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Indoor Mold and Moisture Basics

John Poullain, P.E.

Course Outline

This two-hour online course provides guidance on the basics of mold growth in buildings and the sources and typical conditions that cause mold problems. The course discusses how vapor pressure and surface temperatures interact to cause mold, discusses the relationship between temperature, relative humidity and moisture and shows their combined influences with an ASHREA psychometric chart. The course presents methods to identify sources of mold; mold related health effects, ways to avoid mold growth and reduce exposure to mold and how various building wall cavities can be protected. The course has references to informative sources in the text topics.

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, mold remediators and IAQ professionals.

Benefit to Attendees

The student will become familiar with the causes of mold, health risks, and people who are more susceptible and steps to take for indoor mold problems. Information is provided on how mold problems are identified and basic methods for improving indoor air by controlling the sources of water and moisture. Reference sources are provided for additional information pertaining to indoor air problems, health effects linked to the molds and appropriate steps for reducing the exposure to occupants.

Course Introduction

Molds and mildew and fungi are different groups of organisms that lack roots and leaves and instead have threadlike hyphae, which gives them a furry or powdery appearance. Along with yeast, rusts and mushrooms they are members of the fungus family of organisms which feed on dead or living organic matter, which is often a sign of decay or dampness. Molds do not have the characteristics of either animals or plants although the slime mold demonstrates animal like movements by flowing over and engulfing its food. Molds are not photosynthetic, a few plants have lost this capability too, they do not require sunlight to grow and sunlight will often destroy it. Presence of light does not hinder all molds.

Mold and Mold Spore Basics

Mold is important for breaking down matter and can be found anywhere, outdoors and in buildings. They provide many useful products, foods, drinks and medicines including cheese, yogurt, wine, beer, sour cream, buttermilk and pickled meats. Mold found indoors however presents health problems to the occupants and causes destruction of furnishings, building materials and wood frame structures. Some species produce mycotoxins or mold toxins. 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. Mold spores can survive in cold climates and dry conditions but will not grow unless moisture is present.

The most important ingredient for mold growth is moisture or water. Indoor mold needs a nutrient and organic material to grow. Mold grows on surfaces in threadlike masses and can produce thousands of spores in less than a week. The spores may move about by air currents or by sticking to animals, insects or water. It will grow on wood, insulation and dirt when moisture is present. Since moisture is necessary for mold growth, typical sources of unwanted moisture are locations that have leaky roofs, and plumbing, leaks into the building from cracks, gutters or poorly planned exterior drainage. Because mold doesn't need daylight, hidden mold is a serious source of contamination and locations that are most likely to grow mold must be recognized. A building having visible mold, water damage, or moldy odors should be assessed without delay. Sources of water or leaking water pipes, especially in pipe chases, should be stopped and repaired to deter future mold growth.

The US EPA has several publications on IAQ and mold. The following list of guidelines for dealing with mold problems, "Ten Things You Should Know About Mold", was taken from their publication, "Mold Resources".

1. Potential health effects and symptoms associated with mold exposures include allergic reactions, asthma and other reparatory complaints.
2. There is no practical way to eliminate all mold and mold spores in the indoor environment: the way to control mold growth is to control moisture.
3. If mold is a problem in your home or school, you must clean up mold and eliminate sources of mold.
4.Fix the source of the water problem or leak to prevent mold growth.
5.Reduce indoor humidity (to 30-60%) to decrease mold growth by using vents, A/C, dehumidifiers, ventilation and exhaust fans.
6. Clean and dry any damp or wet building materials and furnishings within 24-48 hours to prevent mold growth.
7. Clean mold off hard surfaces with water and detergent and dry completely. Absorbent materials that are moldy may need to be replaces.
8. Prevent condensation: Reduce the potential for condensation on cold surfaces (windows, piping, exterior walls, roof or floors) by adding insulation.
9. 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.
10. Molds can be found almost anywhere; they can grow on virtually any substance, providing moisture is present.

Sources of Mold Affecting Indoor Air Quality

Today's construction technology has made many homes, offices and school buildings more energy-efficient than they were 20 - 30 years ago due to better sealing, caulking, and better 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, exterior to 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 is now 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 once helped to remove moisture and improve poor indoor air quality. Much of today's construction uses particleboard, drywall and other cellulose materials that easily support mold growth.

A brief list of mold source includes:

1. HVAC problems:

a. 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 the inside surface temperatures above the dewpoint temperatures to prevent moisture condensation.

b. Mold contaminated HVAC systems, humidifiers, fresh air equipment, and air ducts and air diffusers.

