Photograph: Mold on drywall - Daniel Friedman Mold on Walls, Drywall, or "Sheetrock®"
Causes of mold growth indoors
How to Find & Test for Mold on Walls


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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?

This article series 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.

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How to Test or Sample Building Drywall, Gypsum Board, "Sheetrock" and other Building Surfaces for Mold Using Clear Adhesive Tape

Severe indoor mold contamination (C) Daniel FriedmanThis article discusses:

  • Why & where mold grows on drywall: how much moisture, water, or leakage is necessary for mold to grow on drywall - mold growth below leaky skylights & windows
  • Moldy drywall sampling mistakes to avoid; proper use of a flashlight finds "hidden" mold on drywall
  • What mold looks like in different areas or on different surfaces of drywall
  • Why are different mold genera/species found at different heights on building walls? We illustrate how three tape at three nearby locations on drywall can collect three completely different mold genera/species
  • How should mold test samples be collected from building surfaces?

As I've explained in various articles and at DUST / MOLD TEST KIT INSTRUCTIONS, 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.

The photo at page top 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 HIDDEN MOLD in CEILINGS / WALLS we discuss finding mold on the wall cavity side of drywall.

This article is part of our '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.



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.
  • Photograph: Moldy drywall supports different mold genera and species at different moisture levels in the same area - Daniel Friedman

    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.

    At MOLD LEVEL REPORTING we explain the errors you can expect if you do not choose a properly-representative area of a surface when collecting mold or dust samples and
    at MOISTURE GRADIENTS & MOLD we explain why we find different mold genera/species at different locations on moldy drywall.

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

Photograph: Multiple tape samples on one zip-lok bag - Daniel Friedman

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?

Mold growth pattern on drywall (C) Daniel Friedman Mold growth pattern on drywall (C) Daniel Friedman

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.

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.

How to Relate Mold Growth Pattern to Building Leaks or Moisture Vairations

Drywall mold on ceiling (C) InspectAPedia - LYReader Question: why is this black mold growing on these drywall ceilings & walls?

I attached three mould photos for you. Can you help me to find out the possible reasons? - L.Y. 6/4/2013

Reply: Summary of common reasons for mold growth on drywall

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider:

Possible reasons for indoor mold growth ?

In all cases of indoor mold growth the mold needs food (wood, paper, organic material), oxygen, and water; some molds thrive on light, some are tolerant or intolerant of UV, and other factors.

Moisture, water, leaks, condensationa are key determinants in indoor mold growth

Of all of these, water is the key necessary ingredient which we can track and which is inviolate - that is, no water, no moisture = no problem mold growth.

Indoor  mold growth on drywall ceiling in framing pattern (C) InspectApedia LYI have no context for the photos, no age, no building history, occupancy, use, or other key information that an onsite inspection would disclose.

But I note that your middle photo looks as if it is a ceiling and because there is a vent therein I suspect this may be over a bathroom; Common moisture sources for such an area might include

  • leaks into the ceiling above
  • high indoor moisture from in-building sources such as a steamy shower or clothes dryer
  • leaks from the ducting for the ceiling fan

The density of mold growth in your photos suggests that there have been leaks from above. But in some cases even without actual water leaks into a ceiling or wall we can see dense mold growth on building drywall caused by other wet building conditions (flooding of the floor for example, or a heating or steam pipe leak).

Ceiling mold (C) InspectAPedia LYMold growth tracking ceiling or wall framing

The fact that the mold seems to track or be more dense along lines resembling either drywall joints or actual locations of framing would argue for either leaks from above penetrating at drywall butt joint seams or quite often, temperature differences (wood framing in contact with drywall produces cooler temperature than insulated ceiling cavities).

Temperature variations can in turn invite higher levels of condensation on cooler surfaces in those areas. We also see this effect

So even when we don't see leak stains showing a water source, the mold growth pattern might track moisture gradient in the materials and thus suggest something about its cause.

You will need to remove the moldy drywall in any case, as well as any insulation that has been wet or exposed to mold.

That will let you inspect the wall or ceiling cavity for leaks as well as to check that the bath vent fan is actually venting and is properly used.

Reader Follow-Up:

Mold behind plastic (C) InspectAPedia LYBut how to explain the reasons for this photo? It is the most black area (I showed in previous email)when my landlord removed the ceilling, there has plastic.

`Actually I know very little knowledge about mould. My landlord said the mould was caused by the lack of heat. Can you show me some ideas?

Does the water stain of the second picture show water leakage? This house is more than 40 years old.

There is no indoor ventitaltion system, the fan in the bathroom has been blocked for long time and the fan had not been insulated well.

There's no fan to transfer gas to outside in the kitchen. - Y. 6/5/2013


Mold is is not directly caused by lack of heat - cooler temperatures in fact retard mold growth, if all other conditions are equal. But lack of heat that leads to condensation problems or to a frozen burst pipe and leak can indeed cause or encourage indoor mold growth.

And I would agree that heating a building, as it dries a building, would avoid problem mold growth provided that there are not leaks or abnormal moisture sources.

If there is mold there is a condensation or leak problem to find and fix. If the mold is on the wall cavity side of the plastic then there are leaks into the wall cavity - either water leaks or air leaks, perhaps combined with incomplete or improper insulation. I'd have to see more detail to have a more confident opinion. From your photo I'm not even sure what's ceiling and what's wall.

Some speculative examples of what could be going on include:

  • water leaks in to the plastic-covered cavity
  • moist air leaks into the plastic-covered cavity from either side of the wall, ceiling or whatever we're looking at

The suggestions I made earlier about tracking down leak and moisture sources should, perhaps in this case, be combined with inspection for air leaks into the cavity from indoors or from outside.

At What Moisture Level Shoudl Drywall Simply be Removed?

Reader question: 1/30/2014 Phil said:

What % of moisture or water penetration into Drywall can it reach before it can not be dried and has to be removed. Also, if there is a time limit on when you have to start the drying process.


Phil, that's an interesting question and I haven't thought about wet drywall quite that way. Let's try this:

If drywall is wet above 18% or so, even if just measured on the drywall surface, then conditions are ripe for mold formation; Generally the rate at which mold will grow on wet drywall depends on moisture levels and temperature; While there are molds that will grow under just about any condition, in buildings drywall mold of any of about 40 common genera/species will show up in 24-48 hours.

Wetter drywall grows different species (such as Stachybotrys chartarum) than less-wet but still too-wet drywall (18% say) (such as Aspergillus sp.).

So I would say that regardless of how wet the drywall is to start with, if we could get it dry - say at 10-2% or less throughout its thickness - in 24-48 hours and if we don't already see mold growth on it, we're probably OK.

Watch out: however if drywall is really soaked we have to ask where the water came from, where it flowed in the building, and what else got wet. For example, if drywall is soaked because of water flowing through a ceiling or wall cavity, if we do not open that cavity, remove insulation, and get the entire cavity and all materials dry quickly enough, we're likely to have a costly mold contamination problem there. In such cases, deciding to keep or leave drywall in place because of its surface or room-side moisture could be a mistake.


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DRYWALL MOLD TESTING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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