Mold   In  Your  Home

By Douglas Pencille


Introduction | Brief History of Home Construction | What Is Mold? | Ventilation & Air-Flow Basics |
Potential Mold Producers | Strategies For Reducing Mold Growth | Selected References



1. Introduction
Increasing attention is being focused on the importance of good indoor air quality and its affect on health.  The EPA has named indoor air pollution as one of the most urgent national problems (see Industry Snap-Shots).  While many factors can contribute to poor indoor air quality, much attention has recently been given to the detrimental affects of mold.  A recent Mayo Clinic study published in 1999, cited mold as a leading cause of many respiratory problems, and a contributor to nearly 100% of chronic sinus infections.

In a recent study by the Canada Housing and Mortgage Corporation, Don Fulger, a building science researcher, evaluated 400 houses chosen at random in Ottawa.  "We found that 50% of the basements had evidence of moisture damage at some time," said Fulger.  "Wherever you had chronic wetting, you had mold.   Wherever you had mold, you had at least one toxigenic mold."

As a professional home & commercial building inspector (and previously as a building contractor) I have noticed, with what I believe to be increasing regularity, evidence of mold and mildew inside of homes.  For example, I am often called upon to assist in the design, layout, and implementation of renovations in homes.  It is not an uncommon occurrence to find mold inside a wall cavity as a new window is installed - many times in homes as new as one or two years.  I have also closely followed older homes, as they have been upgraded step-by-step, in a homeowners effort to make them more energy efficient.   As new siding, windows, doors, and insulation are installed, what were once "loose" houses that breathed, have become "tight" houses virtually overnight with very little fresh air flow. 

The following article is a condensation of a series of seminars and workshops on "Mold In Your Home", that was presented by DSP in 1998 and 1999, to a wide variety of construction and real-estate related professionals.

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2. A Brief History Of Home Construction - Methods & Materials
Previous to World War II, there was not much change in the way houses were built.  Of course, indoor plumbing, central heating, and electricity were added during this period and became the norm, but the basic building materials and overall design philosophy remained basically the same.  For the most part, dwellings were constructed out of the native materials available in and around a very localized area. 

Considerable changes occurred in building practices after World War II. The invention of modern adhesives and synthetic plastics offered new and exciting materials such as plywood, particle board, affordable (synthetic) carpeting, micro-laminated beams capable of spanning great distances, caulk, etc.  Also, better plumbing and upgraded electrical service greatly added to the comfort and safety of a home.   Still, while great changes and upgrades in building materials were made during this period, much of the overall design philosophy of homes remained the same.

With the energy crisis of the 1970's, an entirely different building design philosophy began to take hold.  In order to save energy, building "energy efficient homes" was the craze, and several government-subsidized  weatherization programs were instituted virtually overnight.  In the zeal to button up every crack, we sealed our homes without thought of the importance of fresh air and proper ventilation.

Many of the newly invented building materials in use, have ingredients that "out-gas" (give off fumes).  The new, tight, energy efficient homes we started to build did not allow for the escape or dilution of these indoor pollutants, thereby creating a whole host of problems.   Also, hidden moisture problems due to overly tight construction (inside wall cavities, attics, etc.) began to surface, which were leading to an increasing incidence of mold growth, related health problems, and deterioration of the affected building materials. 

Tight, energy efficient construction is an excellent idea, and saving energy is also a good idea.  But fresh air and no interior moisture problems are even better ideas!  All three of these concepts can be employed together, if good building practices and proper planning is done.  For example, submarines and space-capsules are among the tightest and best insulated structures ever built, yet they have very good air quality.  NASA and the US Navy have both used healthy materials combined with good air-purification techniques.   Obviously, when building a home you cannot afford, nor do you want to build a space-capsule, but some of the same basic design philosophies can be adopted.



