Photograph of mold found behind paneling in a bathroom.How to Find Mold Contamination in buildings

  • FIND MOLD, ESSENTIAL STEPS - CONTENTS: How to find hidden or less obvious mold contamination sources in buildings. Mistakes to Avoid when Locating Mold for a Mold Cleanup Project. What a Mold Inspector and Mold Test Consultant Need to Know. Essential Steps to Follow When Screening or Testing a Building for Mold
  • POST a QUESTION or READ FAQs about how to find & test for mold contamination in buildings

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How to find indoor mold contamination sources: This document is the starting point of our mold guide that describes the essential steps needed when searching for problematic mold in buildings. This article 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 FIND MOLD: How to Inspect Homes and Other buildings for Mold - the Basics of How to Find Problem Mold Indoors

Photograph: mold hidden behind basement wall panelingMold Testing & Cleanup Mistakes to Avoid

Much too often we are contacted by a property owner who has just paid a substantial sum to "remove black toxic mold" from a building only to learn that conditions in the property are no better, sometimes worse, than before. What went wrong with the mold cleanup project?


  1. Incomplete mold cleanup projects: The owner or mold remediator was quick to address a visible mold problem without first making sure that all of the important mold reservoirs in the building had been found and listed for cleanup as needed.

    The result unfortunately is that another cleanup may be needed. Worse, we sometimes find people addressing the wrong mold problem in a building, for example scrubbing off cosmetic black mold that was harmless while leaving a real problem with mold-contaminated building insulation that looked "ok" to the eye.
  2. In-valid mold testing: relying on a quick air test, swab, or culture to locate the problem mold or to pass a post-mold-cleanup clearance test without including a decent inspection of the premises, the remediation containment system, checks for cross contamination, etc.

    Sometimes this happens because the same people are providing the mold assessment, the mold cleanup, and the post-cleanup inspection and testing. These conflicting interests are risky for everyone concerned.
  3. Questionable mold cleanup procedures: were followed such as inadequate dust containment, or reliance on simple biocide sprays where physical cleaning or removal of moldy materials was needed.

What does an Effective Mold Investigator Need to Know?

  1. Building science: where do leaks and moisture problems occur in buildings, how do air and moisture move in a building
  2. Mycology: what molds are harmful, which are cosmetic, how, when, where, and why does problem mold grow n a building
  3. Industrial Hygiene or Sampling Methods: what sampling and test methods are sound and produce valid results, and which are magic and speculative.
  4. Field Investigation & Mold Test Lab Experience: Where do problems occur in buildings, how do we find them, what test methods are valid, what do labs do with mold samples, what do lab results mean, how do we interpret "mold counts" or other measures of moldiness in buildings?
  5. Listening & looking skills, writing skills: listen to the experience and concerns of the occupants, owners, clients; look carefully everywhere at the site. Write down mold investigation findings and recommendations that are sound, clear, and easy to follow.

Essential Steps in Screening a Building for Harmful Mold Contamination:

  • If you don't see water stains don't assume there was never a flood or a leak.
  • If a building has had flooding, plumbing leaks, roof leaks, A/C condensate leaks, hidden mold may be at serious levels.
  • Check HVAC equipment and duct work for presence of mold or other allergens. Pay close attention to duct work downstream from air filters and blowers; check blower compartments and duct work for contamination (including dead mice), and check other areas where condensate may have accumulated in duct lines, supporting mold growth. Clean ducts in one area don't assure clean ducts everywhere; a "clean" air test does *not* guarantee no duct contamination either as variations in temperature, moisture, and mechanical disturbance can suddenly release mold spores into the building air.
  • If you don't see mold don't assume a severe mold infection can't be present (behind walls, under carpets, under insulation, in HVAC equipment.) I've found serious toxic mold colonies in walls which showed no external moisture stains nor external mold growth - the clue was other evidence of a history of leaks into the subfloor, confirmed by an impaction air sampler result.
  • Small changes in building conditions can themselves make huge changes in the detected level of airborne mold, from not-detected to severely contaminated. Mold may be present at problem-levels in house air depending on variations in humidity, temperature, season, air movement, and physical activity. Not finding it at a given moment is not an excuse for visual and in some cases invasive inspecting.
  • A home inspection is not an environmental check for unhealthy mold or other bioaerosols or allergens. But if you see moldy conditions, or if there is evidence of a history of building leaks, plumbing backups, moisture problems, or visible mold, further more expert investigation is probably warranted.
  • An industrial hygienist or mycologist may not know enough about how buildings work to complete a reliable building inspection and test. Industrial hygiene is not residential hygiene. Mycology is not building science.
  • Do not be hasty to assert that a specific illness or complaint is caused by mold. The four tests (proposed by Burge, Harvard School of Public Health) are stringent beyond your means as an inspector. Mold at high levels may cause and almost certainly aggravates or contributes to a wide variety of complaints. If a significant reservoir of problematic mold is found in a building it should be removed, independent of any efforts to approve causation between the problem mold and occupant health complaints.

List of Detailed Articles on Finding Mold in buildings

If you are looking for evidence of a mold problem in a building you should review these articles.

List of Articles on In-depth information on mold investigation in buildings

  • Mold Investigation Tips for Home Inspectors how to find mold, where to look, what is likely to be important. Advice to building inspectors intending to inspect or test for toxic or problematic mold indoors, mold inspection methods, and mold test methods which are valid or invalid
  • Mold Testing Methods - Brief Tutorial: Toxic Mold and Toxic Gas Testing Methods Compared - valid vs. invalid tests, recommendations. Lists mold testing methods and protocols, links to longer articles describing air tests for mold, surface tape or bulk mold tests, and gas testing such as MVOC's or toxic gases. Longer articles explain the shortcomings and discuss mold testing protocols.
  • Mold Sampling Methods in the Indoor Environment - In-depth article: detailed critique of popular mold testing methods - Is your mold test kit worth the bother?
  • Classes of Mold: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
  • Levels of Mold: How much toxic or allergenic mold constitutes a problem?
  • What Mold and Allergens Look Like: mold identification photos to help identify mold - choosing what to sample in buildings
  • Mold Sampling Methods in the Indoor Environment - In-depth article: detailed critique of popular mold testing methods - Is your mold test kit worth the bother?
  • Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
  • THERMAL IMAGING MOLD SCANS - how reliable is thermography when used to find hidden mold contamination in buildings?

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