HOW TO LOOK FOR HIDDEN MOLD - How to Find and Test for Hidden Mold in Buildings

HIDDEN MOLD, HOW TO FIND where & how to look for hidden mold contamination in buildings
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How to inspect a building for important but hidden toxic, allergenic, or otherwise harmful mold contamination. This article begins our series of instructions on how to track down the source of hidden mold contamination in buildings. We include hidden mold investigation techniques and we provide procedural suggestions to avoid creating an unsafe condition or moldy mess during the mold investigation process. The fact that mold is "hidden" in buildings does not mean one cannot find it. We look by context: where do we see leak stains, or where do we see building practices most likely to have produced a hidden leak or moisture problem? Ice dam leaks in walls, hidden plumbing leaks, roof spillage by the foundation, are all common clues that often track to a wet building wall or ceiling cavity and from there to a hidden mold problem which may need to be addressed.

HIDDEN MOLD in Buildings, Insulation Mold, and Other Hidden Mold Reservoirs in Buildings

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.

In the photo at page top we see the results of a wall cut through drywall to expose wet moldy insulation, mold on the cavity side of drywall, and surprise! a leak in a pipe that the owner did not know was in her basement wall. However there was good evidence in the form of stains on the exposed side of this drywall. Look at the tan stain which is in the photo in the half-round shape directly above the wall cutout.

Don't try to investigate a building by dashing about with an axe cutting holes willy-nilly. That is an unnecessarily and inappropriately destructive approach to studying a property. But when building history, occupant complaints, or direct site observation of site and building conditions raise the level of probability of an important hidden leak or other damage, directed exploration, often with very modest means, can be very productive.

This article provides a Photo Guide to Finding Hidden Mold in Buildings, but first a mold safety warning about looking for hidden mold.

See TRAPPED MOLD BETWEEN WOOD SURFACES for a discussion of the question of need to remove mold from mated building surfaces.

Also see FIND MOLD in BUILDINGS, HOW TO and see MOLD GROWTH on SURFACES for an index of what mold genera/species are frequently found on various building surfaces and materials, and see Mold Related Illness: Index of Symptoms and finally, for an atlas of building molds and for more microphotographs of building mold samples observed in our laboratory, see Mold Atlas of Indoor Clinical Mold, Pathogens, Allergens & Other Indoor Particles. And MOLD BY MICROSCOPE shows what mold looks like under the microscope.

Do Not Open and Spread a Large Mold Reservoir While Looking for Hidden Mold

Making a small 4-inch test cut, using a borescope or a hole saw to look into building cavities, behind trim, or behind wallpaper, even pulling up a few inches of carpeting won't release a tremendous amount of toxic or allergenic mold into a building, and such openings can also be re-sealed or taped pending further investigation or cleaning.

But wholesale demolition, knocking down walls, removing wall to wall carpets, removing building insulation, or peeling back larger areas of wallpaper could indeed be unsafe if it exposes a large mold reservoir. Here is a warning from the US EPA on peeling back wallpaper or other hidden mold exploration methods:

Investigating hidden mold problems

Investigating hidden mold problems may be difficult and will require caution when the investigation involves disturbing potential sites of mold growth. For example, removal of wallpaper can lead to a massive release of spores if there is mold growing on the underside of the paper. If you believe that you may have a hidden mold problem, consider hiring an experienced professional.

Where to look for hidden mold on and under carpeting and flooring

Lift carpets to inspect carpet tack strips and carpet padding and carpet underside.

Water stains on carpet tack strips and rusty carpet tacks on those strips are other indicators of the leak history in the area. (See the MOLDY CARPETS link at left.)

Remember that mold may be present on the under-side of the carpet itself or on the top or under-side of carpet padding located below the actual carpeting. Hidden mold may be found on the floor surface below the carpeting, or even between layers of finish flooring and subflooring.

We also find mold quite often on the under-side of subflooring. Look up at subfloors from basements or crawl spaces; where a wooden subfloor is installed on furring or "sleepers" placed over a concrete slab, it may be necessary to cut small test openings to explore these hidden cavities.

The best place for a test cut to look for mold over an inaccessible floor or any other building cavity would be where we suspect that water has been most likely to be present, but openings may also be made in unobtrusive spots nearby, such as in a closet or under shelving where the cosmetic damage will be minimized.

A Photo Guide to Use of Borescopes to Look for Mold in Building Cavities

Use borescopes or small test openings: in areas where we suspect leaks into building wall or ceiling cavities we may use a bore scope to peer into the cavity through an opening about the diameter of a pencil.

