Moldy subfloor and joists may suggest additional hidden trapped mold contamination -  © Daniel Friedman Hidden or Trapped Mold in Building Framing or Sheathing
What to Do About Trapped Mold Growth or Spore Contamination in Buildings

  • HIDDEN MOLD in FRAMING / SHEATHING - CONTENTS: Mold between floor framing & subfloor surfaces. Mold suspected between layers of subfloor and finished flooring. Mold growth between rafter edges and roof decking. Mold growth suspected between wall studs and exterior wall sheathing. OPM: use of other people's money to reduce the consultant's risk? Don't mistake pre-existing, harmless cosmetic black mold for new problematic mold growth. Where may mold be hidden in buildings.
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How to find hidden mold in buildings: trapped mold between building surfaces or materials.

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Hidden Trapped Mold in Buildings: what to do about mold between wood framing, sheathing or other building surfaces

Test cuts for mold between flooring layers (C) Daniel FriedmanThe 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?

Our photo (left) illustrates how we made a test cut through subfloor from below, without damaging the finish flooring above, in order to inspect for trapped mold between layers of building floor materials after a flood. Details of this procedure are discussed later in this article.

[Click to enlarge any image]

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.

Our page top photograph shows heavy mold contamination on floor joists and on the visible under-surface of subflooring? What about mold that may be present on the hidden top edge of these floor joists? Is that a problem that justifies demolition of the subfloor?

This document describes both opinion and direct field experience in finding, testing, and handling mold on hidden building surfaces such as between the upper surface of floor joists and the underside of the area of subfloor contacting the joist surface. Similar mold traps that form potential hidden mold reservoirs in buildings include:

  • Mold growth on the underside of wall sill plates or on the area of subflooring beneath wall sill plates
  • Mold growth or suspected on the surfaces between wall studs and exterior building wall sheathing
  • Mold growth or suspected mold on contacting wood surfaces such as the upper side of rafters and the underside of roof sheathing
  • Mold growth or suspected mold between multiple layers of subflooring, or between the upper surface of subflooring and the underside of finish wood flooring in a building

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.

Guide to Testing, Handling, Killing, or Removing Hidden Mold Between Building Material Surfaces

What are the risks or other factors that provide justification for investigating a building for hidden mold contamination, and in particular, what factors might lead us to go to the trouble of invasive or destructive inspection, tests, or other studies to find hidden mold trapped between wood framing surfaces? Here are some key questions to ask about trapped mold in buildings?

  • What is the history of visible mold in the building, mold contamination, or previous mold cleanup projects?
  • Harmful vs cosmetic mold: what is the evidence that the mold contamination that was or is visible, or that was detected by surface, dust, or air tests is a harmful material?
  • Potential release of trapped mold: If mold cleaning has been completed but potentially moldy areas such as mating surfaces between a sill plate and subfloor surface or between the top of a floor joist and the touching underside of subfloor materials, what are the chances that mold from such an area is capable of entering the living area? This question presumes that we have some reason to believe that the hidden mold is of a harmful genera/species.
  • If mold remains trapped between building materials, what is the potential for movement of existing moldy material, hyphal fragments, or mold spores, out of the trapped area into the occupied space? How much air and moisture movement is there between the trapped space and the rest of the building?
  • Size of remaining mold reservoir: if mold remains in a building because it has not been found or after a mold cleanup project, how much is present? What is the worst-case potential mold reservoir size? More than 30 sq.ft. of contiguous problem mold? A potentially harmful quantity?
  • What simple inexpensive steps can be taken to reduce risk of contamination inside of a building from trapped mold between wood surfaces? Examples are application of sealants and steps to reduce future moisture uptake in wood materials. What risks remain after such steps are taken?

Is Mold Trapped Between Wood Framing Surfaces a Risk to Building Occupants?

Roof mold photo (C) Daniel FriedmanIs trapped mold present at all? First, we don't know whether mold is present on mating wood surfaces such as between floor joists and subfloors, without destructive inspection to determine that presence as well as its extent.

We can inspect for mold between layers of subfloor or between subfloor and finish flooring by making test cuts from the underside of such surfaces, described at Photo Guide to using a round hole saw.

Our photograph of black mold on roof sheathing in an poorly-vented attic (left) displays a diagnostic history of the building, its moisture, and its attic mold. The vertical white stripes between areas of black mold on roof sheathing mark the location of previous wood framing that was removed (the roof was re-framed).

Because the light stripes mark areas where attic mold had not grown, we can see without doubt that the potential mold trap between the upper surface of the (now removed) rafters and the under-side of the roof sheathing (the white stripes) was precisely where attic mold did not grow.

The explanation of these white stripes on roof sheathing gives insight into the probability that troublesome mold growth is likely to be present or absent between the surface of a rafter or joist and the sheathing or subflooring that is attached to it. An understanding of the moisture source and the moisture or water movement path in a specific building allows us to estimate the chance of hidden mold in the mating surfaces of wood framing and sheathing.

In this attic the moisture source was the building below. Poor attic venting trapped warm moist air in cold weather, leading to condensation on the exposed attic surfaces. Because the mating surfaces of rafter upper-sides and roof sheathing under-side were not exposed to attic air and attic condensation, the risk of mold formation on those mating surfaces was much less.