3. What Is Mold?
Mold is one of a wide variety of biological contaminants that can be found in a home, and which can potentially cause health problems.  Some of these include - algae, viruses, pollen, dust mites, dander, bacteria, and fungi (includes yeast, mold, mildew, mushrooms, etc.).  Shown below are three of the more common molds that can be found in homes:

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Aspergillus - Can infect the entire body of an individual, especially if the person has lung damage or another serious underlying illness.

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Cladosprium - Typically forms on surfaces where water condenses (windows, doors, etc.).  Can cause mild allergic responses in some individuals.

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Stachybotrys - Presents the most serious potential health threat.  Can cause internal bleeding of the lungs, and pulmonary hemorrhage.  Has been implicated as one of the causes of "sick building syndrome".
While many forms of mold can present health risks, Stachybotrys chartarum rises to the top as presenting the most serious risk to humans and animals.  In 1993-1994, it was linked to an unusual outbreak of pulmonary hemorrhage in infants in Cleveland, Ohio, where stachybotrys was found growing in the homes of the sick infants.  Due in part to this incident, the medical community has an increased awareness of the potential dangers of molds in homes.   For example, in 1997, the Journal of the American Medical Association carried a news article entitled, "Floods carry potential toxic mold disease".  For some good  information on stachybotrys mold, check out
Mold needs five basic ingredients in order to grow:
  • Food Source
  • Air
  • Surface to grow on
  • Suitable temperature
  • Moisture
If any one of the above is removed, all growth will subsequently stop.  However, mold can lay dormant for many years, and if the above are re-introduced it can continue to grow again.  This is why it is so important when remediating a mold problem to:    1). identify the source and cause,   2). repair the problem,   and    3). remove or kill as much of the mold as possible.


4. Ventilation & Air-Flow Basics  -   Its Effect On Mold In The Home

Over-Pressurization | De-Pressurization | Upgrading An Existing Home

tighthouse.gif (18241 bytes) Most newer homes today are constructed using materials and methods that make them very air-tight.  Many older homes that have been upgraded by installing new energy efficient windows and doors, siding, insulation, etc., can become "tight" virtually overnight.  The effect can be like a giant plastic bag placed over a house, where no air can enter or escape.

Both over-pressurization and de-pressurization of the interior of a house can

lead to inner wall and ceiling damage.  Many appliances such as wood burning fire-places, may not operate properly due to drafting problems, etc.  One characteristic of air-flow and   pressure-related moisture damage to a house and its components is that the damage being caused is often hidden from view, and not always readily visible.  Since many of the problems occur inside a wall or ceiling cavity, they can continue un-diagnosed for a long period of time.



Over-Pressurization can take place when more air is introduced into certain areas of a "tight house", than can escape through normal leakage or through duct-work, etc.  The inside of the house will be at a positive pressure, relative to the outside. Over-pressurization can be localized on a room-by-room basis, or it can be house-wide.

Some ways in which a house can be over-pressurized are:

  • Insufficient return-air venting.
  • Leaky heating & air-conditioning ducts.
  • Closed doors.
  • Upgrading the exterior "shell" (siding, windows, doors, soffits, etc.), in an older home.

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A potential problem with an over-pressurized house is that moisture laden air can be forced from the living space, into the wall or ceiling cavity.  If the "dew point" (the temperature at which air is saturated with moisture - 100% relative humidity - and below which, condensation will occur) is reached inside the wall cavity, condensation will be released into the wall (see diagram below).  A perfect breeding ground for mold!  For a more detailed description of over-pressurization go to "The Basic Dynamics of Over-Pressurization and De-Pressurization".


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Click on the thumbnail image above to see an expanded view of how excess moisture collecting inside a wall cavity can cause wood decay & mold growth. 

The house shown in the photograph on the left, was only several years old when the owner called me for advice. He was noticing some staining on the drywall along the inside walls, and the paint was starting to peel and blister on the outside siding.  When I tested the moisture levels of the interior of the wall cavity using a digital wall probe, the levels ranged from 15% - 31%.   After having the drywall removed, I found mold already in advanced stages of growth along the OSB sheathing (shown in picture).  The fiber-glass insulation was wet with mold growing in it, as well.  All the damaged insulation had be replaced but fortunately, no lasting damage had been done to the structural components.  All visible mold growth was removed, and the affected areas completely sanitized. 

