How to Prevent Mold in buildings
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How to prevent mold in buildings
Discussion of each major topic in mold prevention:
How to prevent attic condensation & mold
How building leaks cause mold
Building vapor barrier or insulation problems cause mold
Heating & air conditioning systems, ductwork, & mold causation
Air purifiers & mold advice
Links to articles with more details on mold prevention
Questions & answers about preventing toxic or other mold growth & contamination in buildings

This document which explains how to prevent mold contamination or mold growth in buildings, is a section of our Mold Action Plan which provides an easy to understand step-by-step guide for dealing with toxic or allergenic indoor mold and other indoor contaminants: what to do about mold. The steps in this document will be sufficient for many building owners who want to do their own mold investigation, mold testing, mold cleanup, and mold prevention in their home or office. Extensive, technically detailed in-depth articles are also organized at our Mold Information Center.

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How to Prevent Building Mold Contamination, Correct the Causes of Mold and Prevent Indoor Mold or other indoor environment problems

There is no point in "cleaning up" or "removing" mold if the underlying causes of a mold problem are not also addressed. Unless the causes, such as leaks or high moisture are corrected, mold will simply recur. Band-Aid approaches like trying to "kill" or "prevent" mold by using chemical sprays, coatings, or air cleaners are insufficient and ineffective at preventing future mold and in some cases (such as ozone) these approaches create their own hazards.

The following articles discuss measures which will substantially reduce the risk of mold growth in buildings. Don't miss Mold-Resistant Building Practices.

In the photo the contractor is pointing out to the owner that wind-blown rain running down the building's wall will enter the wall cavity at the air conditioner penetration because of improper installation and sealing there. Inside we found that the wall cavity was wet and moldy. Proper installation and sealing at building wall and roof penetrations such as roof vents, windows, doors, and trim can prevent many building leaks which in turn reduces the chances of a future mold problem.

Attic Condensation and Ice Dam Leaks: If roof leaks or attic moisture condensation due to a combination of inadequate attic ventilation and a building moisture source (wet basement, plumbing leaks, roof leaks) causes excessive moisture or actual wet conditions in an attic, conditions are ripe for extensive mold growth. Visible mold may appear on wood surfaces in an attic such as on rafters or roof sheathing.

Hidden mold may be present and may be even more of a problem if it forms in insulation or in the ducts and air handler of an air conditioning or heating/air conditioning system. Typical building air convection currents tend to move air up and out from lower to upper building levels, so one would not think that much mold would move down from an attic into the living area. But important exceptions to this can quickly move problem mold from an attic into a living area. For more details on this topic see Attic Condensation & Ice Dams, Detection and Correcting Venting and Condensation Problems in buildings.

Conditions moving mold downwards from an attic include the following:
- Mold growth in HVAC ducts or air handlers found in an attic
- Mold on any attic surface or in attic insulation if it is a species producing airborne spores and if the building uses a whole house ventilating fan, especially if there is inadequate exit venting for the fan operation. This condition pressurizes the attic and moves mold down through various openings into the floors below.
- Mold on building surfaces in an attic or attic knee wall space which opens onto or has a knee wall common with an upper floor living space such as a bedroom.

Building Exterior Leaks and Mold No mold cleanup project will be successful unless you correct the conditions that caused mold growth in the first place.

An expert inspection and report should find and suggest remedies for site and building exterior conditions that produce mold or for building areas that serve as a mold reservoir or as amplifiers for allergens, mold, mildew, excessive pollen or pet dander.

The basic steps: find all unwanted moisture sources, correct appropriate building, site, landscaping, & construction details. 90% of the wet basements and crawl spaces I see are caused by bad or missing roof gutters and downspouts. An IAQ investigator who has training and experience in building science, mycology (mold science), and IAQ, or in some cases an experienced ASHI-Certified home inspector or sick building investigator who is who has a similar in-depth understanding of construction failures can be helpful at this step.