You may not have access to this equipment but there are other useful steps that can be taken to look for hidden mold.

Where there is a greater probability of leaks into a wall or ceiling, or where there are stains, we may use a larger hole saw, such as shown below This article describes the inspection of building wall, ceiling, or floor cavities.

A Photo Guide to using a round hole saw to make test openings in building ceilings, walls or floors to look for hidden mold

Wall cavity test cuts for mold: The photo at right shows us cutting a round hole in a wall to expose the wall cavity. We cut high enough above the floor to avoid the sill plate and to give access to the wall cavity itself. We cut close to but not on top of a wall stud, so that when examining the wall cavity we can examine:

The cavity side of drywall plug which was removed, checking it for visible mold
The condition of insulation in the wall cavity - if any. It may be necessary to vacuum-test wall insulation to test for mold since fiberglass insulation can be moldy but look quite clean.

See Mold in Fiberglass Insulation for details of the occurrence of mold contamination in building insulation, and see Vacuuming building cavities as a screen for building mold as well as Vacuuming exposed insulation for finding mold-contaminated fiberglass.
The condition of nearby upper surface of horizontal sill plates to check for mold growth or evidence of leaks
The condition of nearby vertical surface of a wall stud to check for visual evidence of mold growth or leaks
The condition of the wall cavity-side of the opposing drywall or exterior wall sheathing.

Do not fail to check all of these surfaces. It is common to find more severe mold growth on the drywall of one side of a wall cavity than on the other. This happens when water has run down one surface but not on another.

When cutting a test opening to explore a wood floor installed on sleepers over a concrete slab, we use two diameters of hole saws (see photos in the next section.)

First we cut a larger-diameter round hole (say 2.5") through the finish flooring using our hole saw's larger cutting blade. We remove and save the wooden plug created by this step.

Second we cut a smaller-diameter round hole (say 2") through the subflooring to expose the cavity below.

We examine the exposed and inner or hidden surfaces of each plug which is cut, as well as any rosin paper or other material that may be installed between layers of flooring.

Finally, using a flashlight and often a borescope as well, we explore the floor cavity itself for evidence of visible mold, organic mold-friendly debris, insect activity, or evidence of leaks or flooding.

How to Make Small Test Cuts to Check for Hidden Mold in Building Walls & Ceilings - Using a Drywall Knife

Small homeowner test cuts in walls: Make small test cuts in drywall in areas most-likely to have been wet. Don't make a big dusty (or moldy-dusty) mess. Keep it small and clean.

You may want to re-seal test openings after using them, pending cleanup or further action.

Pull off floor trim at the floor/wall juncture in basements on walls suspected of harboring leakage - look on the back of the trim and on the newly exposed drywall for hidden mold growth.

When to Make Larger Wall Test Cuts to Check for Hidden Mold in Buildings

Larger wall cut openings: We may make a larger cut if external evidence such as staining or rot are strong suggestions of a wet, damaged, or moldy wall cavity.

In the example shown you can see a water stain just above the upper horizontal line of this test cut in a basement corner. We cut the drywall because of a combination of factors that elevated the risk of hidden mold in this area:

Downspout spillage outside the wall in this area
Inside stains on the drywall
Nearby mold odors
Occupant complaints of respiratory difficulties in this area

In the wall we found evidence of a history of plumbing leaks from a drain that the homeowner did not know was even in this wall (center of the cut out area); we found evidence of a severe rodent infestation (right stud bay of the cutout area); and we found visibly wet, moldy insulation in the test cut area (left stud bay in the cutout area).

If there is a large mold reservoir discovered in an otherwise "clean looking" building, you'll want to close up the test cut made above, taping the seams with masking tape, to avoid exposing building occupants to problematic mold while waiting for further investigation and cleaning by a professional.

How to Make (or not make) Random Test Cuts to Check for Mold in Building Cavities

Random wall test openings: We have little confidence in and are reluctant to simply make random test cuts in buildings. Since water can take peculiar paths through hidden openings, such as wall plate holes drilled for pipes or electrical wiring or between single pairs of studs or ceiling or floor joists, cutting a hole that does not reveal a problem is no assurance at all that no hidden mold problems exist.

When to Make Strip Cuts to Look for Hidden Mold in Building Ceilings or Walls

When there is an ongoing building complaint that makes us suspect hidden leaks or mold, if we strongly suspect a hidden mold problem but have not found its precise location, on occasion it is justified to make a "strip cut" across multiple wall studs or ceiling joists, exposing multiple wall cavities.

