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Guide to testing mold on building drywall: This article describes proper procedures for sampling mold on drywall in buildings. Because moldy drywall or "sheetrock" is often the consequence of a building flood or wet floor, the moisture gradient in drywall varies at different heights above the floor surface. Because different mold genera/species vary in their hydrophilic nature (some molds love water more than others), different mold genera/species are likely to be found at different heights on a building wall. Which molds are most important to sample?

The photo above shows several colors of mold on a drywall surface. Still more mold may be present but still lighter in color and harder to see. Each of these may be a different mold genera or species. Which molds that we see on a building surface should be sampled? We explain the answers here. At Wall test cuts to spot hidden mold we discuss finding mold on the wall cavity side of drywall. Also see MOLD RESISTANT DRYWALL for a discussion of that product type as well as a list of drywall or gypsum board industry standards and drywall product MSDS sheets.

This document describes how to find mold and test for mold in buildings, including how and where to collect mold samples using adhesive tape - an easy, inexpensive, low-tech but very effective mold testing method. This procedure helps identify the presence of or locate the probable sources of mold reservoirs in buildings, and helps decide which of these need more invasive, exhaustive inspection and testing.

This chapter is part of a 'how to' photo and text primer on finding and testing for mold in buildings using simple clear adhesive tape on suspect or visibly moldy surfaces. Also see MOLD GROWTH ON SURFACES, PHOTOS and MOLD APPEARANCE - WHAT MOLD LOOKS LIKE.

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SAMPLING Building DRYWALL Gypsum Board, "Sheetrock" and other Building Surfaces for Mold Using Clear Adhesive Tape

As I've explained in various articles and at our instructions for collecting and mailing a tape sample to our lab, different mold genera/species will be found growing on the same or nearby sections of drywall on a building surface, depending on several variables.

If the largest contiguous mold area in a building is trivial in amount, say 1 sq .ft., we would not test it unless we thought that the mold we see is representative of a larger mold problem I cannot see. Small areas of mold should simply be removed.

For larger areas of mold (certainly if more than 30 sq .ft. of area is moldy or if mold is growing on many surfaces in a building), you are looking for the dominant species present and particularly allergenic or toxic species present in the environment.

How to Decide Where to Sample for Mold and How Many Mold Samples To Collect

Collect one mold tape sample per location; do not use the same tape to sample from multiple locations.

• Choose a representative sample spot: select a representative spot of mold growth on a surface such as a wall, cabinet, ceiling or floor.
  
  This means that if you see what appears to be a single coating of mold-suspect growth on a surface, all rather consistent by color, texture, and what it's growing-on, you need only one sample of that material. Variations in appearance or texture or growth surface or mold growing in different building areas or floors are reasons to sample more than one thing.
  
  In our photo (above left) of severe indoor mold contamination in a home, many different mold genera/species were present on the drywall (sample by color or texture) as well as still other genera/species that varied by growth surface, type of wood, painted surfaces, other materials.
  
• Color: Sample molds of different colors: black, white, green, red, gray, brown, yellow, pink - are often (not always) different species.
• Texture: Sample molds of different textures: hard lumpy big grainy versus fuzzy and easily blowing into the air - are often (not always) different species.
• Growth Surface: Sample molds growing on different building materials. This is quite important. Completely different mold genera and species may be found growing in the same building on different growth substrates: drywall room side, drywall cavity side, plywood sheathing, wood stud or joist framing, painted surfaces, exposed fiberglass insulation kraft paper vapor barrier - are often (not always) different species. Even on the same growth surface (drywall for example) different mold species appear at different locations according to variations in moisture level - explained just below)
• Building area: basement, crawl space, living area, and attic all have different moisture conditions, often different building materials, different patterns of air movement and exposure. The "green mold" found on wood subflooring visible overhead from inspection in the basement is very often a completely different genera and species from the "green mold" found on the roof sheathing in the attic of the same building.
• Representative dust samples: we will sometimes screen areas where there is no visible mold by collecting settled dust particles from a horizontal surface. If you are going to collect a single dust screening sample, collect it either from the area of which you are most suspicious (a flooding basement), or from the area where building occupants spend the most time (perhaps a bedroom or family room).
  
• Variations in moisture gradient in the drywall - so if a floor was flooded, water-loving molds grow closest to the floor (such as highly-visible black molds like Stachybotrys chartarum), while molds liking the drywall to be a little less wet grow a little higher (such as Cladosporium sp., Cladosporium sphaerospermum, Cladosporium cladosporioides, Ulocladium chartarum), and molds liking the drywall to be still less wet grow higher still on a vertical wall (such as Aspergillus sp., Aspergillus glaucus, Aspergillus flavus, Penicillium sp., etc.). Therefore where the tape sample is collected can make a big difference in what you find.