The likely culprit here?  The homeowner had finished off his own basement, but had forgotten to install return-air vents and a proper vapor barrier along the outside walls. Over-pressurization due to a lack of return-air vents forced moisture-laden warm air into the wall cavities where it condensed, causing significant damage in a short time.



De-pressurization can take place when more air is exhausted from a house than is returned back into it.  The inside of the house will be at a negative pressure, relative to the outside.   De-pressurization can be localized on a room-by-room basis, or it can be house-wide.

Some ways in which a house can be de-pressurized are:

  • Wind blowing over & around house.
  • Mechanical exhaust venting (clothes dryer, range hood fan, bathroom exhaust fan, etc.).
  • No combustion air venting present for furnace, water heater, fireplace, etc.
  • Leaky heating & air-conditioning ducts.
  • Closed doors.

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Some potential problems with a de-pressurized house are:   provides the potential for "back-drafting" down chimneys of fuel-burning appliances creating a carbon monoxide risk; can increase the radon levels in a home; moisture laden air can be drawn from the outside, into the wall or ceiling cavity; etc. 

If the "dew point" (the temperature at which air is saturated with moisture - 100% relative humidity - and below which, condensation will occur) is reached inside the wall cavity, condensation will be released into the wall (see dew point diagram above).  A perfect breeding ground for mold!  For a more detailed description of over-pressurization go to "The Basic Dynamics of Over-Pressurization and De-Pressurization".



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Click on the thumbnail image above to see an expanded view of how a clothes dryer exhaust can de-pressurize a house.  In this case, causing back-drafting of the water heater chimney vent.



De-pressurization can affect a structure and its inhabitants due to  the hidden moisture potential it can create inside wall and ceiling cavities.

De-pressurization has also been shown to be a significant factor in increasing the radon levels found in many homes. Radon gas will be have more of a tendency to be drawn into the house, due to the differences between inside and outside air pressure.  See Other Services for more information on radon.



Upgrading An Existing Home

Many homeowners living in older houses want to protect their investment by upgrading it, when and where they can.  Some of the most frequently upgraded areas are: permanent siding along with new soffits & fascia; new shingles; new energy-efficient windows and doors; upgrade attic insulation; etc. This is all very commendable, but watch out!  Many problems I see on a daily basis when inspecting homes, are directly due to one area of a home being upgraded, but a homeowner or contractor neglecting to see how the upgrade would impact another area.
Something you need to be aware of before making upgrades to your house is that your house is made up of many different systems (heating, cooling, electrical, ventilation, etc) all working together to perform their intended functions.   A critical mistake many specialty contractors and homeowners make is not to view a house as a whole.  Very seldom can you upgrade or make changes in one area of your home, without it impacting another area.

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For instance, a homeowner recently spent a bundle making upgrades to his house.  He completely re-sided with new steel siding, new aluminum soffit and fascia, new shingles, upgraded attic insulation, and all new "top of the line" energy efficient windows and doors. . .spending in excess of $25,000 in the process.   Starting with the very first winter after the upgrades were made he began experiencing water staining on his ceilings, rusting on his ceiling light fixtures, and water running out of his wall receptacles.  He put up with it for one year, but mid-way through the second winter, he called me for help in diagnosing his problem.

