Mold Testing Services
Mold Testing -- Sioux Falls, South Dakota
Checking for Mold
This section emphasizes use of visual and nasal observations. The use of moisture meters is covered in another section. There are many ways to check for mold. Mold consultants, home inspectors, cleaning firms, plumbers and others can often help building owners to check for mold. MTS offers the following for persons wanting to check for mold themselves. To check for mold is also to check for moisture sources (past or present). Mold may occur as visible mold, airborne mold and as musty odors.

However, the main concern from mold is AIRBORNE EXPOSURE. Airborne mold is too small to see with the naked eye and requires sample collection (testing) and microscopic analysis. It should be emphasized any mold check that does not include airborne mold tests may be incomplete. MTS has worked at many buildings where no visible mold was obvious yet very high airborne mold levels were present. Testing for mold is discussed in the TESTING section.

Health Symptoms
When checking for mold, an obvious question is does anyone have health complaints? Health complaints are often the reason a mold investigation was requested. Asking the persons affected can provide useful information such as: when did the problem start, what parts of the building are symptoms or odors most noticeable? Did symptoms start after cleaning up the basement or after a water leak, etc? See the HEALTH section for more information on symptoms.

Does the person feel better away from the building? If people feel sick in a building but better when away, this is a strong indicator that some of building problem exists (although not necessarily due to mold).

People vary enormously in their sensitivity to mold and allergens. In many cases, only one or two persons in a building may be experiencing symptoms while others may not. This does not mean a mold concern (or other problem) does not exist. Such persons are often assumed to be “complainers” when they may simply be more allergic than others. It is preferable to start with the assumption that any complaints may be real. In fact, in many cases, workers may actually be reluctant to complain at all for fear of losing their jobs. When asking questions, a non-threatening neutral and tone should be used along with open ended questions. Leading questions, such as when did start getting these mold related headaches, should be avoided. Rather, when did you first notice anything? What were your symptoms? Questionnaires can be useful but building managers are typically reluctant to allow their use.

Odors often play an important part in complaints. Women typically have a much more sensitive sense of smell than do men and often notice odors where men do not. Any odors noticed by persons (while perhaps not noticeable to others) should be assumed to be real until proved otherwise.

Using Your Eyes – Visual (Non-Destructive) Check for Visible Mold
The absence of obvious visible mold does not mean that a mold concern may not exist. On the other hand, the presence of large amounts of visible mold is a good indicator that a mold concern may exist. When checking for mold, be careful not to stir up lots of mold. The presence of mold and moisture are closely linked so the most likely areas of mold growth are sources of dampness and/or cold spots. The basement is usually the most common place to find visible mold. Bathrooms, kitchens and laundry rooms are other common locations. The  corners of closets on exterior walls often show mold, especially if the walls are blocked by contents in the closet.

A visual check should include the use of a bright flashlight (and camera). Shining a bright light at a low angle next to wall surfaces can often make mold much more obvious. In some cases, it may help to close shades or shut off main lights to make better use of a bright flashlight in a darkened room. A small pry-bar or painter’s multi-tool is useful to gently partly pull back baseboards and to lift up carpet corners (especially along the exterior walls and corners of basements).  Look for dark and multi-colored spotting. Light spotting and fuzzy growth may be mold.

Water stains, efflorescence (mineral) deposits, and warped wood surfaces (paneling, baseboards, windows, etc) may indicate past water damage. It should be emphasized most visible mold is actually hidden away behind walls.

Check the Drainage.
To check for visible mold, a good place to start is outside. A walk around a house or office building should include looking for low spots and poor drainage. Many mold problems are related to poor drainage. Are there extensions or splash plates for rain gutters? Are there rain gutters at all? If so, are the gutters filled with debris? Are the foundation walls cracked? Does the ground slope away from the house? Are there covers for the window wells? Visible mold is often found in basements at the building corners where rain gutters are not drained away or where low spots occur. If sump pumps are used, is the drainage hose far enough away from the house? If in-lawn watering systems are used, are they too close to the foundation (or used too much)?