2. Water sources: leaks from pipes, fountains, restrooms, leaks through exterior walls, roofs, cracks in walls and water in drip pans and water spills.

3. 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.

4. Outdoor mold spores, which infiltrate the building or are discharged by mechanical ventilation using outdoor air supply.

Visual Inspection for Mold

The mold situation in a building can be determined by a thorough visual inspection. A visual inspection may be combined with surface and/or air sampling as deemed necessary to accurately assess a mold problem. Visual inspections should consider:

a. Mold growth and locations. Mold should be visually assessed first for signs of growth and any possible hidden mold should also be considered. If hidden mold is overlooked it will add to poor air quality later. And some molds, light colored as opposed to black molds, can be overlooked and not sampled.

b. Sources of moisture and/or liquids. The extent of any water damage to building materials such as ceiling tiles, dry wall, cardboard as well as furnishings and the ventilation system should be visually inspected. 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 hidden sources of mold growth and potential moisture problems in construction materials and determine the extent of contamination without the destructive exploration of wall cavities.

Moisture Transport Basics

Mold may be indicated on room surfaces but may also be growing inside wall and ceiling cavities. 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 and cause mold problems. In some cases condensation may be severe enough to also cause certain types of wall insulation to collapse and settle inside wall cavities. 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 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.

Air Transported Moisture:

Air transported moisture is a very destructive mechanism. Air transported moisture may be a bigger problem than vapor diffusion because diffusion is a slow process and does not contribute the vast amount of moisture as air transported. Air often has a considerable amount of moisture in a 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 and depends on whether it's the heating or cooling season time of year. Pressure differences may result from wind or stack effects on the building envelope. If a surface within the wall or ceiling cavity is below the dew point temperature, condensation will form, mold will grow and insulation and structural damages may occur from repeated cycles. Condensation may collect behind the siding and absorb chemicals from the siding and stain it. If the siding may become 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 can help to retard vapor diffusion.

The basic solutions for mold and moisture problems indoor or in a wall are:

1. Keep water out; wherever it rains it will get in. Maintain RH 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 and for natural drying to occur.

Wall Assemblies for Condensation Control (Wall Figs 8, 9 and 10)

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 such as shown on figures 8, 9 and 10 have been developed to meet the climatic conditions, i.e. hot, cold, very cold or subarctic.

Condensation is controlled by limiting it to the interior side of a 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 the 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 helps to close 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 a wall's cavity.

The barrier must be made of non-water sensitive materials and not promote mold growth in side 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. 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 are effective if the joints are sealed.


Indoor air quality is a complex problem which is hard to target because of ever changing types and levels of pollution and the individual's susceptibility and perception of indoor conditions. Indoor air quality is affected by mold sources, operation and maintenance of ventilation systems and moisture and humidity control. Because air and surface cleaning alone cannot remove all indoor mold, mold spores are most effectively controlled by a regime of activities which includes control of moisture and water problems, adequate ventilation and moisture control and adequate air filtration.

1. Effective source control. Manage the sources of mold growth by removal or control of moisture or water 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 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. Mold also is perceived to 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 US EPA publication, "Appendix H - Mold and Moisture", EPA 402-K-95-001, (2005 Edition, 6 pages), PDF file. The course is also based on part of the US EPA publication, "Appendix C: Moisture, Mold and Mildew", EPA 402-F-91-102 (2003 Edition, 3 pages) and also Wall Figures 8, 9 and 10 from the US Department of Energy publication, "Vapor Barrier Journal Paper" (2004 Edition, 3 pages) PDF file.

The links to the course materials are:

US EPA publication, "Appendix H - Mold and Moisture", EPA 402-K-95-001

US EPA publication, "Appendix C: Moisture, Mold and Mildew", EPA 402-F-91-102

US Department of Energy publication, "Vapor Barrier Journal Paper"

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

Course Summary

This course considers mold sources and the health risks of exposure to indoor mold. It presents the conditions that promote mold growth and how to recognize those problem areas in a building and actions to take if indoor mold is a problem or you suspect it may have affected the health of the building occupants. Informative lists for the sources, effects on health, and necessary steps to control and reduce exposure to indoor mold.

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 for building professionals and other in IAQ for commercial buildings. Guidance with module presentation for IAQ problems and how to solve them.

Mold/Moisture Problems in Warm Climates


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.