Listed below are some of the problems I found:

  • The upgrades made to the house had the effect of tightening up the exterior "shell" to the point that what was once a "loose house" that breathed  -   was now a "tight house" that didn't, virtually overnight.
  • No extra attic or roof ventilation was installed to compensate for the possible increase in moisture being forced up into the attic (moisture that used to leak through windows, walls, etc.).
  • When the new metal soffits were installed, the contractor did not align the new soffit vents with the old, thereby greatly restricting air-flow.
  • No fresh air intake was installed in the furnace room, to assist in balancing indoor and outdoor air pressure.
  • Mold growth was present on the roof decking & rafters, with water damage to the insulation & drywall ceilings.  (To see a photo of the damage done in attic of this house, see the bottom picture under the section "Over-Pressurization"   -  "The Basic Dynamics of Over-Pressurization and De-Pressurization" ).

What everyone forgot about when they upgraded the exterior "shell" was, they also should have considered:  1). upgrading the attic & roof venting, and   2). upgrading fresh air (combustion air) capacity.



Achieving a "balance" between the indoors and the outdoors, which will in turn create a "neutral air pressure" in the home, is a key element in reducing moisture problems inside walls and ceiling cavities, and resultant mold growth. 

Two basic concepts are key, in understanding the potential problems associated with improper air-flow and pressurization inside a home:  1). Warm air becoming cold is almost always bad (as it will allow for condensation to occur), and   2). Cold air becoming warm almost never creates a moisture problem.


5. Potential Mold Producers & Hidden Incubators

Micro-Climates | Basement Water Leakage | Sump Basket & Drain Tile

AC Units & Humidifiers | Bathrooms


Micro-Climates In A Home

Micro-climates, and the higher humidity levels they typically have, can be major mold producers in a home. Humidity levels in a house are a key element in reducing and/or controlling the growth and spread of mold and mildew.  The typical comfort range for indoor humidity in northern climates is approximately 30% - 40%, in the winter months.  While indoor "core" humidity levels can be kept relatively stable, and easily sustained at or about 40% RH (relative humidity), "micro-climates" can easily double this.

Please refer to the diagram below, which shows some "micro-climates" that can be found in a typical home.  Click on an item number for more in-depth information about a particular micro-climate:

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Basement Water Leakage

If your house has a basement, then basement water leakage and excess basement humidity levels are likely the number one contributors to mold growth in your home.  It has been documented that at least "98% of all sub-surface basement areas will leak, at some point in their life."  In the over 5,000 home and commercial buildings I have inspected over the years, I certainly have found this statement to be true.

There can be many reasons for a damp or leaky basement (for more information on basement water problems, check out "Basement Water Problems" under Home Resource Materials), and they all add up to problems for you as a homeowner, if not corrected.  Actual water leakage should not be confused with the high relative humidity typically found in basements.   Both can pose serious problems, but need very different approaches to affect a cure.

Moisture trapped between the foundation wall and the finished basement wall can allow mold to grow unnoticed. Long-standing moisture problems can lead to decay, resulting in structural damage to the house.

Please click on the thumbnail images below to expand to full view.
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View of an exterior basement wall, where water had been leaking in unnoticed, from a window well.  All the insulation and damaged framing had to be replaced, extending 10' on either side of the window.

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View of an exterior finished basement wall, where a long-standing exterior drainage issue was left un-corrected.   The entire basement had to be gutted, in order to fully remove all the mold growth from the home.

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View of mold growing on an unfinished block foundation wall.  Normal condensation on perimeter walls due to temperature differences, can lead to extensive mold/mildew growth.  Some type of de-humidification in a basement is a must.


Sump Basket & Drain Tile Systems

The perimeter drain tile system underneath a basement floor and the sump basket it drains into, both serve a very useful purpose. This system provides a key element in helping to protect your house from water damage, by providing an artificial "low level" drainage and collection point under your basement floor.  If the system is working properly, water should drain from the tile into your sump basket.  You can then pump the excess water collected in the sump basket, outside.

Since by its very design it is intended to carry and hold water, this system also provides an ideal breeding ground for bacteria, molds, and mildew.

Please click on the thumbnail images below to expand to full view.

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Water seepage into the basement level can saturate the foundation walls, basement floor, and finishes.  Also, the drain tiles can hold moisture for a long period of time, allowing mold to grow unnoticed.

