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Air and Vapor Barrier Basics

John Poullain, P.E.

Course Outline

This three-hour online course provides guidance on the basics of moisture and condensation problems in buildings and the typical causes. The course discusses how vapor pressure and surface temperatures interact to cause condensation, discusses the relationship between temperature, relative humidity and moisture and shows their combined influences with an ASHREA psychometric chart. The course presents ways to identify the sources of moisture, mold related health effects and how various building wall cavities can protect against condensation. Sources for additional information on the course topics are provided.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Intended Audience

This course is intended for building managers and IAQ professionals.

Benefit to Attendees

The student will become familiar with how climate, season of year, temperature and moisture factor into the development of various wall assemblies for the protection of a building and occupants. The differences between air barriers and vapor barriers are discussed along with their compatibility. Moisture and condensation problems, their causes and the relationship between relative humidity and temperature are also shown. Basic methods for improving indoor air by controlling the sources of water and moisture are provided. Reference sources are provided for additional information pertaining to condensation and indoor air problems, health effects linked to the molds and appropriate steps for reducing exposure of occupants.

Course Introduction

Moisture and condensation problems are often indicated by the presence of mold and mildew. Mold and mildew are different groups of organisms that lack roots and leaves and instead have threadlike hyphae, which gives them a furry or powdery appearance. They do not require sunlight to grow and sunlight will often destroy it. They are members of the fungus family of organisms that feed on dead or living organic matter, which indicates decay or dampness problems. Presence of light does not hinder all molds.

Moisture Condensation and Mold Problems

Mold found indoors and in walls causes unhealthy buildings, health problems to the occupants and causes destruction of wood frame structures, furnishings and building materials. Spore growths can attach to a person's respiratory track causing asthma attacks, allergic reactions and can also cause eye, nose, throat and skin irritations. It should be noted mold problems are only corrected for the short term by removing visible mold. They will reoccur until moisture and water problems, which promote mold growth, are also corrected. Typical sources of unwanted moisture are leaky roofs and plumbing, leaks into the building from cracks, gutters or poorly planned exterior drainage.

Another source of water vapor comes from subgrade concrete walls and slabs. Concrete is naturally porous with networks of very fine capillaries which draw water from the ground by wicking or from hydrostatic pressure. Some vapor barriers are not permanent and will deteriorate from lime leached from the concrete. Unless the barrier is durable, water vapor from the ground can damage moisture-sensitive floor finishes, cause staining, loss of flooring adhesives and mold growth.

Hidden mold is a serious source of contamination and locations that are most likely to grow mold must be recognized. Sources of water or leaking water pipes, especially in pipe chases, should be stopped and repaired to deter future mold growth. The following list of guidelines for dealing with moisture indoors was taken in part from a US EPA publication on IAQ.

1. Potential health effects and symptoms associated with mold exposures include allergic reactions, asthma and other reparatory complaints. There is no practical way to eliminate all molds in the indoor environment
2.Fix the sources of water problems or leaks to stop condensation and mold problems.
3.Reduce indoor humidity (to 30-60%) by using vents, A/C, dehumidifiers, ventilation and exhaust fans.
4. Prevent condensation: Reduce the potential for condensation on cold surfaces (windows, piping, exterior walls, wall cavities, roofs or floors) by adding insulation.
5. In areas where there is a perpetual moisture problem such as drinking fountains, classroom sinks or concrete floor with leaks or condensation, do not install carpeting.
6. Molds can be found almost anywhere; they can grow on virtually any substance, providing moisture is present.

Sources of Moisture Affecting Indoor Air Quality

Today's construction technology has made many homes, offices and school buildings tighter and more energy-efficient than they were 20 - 30 years ago due to better sealing, caulking, and better fitting doors and windows. The use of sheet materials for sheathing, walls, ceilings and interior finishes have reduced the amount of air exchange in walls and ceilings.

Insulation in walls, roofs, under floors and walls, building envelope wraps, have helped to seal buildings more tightly while also trapping air and moisture inside. This has resulted in very air tight buildings and the benefits of better protection from outside weather has become a disadvantage since any moisture generated inside may be trapped inside. Moisture can be generated from occupants exhaling, cooking, and showering and from roof, wall, window and plumbing leaks. Natural ventilation or leakage through loose building structures and windows once helped to remove moisture and improve poor indoor air quality.