Check the Roof.
Is the roof in good condition? Do the eaves include soffit vents? Does the roof have venting? Good attic ventilation is important to let moisture escape from a house. Is there adequate flashing on the roof, especially around chimneys and in roof valleys? Buildings with large and complex roofs sometimes cause drainage problems where large roof valleys occur. Rain gutters may not be able to keep up with heavy rains at such intersections. This may cause drainage problems in basement walls below these roof valleys.

Check the Walls & Closets Mold may be present on exterior walls where furniture, filing cabinets or belongings (photo frames, etc) are against the wall.
Mold may occur in ceiling or floor corners or along the upper edges of walls and ceiling immediately next to an outside soffit vent. Closets are a common place to find mold, especially in exterior corners. It may be necessary to move contents out of the way to look at the corners. Vinyl wallpaper (especially in basements and on exterior walls) may show pink spotting. In such cases, visible mold is often present on the back side of the vinyl.

Check the Carpeting.
Especially in basements, it can be useful to pull back carpet in the corners along exterior walls. Does the carpet smell musty? Are the wood tack strips rotted? Note: some staining of tack strips is common (in contact with damp concrete floor) and may not always indicate a concern.

Check the Windows.
Do the windows show lots of condensation or visible mold? It should be emphasized that some condensation on windows in cold weather and bathrooms is normal. However, large amounts of condensation on the windows of a house often indicate excessive dampness. Because windows are colder than walls, it is common for some condensation to occur and to drip down against the window sills. This is often sufficient for minor mold growth to occur on the dust, paint and top surface of wood or drywall on window edges. That is, some minor mold on windows is normal and often not cause for concern. However, large amounts of visible mold as well as warped or rotted windows may indicate a concern.

Check the Bathroom and Kitchen.
Bathrooms and kitchens are frequent sources of visible mold. Mold may occur under sinks and behind toilets as well as next to bathtubs and showers. Some minor visible mold (often called mildew) is common in many bathrooms and is not necessarily a cause for concern. Does the bathroom (and kitchen) include an exhaust fan? Does this fan vent to the outside? It should not vent into a wall or into the attic.

Check the Basement.
Mold problems occur most often in basements. On the ceiling, mold often grows on the rim joists and ceiling joists. A bright light used at a low angle (almost parallel) to the joist surfaces can sometimes highlight visible mold spotting. Mold especially tends to grow in corners and along the lower edges of the exterior walls.  

In finished basements, paper backed drywall along the exterior walls is especially prone to mold growth. If the bottom of the drywall is in contact with the concrete floor, this often allows dampness from the concrete to be wicked up into the drywall and cause mold growth behind baseboards and on the back side of the drywall. Any form of vapor barrier over drywall along exterior walls may allow mold growth. Unintentional vapor barriers include furniture next to walls, vinyl wallpaper or wood veneer paneling or wainscotting (waist high panels). Check behind furniture against walls. The edges of vinyl wallpaper or wood veneer paneling can be gently pulled back to check for mold. A very useful check is to look behind baseboards against drywall along exterior walls. A prybar or painters multi-tool can be used to pry open the baseboards about a ½ inch or so enough to see if mold occurs on the drywall next to the baseboard. It is very common to find small amounts of visible mold behind baseboards in basements and this may not always be a concern. However, large amounts of visible mold behind baseboards may indicate even more mold on the back side of the drywall.

Common places to find visible mold are along the walls behind washers, water heaters, furnaces and under the bottom of the stairs. Mold may occur under windows and behind baseboards under windows.

The fiberglass insulation in basement exterior walls can be an important source of mold. In many houses, plastic vapor barriers are placed over fiberglass insulation on basement walls. These walls may be left unfinished for a few years or more. The plastic sheeting tends to trap moisture inside the insulation and can allow mold growth (especially where the fiberglass touches the plastic). While mold cannot feed directly on fiberglass, they can feed on bindings and construction dust (especially paper and wood dust).

Look at the HVAC system and ductwork.
Does the HVAC system include a humidifier? The humid air from humidifiers often find cold spots (corners, windows, etc.) causing mold growth. If the ductwork is leaky, this may pressurize the basement and help drive airborne mold into the main floor.