How, Where, & Why to Peel Back Wallpaper to Check for Hidden Mold in Buildings

Carefully peel back wallpaper under windows where there has been leakage, or in bathrooms over and near showers.

Mold often grows on paper wallpaper backing or on wallpaper glue in areas that have been wet.

See our warning about peeling back large areas of wall paper - at the top of this page.

Photo Guide to Finding Hidden Mold on Furniture, Bathroom Vanities, & Built-In Drawers or Shelving

Look for hidden mold under drawers in areas subject to spills, leaks, or dampness, such as in bathrooms and kitchens. Inspect the drawer underside and the wood flooring exposed if built-in drawers are removed completely.

Moldy furniture guide: We also inspect the under-side of furniture, especially in areas that have been wet or humid. These "hidden" un-painted or un-varnished wood surfaces often support mold growth before it will be visible on the sealed or painted (and more moisture-resistant) surfaces.

Game tables such as pool tables usually include un-finished surfaces on their undersides. Chipboard, and even vinyl-covered chipboard are often very friendly to un-wanted Aspergillus sp. or Penicillium sp. mold growth.

Photo Guide to Finding Mold Under or Behind Bathroom Vanity Cabinets

If there has been protracted leakage or spillage under built-in cabinets such as bath vanities, there may be a mold cleanup job under or behind these components. We removed this bath vanity after receiving complaints of recurrent moldy odor in this bathroom. No amount of cleaning of other bath surfaces had reduced the mold smell. A slight slope in the bathroom floor had been sending tub spillage behind this bath vanity for decades.

What makes a lot of sense sense is to study the building carefully to decide on the building points at most risk of having been wet from leaks due to construction details or other site observations. That's where one would make a test cut.

We frequently add text and photos to this series of articles. (See "What MOld Looks Like" and "Stuff that is Not Mold"links listed at Related Topics ). Readers should also review Mold in Fiberglass Insulation in Buildings at our The Mold Information Center - What to Do About Mold in Buildings

Small amounts of mold can be removed simply by cleaning or removing infected materials, something most homeowners can handle -- but see the Warning Notice at the end of this article. Some mold species can make you sick.

How to Identify Hidden & Visible Molds that are Cosmetic or Harmless, or Other Low-Risk Molds

Cosmetic-only Ceratocystis/Ophistoma bluestain mold is shown on the floor joists in the new construction framing in the photo just above.

This is a harmless, cosmetic-only mold that does not damage the lumber and is not a pathogen for humans. Here's a good example of the observation that not all "black mold" is "toxic black mold". It will be totally hidden when the ceiling drywall is installed.

The mold shown in the photograph above is plain to see during construction, but will be covered and hidden completely when the contractor installs the ceiling drywall.

One of our clients discovered this mold during a renovation and was quite concerned that a major toxic black mold reservoir had been found in the building. The client was facing a very costly mold cleanup project if this mold had to be addressed as a toxic material. Luckily this was not the case in this instance, as was easily demonstrated both by a simple inexpensive lab test and confirmed by onsite inspection of other framing details discussed at "Cosmetic Molds" linked-below.

So sometimes the mold in your house might be only a cosmetic concern. "Bluestain" or Ceratocystis/Ophistoma is common on framing lumber and we often find it in attics on the under side of roof sheathing. Unless it's in finished portions of living space where it creates a cosmetic problem, no particular action needed to address this black mold.

Detailed advice about how to determine by visual inspection alone whether or not you're probably looking at one of these common framing lumber cosmetic molds is at our Photo Guide to Cosmetic Molds.

Frequently Asked Questions (FAQs) about Finding Hidden Mold in Buildings

Question: I have a hidden mold problem but none of my "mold inspectors" has been able to find it. How should I proceed?

I am convinced I have a "hidden mold" problem in one of my bathrooms, and have had this problem for quite some time. At times I have smelled a moldy odor, and almost always when I enter the room I IMMEDIATELY experience allergy symptoms.

I have made a visual inspection of the surfaces in the room, and see no mold deposits. I also had a chance to look at the under flooring (the plywood base layer for the floor) a couple of years ago, including the area right adjacent to the toilet bowl, when I had the old tile removed and new tile installed. There didn't appear to be any significant mold signs there.

I also had a handyman cut out two 24" square panels of drywall directly underneath the pipes of both sinks in this room, and although we couldn't see much further than the immediate areas, there were no signs of mold on the inside of the drywall or surrounding areas.

I am affected seriously in terms of health by this problem, and am very concerned about finally finding some way to pinpoint this problem, so I can have it resolved. (I can't afford to completely rip apart the room....walls, floors, ceilings...to find this problem, and further, my residence is a condo, with neighbors underneath and on one side on the same floor, so drastic measures would be difficult to take).