The photograph shows a double problem with this wood shingle exterior wall: shingles are improperly installed and falling off of the home, inviting water to enter the wall structure. Because there is no roof overhang or eaves, and not even a gutter on the home, roof runoff runs right down this wall.

Building Interior Leaks and Mold: address interior conditions that produce or serve as reservoir or amplifier for common allergens: mold, pollen, pet dander as well as other possible respiratory irritants such as latex, paints, product-outgassing furniture or carpets, and renovation hazards such as lead paint or chemicals or other environmental issues.

Plumbing leaks that are unattended, water running under kitchen or bathroom cabinets when fixtures are in use, and air conditioning system condensate leaks into the building or into the HVAC duct work are examples of indoor leaks that can lead to a mold problem if not promptly attended.

The photograph shows extensive staining from protracted plumbing leaks in a house that was left unattended. The result was extensive basement mold requiring demolition and cleaning of large area of the home.

Indoor Humidity Control: What indoor humidity should we maintain in order to avoid a mold problem?

A maximum indoor relative humidity of 55 should be OK, 50 better, 45 for an attic knee wall space which is not vented to outdoors. The moisture level you detect varies depending on when, where, and how you measure it.

For normal home use we like modern dehumidifiers such as this Sears ™ unit which permit setting a specific humidity level and also permit connecting the dehumidifier to a drain so that it can operate unattended. The article cited at the end of this paragraph explains the need for maintaining an anti-mold low humidity level in a building, sets moisture level targets, and tells you how to get the indoor moisture level into the desired range.

More detail about how to control indoor humidity is at Indoor Humidity Control:

Improper placement of or damage to building vapor barriers, or insulation can cause not only mold contamination in the building (visible or hidden in building cavities, it can also cause substantial structural rot. See ROT, TIMBER FRAME for a case study.
Mechanical Systems (Heating and Air Conditioning Systems) and Mold: HVAC - Heating, Ventilation, Air Conditioning Systems and Duct Work Cleaning and Remediation Issues & Advice about mold are needed from the same expert. I often find that a building owner has paid for "duct cleaning" only to discover that simply opening the air handler unit (AHU) shows a filthy or moldy blower compartment! Thoroughness is important as is the choice of duct replacement materials to reduce the chance of new mold growth and to make the system easier to clean.

Duct Cleaning for mold or bacterial contamination: Leaks into duct work wet organic dust and debris deposited on the duct interior, inviting mold growth. Metal ducts and air handlers can often be cleaned. Our experience is that fiberglass and fiberglass-lined ducts and air handlers cannot be cleaned effectively and worse, improper cleaning may cause dangerous release of fiberglass into the building air.

Don't just spray ducts with disinfectants and sanitizers. The US EPA advises caution about using these chemicals, the AIHA also advises that occupants may suffer from respiratory irritation from these chemicals, and research on fungicidal treatment of fiberglass HVAC ducts suggests that their effectiveness is limited anyway.

The photo shows stains indicating a history of leaks into this fiberglass lined HVAC duct. We recommended replacing the insulation, cleaning the duct interior (or replacing the duct work if that was less costly), and making sure that the leak source was repaired.

Indoor Air Cleaning Products can help improve indoor air quality, but watch out for products that are ineffective.

At least one study has shown that there at least up to 2005 there was not one portable free-standing home "air cleaner" or "air purifier" on the market that moved enough cubic feet of air per minute to actually clean a home much less a room of problem airborne particles.

Portable air cleaners: The photo above shows a popular portable air cleaner in a room where we ran a time-lapse impaction sampler to collect particles at regular intervals to see what the machine seemed to be doing. We compared the particle levels in this room with the door shut, with the door open, and with other building areas.

OPINION: our clients' experience, confirmed by our own field testing suggest that in a room without severe dust and particle reservoirs (no pets, no wall to wall carpets), some air cleaners might reduce, but by no means eliminate, problematic airborne particles. The particle reservoir forms what is in a practical sense, an infinite particle source.