Check the Crawlspace.
If access is available, check the crawlspace. It may be prudent to wear tyvex or coveralls and a respirator if there is a need to crawl around. Crawlspaces are often damp and allow mold growth. This is especially true for crawlspaces with fiberglass insulation against exterior walls. Check the
ceiling and joists. Does the floor have drainage or a plastic vapor barrier over it? Are there openings in the crawlspace ceiling (sometimes best seen with the lights off) into the main floor (around pipes, ductwork, etc)? Such openings may allow mold and odors from the crawlspace into main floors.

Check the Attic.
In most buildings, because water vapor is lighter than air, it is important that the attic have good ventilation. Humid air from the basement and main floor may be expected to rise through the drywall ceilings (permeable to water vapor) and into the attic. If the attic does not have good venting, this can trap moisture inside the house. Check under the insulation on the attic floor to see if a vapor barrier is present (plastic sheeting, metal foil etc). If so, this vapor barrier may trap moisture inside the house (often causing visible mold in the ceilings of main floor closets). Make sure any exhaust vents, including the furnace and bathroom are exhausted to the outside and NOT into the attic. Dark stains on roof sheathing may be mold but some forms of plywood have also been known to darken due to heat. Check for soffit vents. Make sure they are not plugged over with insulation. Shutting your flashlight off can help to show sunlight coming through soffit vents. Make sure the roof has roof vents and that they are not plugged over.

Looking Behind Walls – Visual (Destructive) Check for Visible Mold  
In many cases, much of the visible mold in a building is hidden behind walls. To look for this often requires some damage, referred to as destructive or intrusive testing. This does not mean that lots of destruction is required but only that some damage (sometimes very minor) must be done to wall surfaces to look behind them.   

One useful minimally destructive test is to cut small holes into the drywall along basement exterior walls behind the baseboards. The hole can be hidden when the baseboard is replaced. A small hole about 4-5 inches wide by 2-3 inches high (large enough to put a hand into and to look inside) can be made. If the back side of the drywall has visible mold, this indicates mold growth may be behind the wall. The wood sill plate and stud, insulation and sheathing within the wall can also be visually checked for obvious mold or water damage. At times, holes can be cut into walls inside closets or behind furniture to minimize obvious damage. In many cases, large holes must be cut into walls to adequately determine the extent of mold and damage.

In some cases, the seams along paneling can be pried open to look behind them (without causing too much damage). Optical borescopes can be used to look into walls through small holes (<1 inch wide) drilled  or cut into walls. However, MTS has found borescopes often have limited usefulness, especially  when wall cavities are filled with insulation.  

Destructive testing may stir up high levels of airborne mold and so should be done carefully, using appropriate precautions if needed (containment, respirators, HEPA air scrubbers, etc). Large scale destructive testing should not be performed in occupied areas. When holes are temporarily opened in walls, it is often preferable to temporarily seam them back up (using duct tape etc).

Using Your Nose – Musty Odors
In damp or wet conditions inside building, growing mold and bacteria give off many airborne volatile chemicals (called microbial volatile organic chemicals – MVOCs). The presence of musty odors, especially strong odors, is a reliable indication of mold. Most women have a much more sensitive sense of smell than men and often notice musty odors when men do not. Persons with allergies often also seem to have a more sensitive sense of smell than others.  

However, while strong musty odors may be present, this does not always mean that elevated airborne mold levels are also present. Lots of mold growth can occur behind walls, especially drywall with fiberglass insulation. The MVOCs (odors) can readily penetrate the drywall while mold spores cannot. For this reason, musty odors sometimes occur while room tests show very low airborne mold. However, in many cases, when strong musty odors are present, elevated airborne mold levels are often present. Whether or not a mold health concern is present, musty odors are emotionally upsetting and should not be ignored.

Musty odors are often noticeable when first entering a building or basement. However, after a few minutes, the odor may seem much less noticeable (this is called olfactory fatigue). A good way to check for musty odors is to check at any openings near the floor along exterior walls (especially in basements). Wall outlets or are useful to check. The faceplate should be removed (obviously be careful around electrical outlets). If the edges of the opening in the drywall  behind the outlet are painted over or sealed, two small holes (with an awl or small screwdriver) can be made into the drywall next to the outlet (hidden by the faceplate when replaced). This will allow enough airflow to detect odors. The person should put their nose close to the outlet. It may be necessary to briefly walk outside (or put your nose into the crease of your elbow for a moment) to let your nose “clear” and best smell the odor. If the room is also musty, you may not notice musty odors inside walls. Another option is to cut a small hole into a wall behind a baseboard and check for odors.