I have had two or three so-called "mold inspectors" out over the last few years, and none was effective. They tried to sell me "mold testing," but to me, this will do no good because

I already KNOW I have mold...the problem is, to FIND it, so I can have it removed. Any suggestions, and any local referrals you can make for services in my area (Arlington Heights, IL, a suburb northwest of Chicago), will be much appreciated. - M.R.

Reply: Be sure that the mold inspection is sufficiently broad in scope, identify risk targets for further investigation, don't rely on "mold tests" alone, and hire an expert who actually inspects

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. It is possible that the inspectors who looked in the past lacked experience or care, or that their inspection scope was too limited. Certainly if you smell mold it would be a surprise for none to be present. And I agree that a "mold test" without an expert inspection is of little value and that it would fail to provide what's needed: a problem diagnosis and a start to an action plan.

Profile the Building's Leak or Moisture History to Track Down Hidden Mold

If you haven't seen it take a look at HIDDEN MOLD, HOW TO FIND and the articles listed immediately under that heading (article links are at Related Topics ). One of those articles, Use a Flashlight to Find Mold adds another dimension, careful use of light to see hard-to-spot light colored molds.

In the case you describe, where you are already sure that there is a mold problem somewhere but it remains hidden, and where someone has already looked in the obvious places, including modest invasive inspecting by cutting openings at nearby plumbing, my suggestion is to perform a thorough, complete building inspection identifying by observation of construction details, building and site visual clues, building age, history, construction materials and similar approaches, all of the places that present a notable risk of a history of leaks or moisture traps.

Moisture Clues Help Track Down Hidden Mold

Moisture is a gating factor in problem mold growth. That inspection should result in selecting the top few highest-risk locations for further investigation, invasive cutting of limited openings included, if needed.

Smell Testing can Help Track Down Hidden Mold

I'd combine that approach with a careful sniff test or smell test (SMELL PATCH TEST to Track Down Odors) to see if you can track mold smell to mold risk points. More about pinpointing odor or smell sources in buildings begins at ODORS, Smells, Gases in Buildings-Diagnosis & Cure

We can't know from the data at hand so far whether or not the mold problem in your building is small or if professional cleaning will be needed. That will be part of the discovery process.

Strategic Dust Sampling to Track Down Hidden Mold

As a part of a thorough mold investigation, I'd collect settled dust samples from suspect areas and from a few interesting areas on each building floor as a screening procedure. We might find evidence of nearby active mold growth (such as Pen/Asp spores found in chains) as well finding that some building areas are more moldy than others.

Movement of Mold Spores & Gases in Buildings Confound Hidden Mold Investigations

Keep in mind that because ultra-small mold spores as well as odors move through buildings like a gas, you may be smelling or detecting mold in areas that are not always immediately adjacent to the actual problem source.

Hire a Mold Investigator who Actually Investigates

About hiring a mold investigator, we list a variety of building inspection professionals beginning at the EXPERTS DIRECTORY (at page top). And near the top of that article you'll find lists of directories of other specialists including mold and environmental inspectors. Before hiring another inspector discuss your case and your concerns with the inspector and ask him/her to describe their procedure.

If you find that the "mold expert" is simply someone who is going to collect a few test samples and send them to a mold test lab, I'd look further for someone who will be more thorough and who has more experience and expertise. A more useful mold investigation includes taking a careful client complaint history, building age and materials and leak/moisture history, a very thorough outside and inside inspection for risk points, and strategic testing.

Variability in Human Response to Hidden Mold Reservoirs may Confound Mold Investigators

At MOLD STANDARDS we provide a survey of mold exposure standards from around the world. You'll see that all of them set pretty high airborne mold spore levels and that not one of them copes with the enormous variability in individual mold genera/species spore size, allergenicity, pathogenicity, or toxicity.

Finally, by virtue of having my own forensic lab I've been able to perform some interesting tests on individual sensitivity to mold and MVOCs in buildings.

By testing individual clients and buildings in which the clients experienced a severe reaction immediately on entry (such as respiratory distress or allergic response) I found that some people are sensitive to very low levels of airborne mold spores of some genera. For example a woman client reacted within seconds of entry into a building where the airborne mold spore count was dominated by Aspergillus sp. at a level of less than 200 spores/M³ of air, a rather low indoor concentration.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

"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
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 - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol

"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
Rodent control issues, including dander, fecal, and urine contamination of Buildings and Building insulation are discussed at our

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
"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)
US EPA: Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol

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.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd


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