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Water is often left standing inside a sump basket, providing an ideal breeding ground for mold.  The sump basket covers are frequently left open and due to normal pressure differentials, air from inside the drain tiles can be "pumped" throughout the house.


Air-Conditioners & Whole House Humidifiers

The condensate drip-pan, located on top of the furnace, can hold moisture for extended periods. Also,  the older drum-type furnace humidifiers can provide the necessary ingredients for mold growth.  In both of these systems their location is in very close proximity to an air-handling unit, which can allow mold spores to be quickly distributed throughout a house.
Please click on the thumbnail images below to expand to full view.

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The AC condensate tray is very difficult to access, consequently it is very rarely cleaned.

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Older rotating drum-type furnace humidifiers are notorious for providing harborage for molds.


Bathroom Related Mold Problems

Many times, mold growth can a direct result of poor maintenance. For instance, if the homeowner had maintained his ceramic tile (photo below left), he likely would not have incurred the expense of having to replace his shower.  Also, I wonder how long the mold on the drywall had been growing, and what health effects he may have been experiencing as a direct result of it?

The photo below right shows an infrequently used shower, with the shower head encrusted with mold. Very little effort could be expended in order to keep this fixture clean.

Please click on the thumbnail images below to expand to full view.

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View of damaged ceramic tile inside a shower stall, that has been leaking through to the drywall behind it.  Extensive mold growth and wall damage required that a completely new shower be installed.

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View of mold growth inside an infrequently used shower head.


6. Strategies For Reducing Mold Growth In Your Home

Tips On How To Protect Yourself | Four Steps To Reduce Mold Growth

Some molds produce mycotoxins, and other active compounds that can be injurious to human (and animal) health.  In parts of Eastern Europe in the mid-1930's there were reported outbreaks of a new disease that affected horses and other livestock.  Subsequently, it was documented that mold in the hay fed to these animals was the likely culprit.  There have also been reported cases through the 1940's, where humans who handled hay infested with mold, or slept in mattresses filled with moldy hay, suffered some of the same symptoms as the livestock.

One type of mold, stachybotrys, presents a special health risk. If areas contaminated with stachybotrys are discovered in your home, or if you find areas in your home that are contaminated with mold and are unsure what type it is, don't attempt to remove it without first following the recommended safety procedures for persons working with toxic molds (i.e. use a respirator, gloves, proper covering for the skin & eyes, etc.).  I would also recommend that you get advice from a certified hygienist, or from your local Health Department.  You can contact the American Industrial Hygiene Association for referrals, at (703)849-8888.


Some Tips On How To Protect Yourself

  • Be aware of any noxious odors.   When a mold colony is growing, it can give off distinctive odors that are the by-product of its metabolism.
  • Be familiar with the symptoms of a mold-related illness. Different mold species can produce different health effects.  Some of the common symptoms of mold allergy include a runny nose, itching of the eyes & nose, congestion, fatigue & general malaise.  Ask yourself, "Do my symptoms get better when I'm away from home for any length of time?".
  • Perform a visual inspection of your home.  Check out some of the more common places where mold could be growing in your home.  Look for any signs of past water leakage, check under sinks, in and around bathrooms, check out your basement (don't forget to look at your sump basket!). If you don't have a sealed cover over your sump basket, get one.
  • Contact a heating/cooling specialist to inspect and clean out your AC condensate tray, clean out and service your furnace humidifier, clean out and service the drip-pan under your refrigerator, etc.
  • Allow for good air flow throughout the interior of your house.  Keep furniture away from walls. Allow for adequate air-flow over your windows & doors, as they will be the largest condensing surfaces in your house and air-flow is needed to help evaporate moisture buildup.
  • Check your attic (roof decking, rafters, etc.) for any signs of darkening that could indicate mold growth.   If you notice any suspicious areas, contact a ventilation specialist for advice.
  • Check for any discoloration on walls or ceilings.  Often, this can be an indicator of moisture buildup, insulation problems, or ventilation problems that could be associated with future mold growth.
  • Be on the lookout for signs of excess moisture.  Do your windows frequently have moisture on them in the winter time?  Excess moisture is the single largest contributor to mold growth.
  • Is your heating/cooling duct work clean?  When was the last time you had your heat vents professionally cleaned and sanitized?