In some cases, musty odors may be masked by fresh paint or air fresheners. Musty odors may also only be really noticeable in damp or humid conditions. For example, a basement may have water damaged with mold growth inside the walls but the dried out. Musty odors may no longer be obvious and yet considerable mold may still be present behind the walls. Many homeowners notice musty odors much more in warm and humid weather than in the winter.

Field Meters
This section emphasizes use of field meters to check for mold, especially moisture meters. MTS offers the following for persons wanting to check for mold themselves. However, field meters often require some experience to use and sometimes give false readings (indicating moisture when it is not present).

Field meters include:
(1) Relative humidity meters (and data loggers); (2) Carbon dioxide meters (and data loggers); (3) penetrating moisture meters and; (4)  non-penetrating moisture meters

However, as note in the Visual/Nasal section, the main concern from mold is AIRBORNE EXPOSURE. Airborne mold is too small to see with the naked eye and requires sample collection (testing) and microscopic analysis. It should be emphasized any mold check that does not include airborne mold tests may be incomplete. Quite often, especially a few weeks after a water event, moisture meters may indicate dry surfaces even though extensive visible mold and elevated airborne mold levels may still be present. The use of field meters alone is often insufficient for an adequate check.

Humidity Meters.
The relative humidity of a room is a useful quick and inexpensive measurement to help determine if there may be a mold concern. It is also useful to see if adequate drying has been performed or if dehumidifiers etc may be needed. Digital relative humidity meters (also called hygrometers) can be purchased over the internet for about $20-30. Cheap plastic and needle dial hygrometers (<$10) are not recommended (much less accurate). Digital RH% automatic data loggers are available that can be left in a room (or wall cavity) for several days or weeks. Relative humidity is a function of both temperature and humidity. When keeping records, both the air temperature and RH% should be recorded.

In warm months (Midwest USA), the normal relative humidity (RH%) in rooms is 40-60% while in the cold months, normal RH% is often 15-30%. As a rule of thumb, it is preferable to keep RH% below 50% in the warm months and below 40% (but not below 20%) in the cold months. For drying out rooms, it is preferable to keep RH% below 40%. It is important to recognize that seasonal and even daily changes in RH% can occur. Just because a room has low humidity in the winter does not mean it may not have high humidity in the summer.

As RH% approaches 60-70% in rooms, cold spots (especially in basements) can often occur allowing RH% on surfaces to exceed 70-90% or for condensation to occur (reaching the dewpoint). Mold can grow on surfaces with RH% >70-80%.  In buildings or basements where the RH% is consistently above 60-70%, mold and dampness problems are almost certain to occur.

Carbon Dioxide (CO2) Meters.
Our respiratory systems act like furnaces. We take in oxygen and fuel (organic matter) and exhale carbon dioxide and water vapor. In occupied buildings, measurement of CO2 can sometimes serve as an indicator of adequate ventilation and if the building is too tight. This does not work for vacant buildings or for large buildings with only a few persons present. Outside CO2 levels are about 400 ppm. We exhale about 4-5% CO2 (40,000 to 50,000 ppm – Wikipedia). In occupied, non-problem buildings, normal CO2 levels are about 700 to 1500 ppm. If the office or house shows high levels of CO2, usually >2000 ppm, this may sometimes indicate the building is too tight (or inadequately ventilated with no outside fresh air provided). CO2 levels vary daily and weekly. This often requires taking several measurements (or using a data logger) over a few days. In offices, CO2 levels tend to peak in mid-afternoon and later in the work week. In houses, levels tend to peak in the middle of the night in bedrooms.

High levels of CO2 are sometimes associated with increased dampness. CO2 meters are fairly expensive, ranging from about $400 to $2000 or more. They usually have limited usefulness in moisture damage investigations but are good for IAQ investigations just to make sure adequate ventilation is provided.