A brief list of moisture sources includes:

1. Inadequate HVAC and filtration and outdoor air supply equipment that do not maintain relative humidity in the ideal 30-60% range and/or do not keep inside surface temperatures above the dewpoint temperatures which would prevent moisture condensation on interior surfaces or inside the walls.
2. Humidifiers, fresh air supply equipment, air ducts and air diffusers.
3. Water sources: leaks from pipes, fountains, restrooms, leaks through exterior walls, roofs, cracks in walls and standing water in drain, incorrectly sloped pans and water spills.
4. Moisture sources: exhalation of occupants, unvented showers and dryers, humidifiers (humidifier fever), outside air supply, wet foundations, poorly insulated ceilings, condensation on wall surfaces, room surfaces or within wall cavities.
5. HVAC systems can promote mold growth in the ductwork, outside air supply systems, filters and drain pans and are always suspected mold locations. Electronic equipment like moisture detectors and boroscopes (wall boring and camera devises) can reveal any potential moisture problems and hidden sources of mold growth in construction materials.

Moisture Transport Basics

Any moisture migrating from the inside and into the wall or ceiling during the heating season may condense someplace inside the walls where dew point temperatures are reached. In some cases condensation may be severe enough to also cause certain types of wall insulation to collapse and settle inside the wall cavity. This is usually caused from poorly insulated exterior walls and windows.

Moisture is transported in a building in four ways. The first way includes bulk types that include rainwater and ground water and the second is by capillary actions such as from concrete floor slabs. Measures to control the first two are relatively easy and straightforward. The latter two, air transported and vapor diffusion, are far more difficult to control and are briefly described below.

Air Transported Moisture:

Air transported moisture is a very destructive mechanism. It is a bigger problem than vapor diffusion because diffusion is a slow process and does not contribute the vast amount of moisture that is air transported. Air often has a considerable amount of moisture in vapor form and as it moves from inside to outside or outside to inside in a building the vapor moves along with it. Air movement is caused by air pressure differences from wind or stack effects on the building and also depends on whether it's the heating or cooling season time of year. If a surface within the wall or ceiling cavity is below the dew point temperature, condensation will form, water may drain to the bottom of the wall, mold will grow and insulation and structural damages may occur from repeated cycles. Also condensation may collect behind the siding, absorb chemicals from the siding and stain it. If the siding becomes wet enough the paint will blister.


Vapor diffusion depends on the differences in vapor pressure across materials, on their permeability and usually goes from higher to lower pressures. All materials are vapor permeable to a degree. Diffusion has less of an effect than air transported moisture; failure of a building envelope from vapor diffusion is rare. Paint with a low permeability, oil based paint or several coats of paint can help to retard vapor diffusion.

The basic solutions for moisture problems inside or in walls are:

1. Keep water out; wherever it rains it can get in. Maintain interior relative humidity in the 30-60% range.
2. Let water out if it gets in or condenses. Prevent condensation within the wall but also provide the means to safely drain it away with drainage planes and to permit natural drying to occur.

Wall Assemblies for Condensation Control (Wall Figs 5 to 10)

Wall assemblies are designed to perform several functions primarily for energy efficiency and for weather proofing the building and its interior. The problems caused by condensation and water must also be considered in order to construct a sound building and solve these problems. Condensation in a building or wall will depend on several factors: climate (hot-humid or dry, mixed, marine or cold), exterior and interior temperatures, relative humidity, annual precipitation and if it's the heating or cooling season. Wall systems as shown in the text have been developed to meet various climatic conditions, i.e. hot, cold, very cold or subarctic. Wall systems vary with different types of exterior materials (brick, stucco, siding or precast concrete), wall constructions (wood, steel or concrete block) but similar types of air and vapor barriers and drainage planes are used.

As described below there are several ways to avoid the moisture and condensation effects on insulation. Basically they require insulation unaffected by moisture, stopping moisture migration with vapor barriers and drainage planes of materials which control the release of moisture from the wall before any damage occurs. Condensation is controlled by limiting it to the interior side of the vapor barrier (cold climate) or on the exterior side of the vapor barrier (hot climate). Because indoor air will tend to move into the wall cavity in a cold climate, the cavity temperature must be maintained above the dewpoint. In order to do that additional insulation may be placed exterior to the cavity which will isolate the wall cavity more from cold outside temperatures by pushing the dewpoint exterior of the insulation.

Moisture driven inwards from brick veneer will condense on the vapor barrier. The vapor barrier then serves also as drainage plane and conveys condensation away from insulation. Such walls allow for the draining action and some degree of natural drying by diffusion. A continuous air barrier decreases air movement into walls and closes any gaps or cracks not otherwise sealed. Insulation sheathing in cold climates avoids the very undesirable situation of having a double vapor barrier that will trap moisture within the wall's cavity.

