulletin has been
prepared to assist the metal building industry to understand and deal with condensation.
Its contents are based on information believed to be reliable. However, the
prevention and elimination of condensation depend on the design and construction of the
building for which neither LFIPA nor its members can take responsibility.
Accordingly, nothing in this Bulletin should be regarded as a recommendation concerning
metal building design and construction.
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