Moisture Meters.
There are a variety of moisture meters that can be used to help determine the extent of dampness in walls or materials. These range from inexpensive $10 meters to very expensive meters. There are two basic types of moisture meters. The first are penetrating meters. These use metal needles or pins inserted into the surface to be tested. The other type are called non-penetrating meters. Depending on the type of meter, these meters send an electrical signal or other form of energy into the material to be tested. The depth of penetration depends on the type of meter but is often 1-2 inches deep (deeper in more expensive meters).  

It must be emphasized moisture meters can detect only moisture in a material at the time the meter is used. In many cases, a wall may have dried out before a moisture meter is used to check the wall a few weeks or longer after the wall became wet. Just because a wall tests dry does not mean that considerable mold may not actually be present behind the wall. If a wall tests dry immediately after a water incident (<48 hrs), then this is good evidence that a mold concern (at least from this incident) is not likely to occur.  

Moisture meters can be used to test a variety of materials but are most often used on drywall, plaster and wood or wood-like materials. As noted earlier, when the relative humidity on a surface exceeds 70-90% then mold growth can occur. For damp carpets, penetrating meters with large pins (metal points) may be needed (such as the Hydrosensor II).

The scales used for wood (and wood products) are different than for drywall and plaster. The moisture content in wood is compared to the dry weight of the wood (called the Equilibrium Moisture Content (EMC). Wood (depending on type) becomes damp when the EMC exceeds about 18-20% (this is equivalent to about 80-90 RH% (Hoadley, 2000,p.113). As a rule of thumb, wood is considered “damp” when the moisture meter reading (wood setting) is >20%. Visible mold and dry rot can occur >20%. Note: some visible mold can also occur on damp surfaces of wood even when the wood moisture below the surface is <20%. For wet rot to occur, EMC must be >30% (Oliver, 1997).

Drywall and plaster with a moisture meter reading (drywall or plaster setting) >70-90% is considered damp and likely to support mold growth. In general, “dry” drywall is typically <50% in warm weather and <20% in cold weather. Note: plaster is typically much less sensitive to moisture and dampness than is standard paper backed drywall.

It should be emphasized that these moisture meter readings are NOT the same as relative humidity (measurement of the humidity in the air dependent on temperature). However, they may sometimes approximate each other. Large seasonal variations in the moisture content of materials typically occur. There is a close interaction with the relative humidity of the adjacent air. In humid weather, it is not unusual for the moisture content in drywall to exceed 50% (and yet still not be a mold concern). By contrast, in cold weather, drywall moisture may drop below 10%.

Technicalities aside, high readings indicate dampness and the potential for mold growth. It is very important to use the appropriate scales on the meters. For example, while a moisture content of 30% in drywall (drywall setting) may be of no concern, the same reading using the wood setting would indicate the wood to be wet.

The moisture content (MC) of materials is measured by comparing by weight the damp material versus the dried material. Prezant, (2008,p.56) note for a “damp material” showing 80% “moisture (water activity)” on a moisture meter, the actual MC is only 0.7% in drywall, 1% in concrete, and 17% in wood. It is important to note that the moisture meter readings in different materials is NOT the actual MC (by weight) of the material.

For concrete, brick, and masonry products, moisture measurement and assessment is much more complicated. Depending on the material, various percentages of water (by weight) may indicate dampness (Oliver, 1997). These materials are also much less prone to mold growth (but can support mold growth on damp surfaces). It is normal to find some dampness in below grade (basement) concrete floors and walls. Fresh concrete can take many months to dry out, releasing lots of moisture in the process. If new flooring is installed over damp concrete, this sometimes causes chemical interactions giving off irritating and smelly odors. Various types of penetrating and non-penetrating meters (and sorbancy tests) are available (Kanare, 2005).