Moisture over time is a very destructive force on walls, more so than mold. Barriers however should be made of non-water sensitive materials to prevent mold growth inside the wall. Mold resistant construction has been developed to withstand destructive moisture events. There are moldproofing construction sprays for pre and post construction applications and can be applied to lumber, particleboard walls, drywall and metal. The sprays seal the material and provide an antifungal property.

Air barriers are distinguished from vapor barriers because they control the flow of air by resisting differences in air pressure. Water vapor will naturally move from higher pressures to lower pressures through air barriers but is resisted by vapor barriers depending on the perm rating. Effective barriers must be installed properly at all doors, windows and any other openings in the building envelope to completely enclose the air within a building. Many air barriers are also vapor barriers and vice versa. Wallboard, plywood and OSB will be more effective if the joints are sealed.


Moisture and condensation are problems that are hard to target because of ever changing types and levels of pollution and the individual's susceptibility and perception of indoor conditions. Operation and maintenance of ventilation systems and moisture and humidity control affect indoor air quality as well as comfort. Because air and surface cleaning alone cannot effectively remove all indoor mold it is controlled by a regime of activities which includes control of moisture and water problems, adequate ventilation and filtration and proper air and vapor barriers installations.

1. Effective source control. Manage the sources of mold growth by removal or control of moisture, water or condensation problems.
2. Adequate ventilation and moisture control. Ventilate the building with adequate ventilation or rates of air exchange.
3. Air filtration. Clean the air and remove mold spores with adequate filtration equipment and filters recommended by ASHRAE.

It has been estimated that people spend as much as 90% of their time indoors at home, office, schools, stores or other commercial buildings. This has become a serious health problem since indoor air quality may be poorer than outdoor air quality. Some people including the young, elderly, those afflicted with respiratory diseases and those with reduced immunity systems are especially susceptible to indoor mold. OSHA estimated 30% of Americans work in buildings, which have some type of air pollution. Asthma attacks are often triggered by mold or mold-laden dust and is estimated as the forth-leading cause of work absenteeism.

The statistics for people affected by allergies, asthma and other respiratory diseases have been increasing for all age groups. Building managers have become aware of the negative effect of an uncomfortable or harmful building environment. Poor indoor air can also impair students' learning ability. Prominent factors of poor IAQ, tobacco smoke and sick building syndrome have received much public attention causing regulatory actions and litigation to push building owners to monitor and improve indoor air quality. In addition to the structural and material damages, mold also can cause serious harm and long term health problems and perhaps may be "the next asbestos".

Sources of Information

1. Vapor Barrier Journal Paper prepared by the Midwest Research Institute for the US Department of Energy, April 2004.
2. Moisture Control for Buildings, ASHRAE Journal, February 2002.

Course Content

This course is based primarily on the publication, "Vapor Diffusion Retarders and Air Barriers" prepared by the Energy Efficiency and Renewable Energy of the US Department of Energy, (2005 Edition, 8 pages), PDF file. The course is also based on the selected parts of the US Department of Energy publication, “Vapor Barrier Journal Paper” (2004 Edition, 18 pages) PDF file and parts of the US EPA publication, "Appendix C: Moisture, Mold and Mildew", EPA 402-F-91-102 (2003 Edition, 3 pages), PDF file.".

The links to the course materials are:

Vapor Diffusion Retarders and Air Barriers

Vapor Barrier Journal Paper

Appendix C: Moisture, mold and Mildew

You need to open or download above documents to study this course.

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Course Summary

This course considers how air and vapor barriers control moisture and condensation in a building, the wall cavities and health benefits for the occupants. The sources of moisture and water problems and the factors of climate, season and humidity are discussed along with guidelines for avoiding conditions causing the condensation. The course also considers mold sources and the health risks of exposure to indoor mold. It presents the moisture conditions that promote mold growth and how to recognize those problem areas in a building. References to informative sources for the topics covered are provided.

Related Links

For additional technical information related to this subject, please refer to:
The site provides an introduction to health and IAQ, sick building syndrome causes, diagnostic quick reference and diagnostic checklists to target the most likely building pollution problems. Information and reference material about IAQ improvement and point source controls are presented.
IAQ topics for buildings, schools, home, FAQ, information and guidance for air quality regulations.
IAQ for large buildings using I-BEAM developed by the EPA to be used by building professionals and others interested in IAQ in commercial buildings. Guidance is provided to prevent IAQ problems and how to solve them when they occur.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.