In the first photo of moldy drywall, three completely different mold genera and species were within a few inches of one another at different heights on this laundry room wall.

This condition often occurs, but the different genera may be as close as inter-mixed and even overlapping in the same area, to growing several feet apart on the same wall, to growing in the same building but on different materials on different surfaces.

In this case, tape sample #1, the bottom mold, was Stachybotrys chartarum, tape sample #2, the middle mold, was Cladosporium sphaerospermum, and the top tape sample, #3, was Aspergillus flavus. Of these three, the Aspergillus is the easily-airborne toxic spore which is more likely to be a problem in the building if it is present in sufficient quantity.

How to Prepare & Save Mold Tape Samples for Mailing to a Mold Test Laboratory



In this photo detail you'll see that using a new and clean zip-lok™ bag, we placed several surface tape samples on the same bag. If you can't assure that the bag surface is clean between tape sampling, use a new bag for each sample.

• Interruptions in the moisture gradient absorption path: for example at a wet floor which soaks the bottom of drywall, moisture wicks up into the drywall material. But moisture wicking may be reduced suddenly at a horizontal drywall joint, resulting in easily-visible borders or lines in fungal growth.
  
  
• Exact pathway of water on a surface or in a building cavity: so tracing the exact water path through a ceiling or wall cavity is very important.

Are you collecting too many mold test samples?

There are nearly always multiple mold species present in any environment where mold producing conditions are present.

We sample surfaces likely to host different molds, focusing on surfaces which appear to represent mold or mold-suspect material growing over large areas in the building. Don't collect and send 50 samples. If you find you want to collect a great many samples it would probably be smarter and more economical to bring in an expert to survey the building and who can sample more strategically.

Interrupted Mold Growth Pattern on Building Drywall - Why Does Mold Growth Sometimes Stop in Straight Lines?

In our photographs shown above the thick black mold growth on drywall in a wet basement appears to nearly "stop" in a neat horizontal line just about four feet from the floor surface. Why?

Stachybotrys chartarum, which dominated the mold on this drywall, really likes wet conditions. As we explained above, the genera/species of mold growth may vary on a surface of the same material as a function of variation in moisture levels in the material.

In our photo at above right we show by having made a test cut into the moldy drywall that mold growth stopped its rapid advance up the drywall when it encountered the horizontal tape joint between the lower and upper runs of drywall in the building. We have found two common explanations for this observation:

1. The moisture wicking upwards in drywall from a wet floor is interrupted where the paper-covered edges of two horizontal runs of drywall abut.
2. Mold growth on joint compound alone is often significantly less in a building than on paper-covered drywall in the same area. This observation describes the success in "paperless drywall" sold for some applications.

Frequently Asked Questions (FAQs) about mold growth on drywall in buildings

Question: What are the necessary conditions for mold growth on building drywall and in the wall cavity?

I had energy efficient windows installed in my townhouse over a year ago. This past spring one of the master bedroom windows leaked after a rain storm because the caulking failed. The company immediately came out and recaulked the window and it hasn't leaked since.

Mold growth in wall cavity below a leaky window

[Photo at left provided by InspectAPedia, taken from a New Jersey home and not the building discussed by this reader, shows mold growth on drywall and on wallpaper beneath a leaky window. A single rainstorm combined with the window having been left partly opened caused this mold growth.

After checking the basement ceiling cavity below this window as well as in the wall cavity itself, our investigation found that the total moldy area was less than 30 sq.ft. and the mold cleanup could be handled as a normal renovation job. More examples of mold growth on and in wall cavities and hidden by wallpaper are shown at MOLD GROWTH ON SURFACES, PHOTOS - Ed.]

My concern is that I now have a water stain under the window on the drywall, and since I have a mold allergy, I'm wondering if there might be mold on the inside of the drywall. I read your article on testing the dry wall but as mentioned in the article would rather not cut into it unless it's necessary. I looked at other articles but didn't see one with a picture resembling the water stain I'm concerned about. What would you recommend?

By the way, this is a very helpful website. I was considering using ozone for any possible mold in my place but see from your article that's not a good idea.

Continued: I emailed you recently about the possibility of mold forming on the inside of drywall under a window that leaked last spring when it rained.

I contacted a bonded company about replacing the drywall. The gentleman I spoke on the phone with told me that he's attended mold seminars and that mold requires an ongoing source of water or moisture over an extended period of time to grow. In his opinion, since my leak occurred only once during a rain storm that lasted an hour or so, there's little or no chance mold could grow on the inside of the drywall under the window. He also said that since the water stain is barely visible, this is more reason to conclude there's no mold on the inside of the dry wall.

I thought it would be a good idea to check this on your website, but didn't find any topic that seemed related to the conditions for mold formation, so I thought I should email you. Is what he says about mold formation accurate? And if so, is it reasonable to conclude that there's probably no mold on the inside of the dry wall under the window that leaked?