Four Steps To Reduce Or Eliminate The Threat Of Mold Growth

John Bower, an expert in the field of designing healthy homes, has written a book entitled, "The Healthy House".   It is recommended reading for anyone concerned about indoor air quality and healthy home design.  In his book, Mr. Bower outlines four basic principles for healthy home design, which can be modified somewhat, and adapted as strategies to reduce and/or) eliminate mold growth in your home.

The following four steps provide a basic outline to follow, as a starting point.  It is important to note that no two homes are exactly alike, therefore no two mold problems are exactly alike, therefore no two strategies for reducing mold in a home are exactly alike.

1.  Elimination
  • Identify the source of the problem and its cause.  Try to a devise a repair strategy.
  • Remove or substitute any damaged or otherwise affected area (carpeting, wood products, etc.).  Sanitize all areas where mold was growing. Use caution when dealing with molds, as some may be very toxic!
  • Eliminate any one (or all) of the five ingredients necessary for mold growth (food source, air, moisture, suitable temperature, and a surface to grow on).
2.  Separation
  • Separate the problem areas from the living space by building a barrier between you and it.  The reasoning is: "If a pollutant can't reach you - it can't hurt you".
  • Paint, seal, and disinfect mold & mildew stains on walls, ceilings, and window & door frames.
3.  Ventilation
  • Ensure that you have adequate ventilation that provides a proper exchange of air between rooms - and indoors & outdoors.
  • Consider installing an air-exchanger or heat recovery ventilator.
4.  Filtration
  • Filtration rarely works well, on its own.  It is best when used in conjunction with one (or all) of the other three steps outlined here (elimination, separation, ventilation).
  • Try to eliminate any problems you may be experiencing using one of the other principles, first.



Selected References
Anachem, Inc. and Sandia National Laboratories.  Indoor Air Quality Handbook.  Albuquerque, NM. Sandia National Laboratories, September 1982.  #SAND82-1773, UC-11.
Bower, John.  Understanding Ventilation: How to design, select, and install residential ventilation systems.   Bloomington, IN. 1995.
Bower, John.  The Healthy House.   Bloomington, IN. 1989.
Environmental Protection Agency (EPA).  The Inside Story: A Guide to Indoor Air Quality.  Washington, DC:EPA, September 1993.  #EPA/402-K-93-007.
Environmental Protection Agency (EPA). Home Buyer's and Seller's Guide to Radon.   Washington, DC, EPA, March 1993.   #402-R-93-003.
Jantunen, M.J.   "Does moisture condensation in air ducts promote fungal growth?"  (Toronto: Proceedings of the 5th International Conference on Indoor Air Quality & Climate, Vol.2, 1990).
Lstiburek, Joseph and John Carmody.  Moisture Control Handbook: Principles and practices for residential and small commercial buildings.   New York:Van Nostrand Reinhold, 1993.
Mann, Arnold.  Mold: A Health Alert.   USA Today. December 3-5, 1999.
National Association of Home Builders (NAHB),   Controlling Moisture In Homes.  Washington, DC: NAHB, 1987.
Nelson, Berlin.  Stachybotrys chartarum: The Toxic Indoor Mold.  1999. The American Phytopathological Society.
The Journal of Light Construction. Moldy Basements Blamed for Infant Deaths.  Washington. DC  Vol. 18, No. 3, December 1999.
Traudt, Jon.  Control of Indoor Air Pressure for Protection of Health, Preservation of Buildings, and Conservation of Energy.   Atlanta, GA: Paper presented to the 16th World Energy Congress and 3rd Environmental Exposition, October 26-28, 1993.


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