Photograph: mold hidden behind basement wall panelingHow to Find Hidden Mold Contamination

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Find & fix hidden mold contamination:

This article series provides a Photo Guide to Finding Hidden Mold in Buildings - 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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Toxic black mold, in this case Memnoniella echinata, exposed by removing apartment drywall, extensive mold contamination visible

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.

We discuss the problem of hidden mold reservoirs (photo at left) and steps to take to find such problems in buildings. The mold contamination show at left is described in more detail

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.

Article series contents - each of the below is a separate article

Watch out: just a light touch is needed. 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.

An Example of "Hidden Mold" in Air Conditioner Air Handling Equipment

Photo of mold in a Florida Air Conditioning air handler unit - Mark Cramer, Tampa

The photograph of a dense colonization of light-colored mold inside of a Florida air conditioning air handler unit above, courtesy of Mark Cramer, Tampa.

While it is certainly possible for HVAC equipment and duct work to form a problem reservoir of mold contamination in a building, we caution that small mold reservoirs in HVAC air handlers are common,

Often, because of the small size and mold genera found in an air handler, such reservoirs are not the problem mold reservoir in a building, and investigators need to look further.

Watch out: 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

Photograph of inspecting below carpeting for hidden mold.

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.

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

Use of Borescopes to Look for Mold in Building Cavities

Photograph of inspecting the wall cavity through a small borescope opening.

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.

Use of a round hole saw to make test openings in building ceilings, walls or floors to look for hidden mold or to collect core samples for analysis of layers of material

Photograph of inspecting for hidden mold with a larger test cut.

The photo above shows us cutting a round hole in a wall to expose the wall cavity. Please see TEST CUTS for MOLD in BUILDING CAVITIES for details of this topic

Photograph of inspecting the wall cavity through a small test cut.

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. Details of this procedure are at HIDDEN MOLD in CEILINGS / WALLS.

Larger Wall Test Cuts to Check for Hidden Mold

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.

Photograph of inspecting below wallpaper for hidden mold.


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

Photograph of inspecting below a bath vanity drawer for hidden mold.

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

Photograph of inspecting below a bath vanity for hidden mold. Photograph of inspecting below wallpaper for hidden mold.

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"a rticles. 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

Photograph: typical cosmetic bluestain mold on new framing lumber, floor joists - © Daniel Friedman

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.

Reader 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, 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 .

One of those articles, USE a FLASHLIGHT to FIND MOLDadds 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 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 m
old 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 EXPOSURE 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.


Continue reading at HIDDEN MOLD in CEILING / WALL CAVITY, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see TRAPPED MOLD BETWEEN WOOD SURFACES for a discussion of the immobility of mold between wood framing and other surfaces.

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HIDDEN MOLD, HOW TO FIND at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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