Central HVAC systems, with a much larger air handling capacity, can be equipped with filtration that can substantially reduce the level of indoor airborne particles. Their effectiveness can be increased still further if the system permits the fan and filtration to run continuously.

Air Cleaning Products are discussed and listed further \ here.

Fresh Air Ventilation / Air-to-Air Heat Exchange: contact local HVAC contractors and see products

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These Articles Offer Details on Mold Prevention

Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
Mold proof drywall? You Must Be Kidding!
Moisture control, dehumidification, ventilation: reduce conditions that encourage mold growth
Condensation: Detecting and Correcting Attic Condensation and Preventing Ice Dam Leaks in buildings
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
IAQ or Anti Mold Products:Allergy/Asthma, Air Quality, Cleaning, Filtering, Ventilating, Dehumidification Products
Contact a home inspector or a local de-watering contractor for inspection and advice

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Environmental Health & Investigation Bibliography - our technical library on indoor air quality inspection, testing, laboratory procedures, forensic microscopy, etc.
Adkins and Adkins Dictionary of Roman Religion discusses Robigus, the Roman god of crop protection and the legendary progenitor of wheat rust fungus.
Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
"A Brief Guide to Mold, Moisture, and Your Home", U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
US EPA - Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
US EPA - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol

Allergies, Allergens, Allergy Testing in buildings - References & Products

Air Conditioning System Blower Fans & Filters Cascading for Optimum Indoor Air Quality
Allergen Tests in buildings advice about how to test, what to look for, in evaluating the level of dog, cat, or other animal allergens in a building
"IgG Food Allergy Testing by ELISA/EIA, What do they really tell us?" Sheryl B. Miller, MT (ASCP), PhD, Clinical Laboratory Director, Bastyr University Natural Health Clinic - ELISA testing accuracy: Here is an example of Miller's critique of ELISA http://www.betterhealthusa.com/public/282.cfm - Townsend Letter for Doctors and Patients
The critique included in that article raises compelling questions about IgG testing assays, which prompts our interest in actually screening for the presence of high levels of particles that could carry allergens - dog dander or cat dander in the case at hand.
http://www.tldp.com/issue/174/IgG%20Food%20Allergy.html contains similar criticism in another venue but interestingly by the same author, Sheryl Miller. Sheryl Miller, MT (ASCP), PhD, is an Immunologist and Associate Professor of Basic and Medical Sciences at Bastyr University in Bothell, Washington. She is also the Laboratory Director of the Bastyr Natural Health Clinic Laboratory.
Allergens: Testing for the level of exposure to animal allergens is discussed at http://www.animalhealthchannel.com/animalallergy/diagnosis.shtml (lab animal exposure study is interesting because it involves a higher exposure level in some cases
Allergens: WebMD discusses allergy tests for humans at webmd.com/allergies/allergy-tests
Animal Allergens: Dog, Cat, and Other Animal Dander - Cleanup & Prevention Information for Asthmatics and regarding Indoor Air Quality.
Atlas of Mold Related Illness Symptoms & Complaints - long list of both documented, studied mold related illness, and complaints ascribed to mold contamination or allergens in buildings
Cat Dander: how to inspect and test a building for past or current presence of cats, cat hair, cat dander, and cat allergens
Clinical Atlas of Mold Toxicity - An Online Description of Toxic, Pathogenic, Allergenic Fungi, Fungal Diseases
Fiberglass Insulation Contains Mold© 2005 comments about a field study in process, & more about health hazards from fiberglass insulation
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Mold Action Guide detailed guide on finding, removing, and preventing indoor mold contamination
Odors, Odor Detection, Smells, & Gases how to find and identify sources of noxious or toxic odors and gases
Other environmental risks, Our much longer list: Asbestos, carbon monoxide, electromagnetic fields, etc.
Ozone: The Use of Ozone Indoors for Control of Odors and Mold Removal in buildings: A Summary of Hazards and False Claims.
Pollen Allergens: identification, plant pollen and indoor air quality
Products to Reduce Mold & Allergy Problems to reduce indoor mold or allergen levels: air cleaners, air purifiers, dust mite covers, vacuum cleaners, crawl space vents
Recognizing Allergens: What various indoor allergens look like - identification photos to help identify pollen, dust mites, animal dander, toxic or allergenic mold - Common Mold and other Allergens, Irritants, Remedies & Advice
Rodent control issues, including dander, fecal, and urine contamination of buildings and Building insulation are discussed at our
Sewage and Septic backup contamination in buildings: inspection, testing, remediation, & references to expert sources
Action Guide: What to do about mold, mildew, and other indoor allergens
...