I'm hoping he's right, because I won't have to replace the drywall. I'll replace it if necessary, though.

Thanks again for your help. G.N.

Reply: Even a single event water leak is likely to lead to mold growth in building ceiling or wall cavities - it depends on ...

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with mold, hidden mold, and with tracking down just how much water leaked into the building and where it went.

First, how disappointing that your new windows leaked - certainly a wet wall below a leaky window is not particularly energy efficient, and indeed it could become a mold reservoir.

The risk of a mold problem that you can't see but that is significant enough to merit removal is not something I nor anyone should guess at by email nor by telephone (that is, with no building inspection) with so little information. MOLD EXPERT, WHEN TO HIRE provides advice on deciding (or not) to hire someone to inspect and test for mold contamination.

Watch out: mold testing alone, without a building inspection, case history, and occupant interview, is not reliable.

I would not rely on "mold tests" - a test, especially an air test for airborne mold, performed without an expert diagnostic inspection of the building is just not reliable in cases where the result is "negative".

Continued reply: G., your bonded company expert is mistaken, as you can see in our example photo above showing mold growth in the wall cavity and behind wallpaper below a window following a single event leak.

A single water entry event can cause mold growth in a building cavity, on the wall cavity side of drywall, in the wall insulation, on the kraft paper insulation facing, on the building structural wall wood studs or cavity side of the exterior wall sheathing.

Your expert would, however, be correct, if s/he asserted that the extent or size or severity of mold growth from a single event leak depends on the size of the leak, the amount of water entering the cavity, where it went, what materials got wet, and the moisture exit rate from the building cavity.

Unlike your expert, beyond that advice I'm reluctant to promise what is or is not in a building cavity on a building I've not studied. Often an expert on site finds evidence of chronic leaks that a normal building owner may never have noticed. Very often very small leaks or wet spots do not produce a significant problem mold reservoir.

Why even a single building leak can cause significant mold growth in 24-48 hours

But because building wall or ceiling cavities dry out very slowly, leaks into a wall or ceiling, even from a single event, can initiate problem mold growth in those areas. In general, if a building interior or building cavity interior contains mold friendly organic materials such as wood, paper, drywall, fabrics, drywall, mold growth can be expected to occur if the area remains wet for 24-48 hours. Low temperatures can sometimes slow down mold growth so a building that was wet during freezing weather may have a bit more time.

What happens to mold in a building wall or ceiling cavity once the area has dried out?

When the building cavity finally has dried, several conditions are likely to occur:

1. Depending on the mold genera species, there may be a very large spore release as the fungus dries. For example I've seen an enormous release of Aspergillus sp. spores during building dryout procedures preliminary to a large scale mold remediation project. And depending on the amount of air communication between the building cavity and the occupied space, this mold reservoir and spore release can be a problem for some occupants.
2. Once the building cavity is sufficiently dry ("sufficiently" depends on the mold genera species and the material on which its colony has formed) the mold colony or reservoir may become dormant - that is, it may stop further expanding growth and it may stop releasing spores to the environment.
3. Future changes in the building condition, ranging from increases in indoor relative humidity to new leaks into or near the same moldy drywall or moldy building cavity can cause a renewal of mold growth and spore release activity at a more rapid rate and on a larger scale than would have otherwise occurred if the pre-existing mold reservoir had not been present or if it had been previously removed and the building cavity properly cleaned (and of course had the cause of building leakage been repaired).

How do we decide if destructive or invasive inspection of building cavities is justified

In general, if there has been anything but the most absolutely trivial leakage into a building wall or ceiling cavity, and if thus we decide to investigate, I recommend choosing the one or two most suspect leak areas, where there is the most evidence or suspicion of the area that received the most water or was the most wet. In those areas I'll make a small test cut, perhaps 2" x 4" into the ceiling or wall, through the drywall. There we perform a visual inspection for visible mold on all surfaces and materials; if there is insulation present we also collect a vacuum sample of that material, and we collect tape samples of any visible mold on any of the newly exposed surfaces.

If that most-suspect area is "clean" of any strong suggestion of mold contamination, we don't cut or dig further without some other compelling reason to do so.

What is appropriate when there has been building leakage and where there is a concern for possibly significant hidden mold contamination is to make a decision about whether hiring an expert is justified in a particular case - see MOLD EXPERT, WHEN TO HIRE for advice on making that decision.

More photographs and examples of mold growth on drywall are found at Mold on Drywall or Sheetrock®. Also see HIDDEN MOLD, HOW TO FIND.

Questions & Answers regarding this article

Questions & answers about mold growth on building drywall surfaces and about mold testing procedures.

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DRYWALL MOLD

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 US EPA
• US EPA - Una Breva Guia a Moho - Hongo - en Espanol

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)
• 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 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 - 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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