Non-Penetrating Meters (NPM).
For drywall and wood, NPM are often preferred because they can provide rapid assessment without causing any damage to walls or surfaces. They use non-damaging radio waves or other forms of electronic signal to detect moisture inside surfaces. The typical depth of penetration for the more common meters is <1 inch below the surface. However, NPM do have some shortcomings. Moisture may occur on the upper surface of a material (paint layer etc) and set off the NPM even though the actual material (under the paint etc) is actually dry. NPM meters often detect metal and may set off false positives (seeming damp when not). The metal mesh in plaster can set off the meter as can wall paper coverings with metallic finishes. That is, while a useful tool, NPM are not always as accurate as penetrating meters.

Special non-penetrating meters that send electronic signals 3-4 inches or more into materials are available for Dry-Vit (EIFS), roofs and other materials with thick surfaces.

Penetrating (invasive) Meters (PM).
For drywall and wood, PM provide a more reliable assessment than do the NPM. Metal prongs (pins) are used to penetrate the surface to be tested. Measurement of the moisture inside a material provides a more accurate assessment than on only the surface. However, they do create small double holes when used. Although these holes are typically very small (thumb-tack like), this can still be upsetting and unacceptable to many building owners. It is often preferable to use PM in inconspicuous areas such as along baseboards, under the edge of wall outlets and along window edges rather than in open areas. The typical holes are only about ¼ inch deep. However, for some surfaces, such as Dry-Vit or to check the sheathing inside a wall cavity, longer (and larger) probes (4-6 inches) may be used. These cause more obvious holes (ice-pick sized). These large probes can often be used behind baseboards to avoid noticeable holes. They can also be installed at an angle behind baseboards. Holes can be back-filled with various materials.

Moisture Mapping – Using Meters and Visible Mold.
When performing moisture mapping, it can be very helpful to use painter’s tape (or masking tape) to map out damp areas on walls and floors. Photos and measurements can then be taken of the damp areas. It is also worth noting that visible mold is actually a “moisture meter” indicating a surface that was damp at one time (but may not be now). Moisture mapping is very useful immediately after a water loss (and after drying) but is less reliable when performed a few weeks or more after a water loss. Very moldy surfaces may be present on the back sides of walls and yet the walls may test “dry” a few weeks after a water loss.

Thermal (Infrared) Cameras.
An increasingly popular tool for detecting moisture is the thermal imaging camera. This camera uses infrared (IR) to identify minute changes in temperature. While the best IR cameras cost about $10,000 or more, costs are quickly dropping and cheaper models are now available for about $1500. Many home energy firms and home inspectors can provide often fairly low cost IR inspections (about $150-500). It is important to emphasize that the IR camera detects changes in temperature. It does NOT measure moisture. The IR camera detects extremely small relative changes in temperature. That is, small changes in temperature may seem visually impressive and yet mean little.

It is true that damp or wet building materials will have greater conductance than other materials (hotter in warm weather, colder in cold weather). Damp materials also are cooler (often much cooler) because of moisture evaporating from them (1 gram evaporated water releases 539 calories of heat – Holst, 2000, p.23).

Colder spots are often more prone to dampness but many cold spots occur in buildings that are not damp. IR cameras are very useful for a quick scan of a building, especially after a recent water loss. But, it is very important any potentially damp areas be confirmed by moisture meters or opening up the wall, etc. IR cameras used alone are unreliable and may be misleading implying a major concern where none exists. IR changes in buildings vary seasonally. The lower walls of a basement may be cool in the summer and yet be (relatively) warmer in the winter.  

Many factors can affect thermal cameras and any interpretation of materials as damp should be backed up by visual inspection and/or moisture meters.

What May Looks Like Visible Mold But Is Not
In many cases, apparent mold reported by a building owner or home inspector has turned out NOT to be mold.  Black soot is often mistaken for mold. Black soot tends to accumulate on cold surfaces such as the studs along walls (called “zebra-striping” or “ghosting” and on metal or electrical surfaces (around wall outlets, return air vents, etc). The most common source of black soot is from burning candles but may also occur from furnaces, stoves, etc. In some cases, dirt, especially below windows in basements, has been mistaken for mold. Rust stains are often black and sometimes mistaken for mold (such as on sheathing in attics). In one case, dark spotting from the splatter from a spilled pop can was mistaken for mold.

Perhaps the most common misidentified substance has been the whitish to yellowish mineral deposits (called efflorescence) that occur most often on concrete walls (especially along grout lines). Mineral deposits seeping through the concrete dry out, leaving behind a powdery or crystalline deposit.