Mold Contamination Testing, Cleanup, Prevention: references & products

The Mold Information Center: What to Do About Mold in buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Aerobiology, Building Science, Microscopy, & Laboratory References, an extensive technical bibliography
Allergens: what they look like in buildings
Associations: Sick House, Sick Building, SBS - Air Quality, Government, Private Associations and Information Resources
Atlas of Clinical Fungi, 2nd Ed., GS deHoog, J Guarro, J Gene, & MJ Figueras, Centraalbureau voor Schimmelcultures, Universitat Rovira I Virgili, 2000, ISBN 90-70351-43-9 (you can buy this book at Amazon) - The Atlas of Clinical Fungi is also available on CD ROM
Atlas of Mold Related Illness: Index of Symptoms and health, physical, neurological, psychological, and other complaint which people suspect may be mold or building-related.
Atlas of Indoor Mold, Online Clinical Mold Atlas, Toxins, Pathogens, Allergens and Other Indoor Particles - Medical Health Effects of Mold (separate online document)
Black Mold that is Harmless Photos of recognizable, usually harmless black mold on wood, bluestain, ceratocystis, ophistoma
Building Floods: quick steps after a building flood or plumbing leak can prevent costly mold contamination
Classes of Mold: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
Clinical Mold References - Detailed bibliography of mold reference texts
"A Brief Guide to Mold, Moisture, and Your Home", U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
"Disease Prevention Program for Certain Vegetable Crops," David B. Langston, Jr., Extension Plant Pathologist - Vegetables, University of Georgia (PDF document) original source: www.reeis.usda.gov/web/crisprojectpages/209797.html
"Disease Prevention in Home Vegetable Gardens," Patricia Donald, Department of Plant Microbiology and Pathology, Lewis Jett
Department of Horticulture, University of Missouri Extension - extension.missouri.edu/publications/DisplayPub.aspx?P=G6202
"Management of Powdery Mildew, Leveillula taurica, in Greenhouse Peppers," Ministry of Agriculture and Lands, British Columbia - Original source: www.agf.gov.bc.ca/cropprot/peppermildew.htm
Environmental Health & Investigation Bibliography - our own technical library on indoor air quality inspection, testing, laboratory procedures, forensic microscopy, etc.
Fiberglass: Mold in Fiberglass Insulation© 2005 comments about a field study in process, & more about health hazards from fiberglass insulation - DJF
Fifth Kingdom, Bryce Kendrick, ISBN13: 9781585100224, is available from the InspectAPedia online bookstore - we recommend the CD-ROM version of this book. This 3rd/edition is a compact but comprehensive encyclopedia of all things mycological. Every aspect of the fungi, from aflatoxin to zppspores, with an accessible blend of verve and wit. The 24 chapters are filled with up-to-date information of classification, yeast, lichens, spore dispersal, allergies, ecology, genetics, plant pathology, predatory fungi, biological control, mutualistic symbioses with animals and plants, fungi as food, food spoilage and mycotoxins.
Fungi, Identifying Filamentous, A Clinical Laboratory Handbook, Guy St-Germain, Richard Summerbell, Star Publishing, 1996, ISBN 0-89863-177-7 (English) (buy at Amazon)
Looking for Mold Procedure: what mold is often found where in buildings - simple technical presentation
Meruliporia: the house eating fungus or "poria"
Mold Action Guide: Step-by-Step Instructions, What to do about mold, mildew, and other indoor allergens
MOLD APPEARANCE - WHAT MOLD LOOKS LIKE Photos of what mold looks like in buildings
MOLD APPEARANCE - STUFF THAT IS NOT MOLD Photos of NOT-mold material that is sometimes mistaken for mold
MOLD ATLAS & PARTICLES INDEX, Pathogens, Allergens and Other Indoor Particles - Medical Health Effects of Mold (separate online document)
MOLD BY MICROSCOPE Mold under the microscope - photo identification of the most common indoor molds found in buildings
Mold FAQs Answers to Most Questions about Indoor Mold, Mold Related Illness, Mold Cleanup, Mold Prevention
US EPA: Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
Mold spores in the Home - a Photo ID Library for detection and identification of mold allergens
Mold Test Kits - How to Collect and Send Your Own Mold Sample to our mold testing lab or to any mold lab you wish
Most Common Indoor Molds Found in buildings, A Table of
Mycology, Fundamentals of Diagnostic, Fran Fisher, Norma B. Cook, W.B. Saunders Co. 1998, ISBN 0-7216-5006-6 (buy this book at Amazon)
Ozone Warnings - Use of Ozone as a "mold" remedy is ineffective and may be dangerous.
Rot concerns in buildings-some building mold such as Meruliporia incrassata "Poria" risks serious rot and hidden structural damage
US EPA: Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol

OTHER IAQ ISSUES: How To Find and Address Other Indoor Air or Indoor Environment Contaminants Besides Mold

Mold or allergens may not be the only or even the main indoor environmental contaminant. Don't let media attention to mold cause so much enviro-scare fear that other, possibly more urgent hazards go un-addressed.

Fiberglass building insulation and HVAC duct work insulation hazards
Sewage and Septic backup contamination in buildings: inspection, testing, remediation, & references to expert sources
Other environmental risks: Asbestos, carbon monoxide, electromagnetic fields, environmental illness, fiberglass, MCS - multiple chemical sensitivity, toxic gases, etc
Indoor Gas Sampling Plan for Residential buildings lists a number of toxic indoor gases which we test for, depending on the building complaint and building conditions
Ozone Warnings - Use of Ozone as a "mold" remedy is ineffective and may be dangerous.
Pet control - if you can't say goodbye to your bird, cat, dog, guinea pig, hamster, tropical fish, then limit the areas they occupy and limit the airflow from that area to sleeping or other areas of the building, use allergenic bedding, eliminate wall-to-wall carpeting, improve housecleaning including use of a HEPA-rated vacuum cleaner. For more details see our article Dog, Cat, and Other Animal Dander - Information for Asthmatics and Indoor Air Quality
Rodents, Mice, Squirrel Control - I find high levels of mouse and rodent dander, fecal dust, and urine-contaminated dust in some buildings, and high levels of these materials in building insulation in those locations. If you have a mouse problem, particularly if mice and their waste (fecals or urine) are contaminating the building HVAC or building insulation, may need both steps to clean up or remove infected materials and steps to stop an ongoing rodent problem. If squirrels are a problem, the cleanup needs to include closing off entry openings into the building. Get some help from a licensed pest control expert.

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How to Prevent Building Mold Contamination, Correct the Causes of Mold and Prevent Indoor Mold or other indoor environment problems

Questions & Answers regarding this article

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Technical Reviewers & References

These Articles Offer Details on Mold Prevention

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Allergies, Allergens, Allergy Testing in buildings - References & Products

Mold Contamination Testing, Cleanup, Prevention: references & products

OTHER IAQ ISSUES: How To Find and Address Other Indoor Air or Indoor Environment Contaminants Besides Mold