A simple tape test (and subsequent microscopic identification) can almost always determine whether a substance is mold or not. For efflorescence, if adding acid to the material in a teaspoon or glass causes bubbling, it is probably a mineral deposit and not mold.

What Are Visible Mold Guidelines?
There are no well defined rules for visible mold in buildings. An international committee in 1992 offered the recommendation that: “visible fungal growth in non-industrial indoor environment is not acceptable on medical (and hygienic) grounds" (Samson,, 1994,p.532). Simply put, as a rule of thumb, the presence of visible mold is generally considered unacceptable in occupied buildings. Ideally, no obvious visible mold should be present inside buildings.

The problem is this statement leaves unanswered how much visible mold must be present to pose a concern? From a practical perspective, slight amounts of visible mold are often found in many buildings and houses and yet often pose no health concern. To come up with an exact limit of visible mold is virtually impossible as conditions will vary from building to building as what may pose a concern and what may not. The presence of visible mold may be an indicator of much more massive growth behind the walls or may indicate growth is only occurring where visible. As a rule of thumb, large amounts of visible mold (regardless of the type of mold) pose concerns while small amounts (regardless of the type of mold) do not. Large amounts of visible mold are often found with even more visible mold hidden behind walls and with high levels of airborne mold.

The question becomes what is a large amount of visible mold?
The New York City Health Dept (2008) offers a guideline that <10 ft2 of visible mold as “small” but still requires appropriate procedures for clean-up. Ten square feet is equivalent to one spot about 3 x 3 ft or the equivalent sum of several smaller mold spots adding up to this amount. Various other organizations also use this guideline of 10 ft2. One organization, ISIAQ (1996) suggested an area <2.15 ft2 (0.2 m2) was sufficiently small that respirators and containment might not be needed (see discussion of these guidelines in Prezant,,2008). Any visible mold guide is of limited usefulness. While some visible mold behind a washer in the basement of a house may not be a concern, the same amount in a restaurant kitchen or operating room of a hospital would be unacceptable.

Building – Outstanding source of information on moisture in buildings. Brilliant and clear information on many topics – basements, roofs, vapor barriers, etc. Visit Website.

Hoadley,  R. Bruce, 2000, Understanding Wood – A Craftsman’s Guide to Wood Technology, Taunton Press, Newton, CT (p.113 wood moisture graph) Outstanding book about wood.

Holst,Gerald, 2000, Common Sense Approach to Thermal Imaging, SPIE Press, JDC Publishing, Bellingham, Washington (technical book, some limited information on IR & buildings)

Kanare, Howard, 2005, Concete Floors and Moisture,  Portland Cement Association. Visit Website. Very good book on concrete floors and moisture problems.

Oliver, Alan, Douglas, J. and Sterling, S., 1997, Dampness in Buildings, 2nd Ed., Blackwell Science, Cambridge, MA (p. 84 – wood decay 20%, wet wood rot 30%). Reviews moisture in many types of building materials.

Oxley, T. and Gobert, E., 1983, Dampness in Buildings – Diagnosis, Treatment, Instruments, 2nd Ed., Butterworth-Heineman, Woburn, MA (moisture meter guide). Very good short reference.

NYCDOHMH, 2008, Mold Growth: Prevention and Clean-Up for Building Owners and Managers, NY Dept Health and Mental Hygiene (p.8  <10 ft2 – small areas). Visit Website.

Prezant, Bradley, Weekes, D. and Miller, D., Editors, 2008, Recognition, Evaluation, and Control of Indoor Mold,  Am, Inst. Industrial Hygiene, Fairfax, VA (Chapter 17 – mold guidelines). Useful reference.

Professional  - Large variety of moisture meters and other meters. Visit Website.

Samson, R. et. al., 1994, Health Implications of Fungi In Indoor Environments, Elsevier, New York ( p. 532 – Visible mold unacceptable in occupied buildings on health and hygienic grounds).

Lab Analysis
Testing for mold refers to collection of samples for submittal to a laboratory for microscopic analysis (or other forms of testing requiring a lab).