Are the Typical Quantity & Mobility of Trapped Mold on Wood Surfaces a Risk to Building Occupants

Unlike large mold reservoirs found in building wall cavities on the cavity side of drywall or in building insulation (these can be a problem in some buildings as air moves in and out of building cavities), mold trapped on mating surfaces such as between floor joist tops and subfloor undersides is very unlikely to be a hazard to building occupants.

We explain this view in more detail below. If however the mold is a species harmful to wood, causing rot and structural damage, further steps might be needed to protect the building. (For an example of mold harmful to buildings see MERULIPORIA MOLD PHOTOS).

Field Experience Inspecting & Testing Mold on Inaccessible Mating Building Material Surfaces

Mold encapsulant spray (C) Daniel FriedmanTrapped mold not detected in building air, surface, or dust samples: We have studied this problem in a number of catastrophic mold cleanup projects where in early years people believed it was appropriate to disassemble such framing to expose the hidden surfaces.

Later we inspected and measured buildings where, for example, a subfloor was left intact, cleaned on all exposed surfaces, but with no attempt to remove subfloor to clean the contact area between the subfloor and the tops of floor joists.

The areas were thoroughly cleaned and thoroughly treated with a fungicidal sealant. In our field and lab investigations since 1986, where mold removal/cleaning and sealants were properly performed and applied, subsequent field tests have not detected mold spores nor other particles entering the building from those locations.

Correcting the cause of mold reduces the risk of further growth of trapped mold: If there was mold growth between a sill plate and subfloor, but the building is now dry, which is highly likely in a dry climate a year after dry-in of the building (use a moisture meter with pin sensors and/or a radio-signal type such as the Tramex™), there will be no measurable movement of mold spores or particles out into the building air from those spaces, regardless of whether you caulk or not.

That has been my field experience with carefully instrumented measurements, including using a vacuum pump to try to pull particles out of such areas - there is just not sufficient air movement in and out of such tight spaces.

Harmfulness of trapped mold may be unknown: Furthermore, seeing "mold" on the bottom of sill plates does not tell us anything about whether or not the mold is harmful or cosmetic - some cosmetic molds enter buildings at the time of construction, already present on wood. Nor does such an inspection indicate the possibility of such spores entering the living area.

Mold anxiety may lead to unnecessary mold cleanup or sealant costs: We realize that leaving "mold" in place even in on sealed surfaces is uncomfortable for people who have already some reason for feeling anxious about a building. Think of it like seeing a few small knots in a 2x4 wall stud. If the stud is able to perform its structural duty, few experienced builders would insist that only clear lumber (free of knots or other cosmetic defects) be used to frame a building.

The trapped mold may be blamed for other, more serious harmful mold or allergen reservoirs: If you continue to have an allergic reaction to such a building, we would suggest that you or your mold expert look for a problem mold or other particle reservoir that has not yet been identified, such as moldy insulation that looks "clean"

Future mold risk in buildings where mold cleanup has occurred: Finally, even after a building is inspected and tested for mold and allergens by a well qualified expert (if such testing is risk/cost justified) that is absolutely no guarantee against a future mold problem.

Proper construction in many details (roofing, insulation, ventilation, plumbing, site drainage, choice of materials, installation details), must be supplemented by proper building maintenance (leak prevention, rapid response to leaks and water entry) to avoid a future mold problem.

See MOLD PREVENTION GUIDE for detailed advice on prevention of mold contamination in buildings.

To evaluate the leak and moisture history of your building, the overall overall building risk for problematic mold contamination is discussed

When to Hire a Professional to inspect or test for mold? MOLD / ENVIRONMENTAL EXPERT, HIRE ? helps consumers decide if it’s appropriate to bring in an mold professional - someone without conflicts of interest and who does not themselves offer mold cleanup or remediation services. If there seem to be particularly high risks (building history, what you see, complaints or health risks of occupants) it might be appropriate to hire an expert

To Do your own sampling of surfaces or dust for evidence of problematic mold see MOLD TEST KITS for DIY MOLD TESTS, a detailed procedure that describes how to collect DIY samples of what you see (you wont' address hidden mold in insulation or in building cavities by this method) using tape sampling.

What about "killing" the hidden mold in place using bleach (see MOLD CLEANUP with BLEACH) heat (mold pasteurization), ozone (see OZONE MOLD / ODOR TREATMENT WARNINGS) , or tenting and gases?

Heating mold to pasteurize it or using other mold killing methods is pretty much junk science sold by companies preying on consumer mold fear, as is "killing" it by any process since many particles remain allergenic or toxic, live or dead.

See MOLD KILLING GUIDE for an explanation of why mold killing attempts are usually a costly mistake.

OPM: Some Consultants Spend Other People's Money to Reduce Risk For the Consultants, not for the Clients

Extensive demolition for mold cleanup (C) Daniel FriedmanWith this as many topics, even smart people but with no actual field test data to support a view, construct mental models of what they think is happening (yet without a single fact nor shred of evidence) and give aggressive cleanup advice accordingly.

This approach is especially dangerous when the advice suffers what we call the "OPM" problem: that is, Ms. X, the consultant give advice that is safest for her (because the client cannot possibly complain in the future that her advice was not aggressive enough), because the safest (for her) advice does not cost her a dime - the client is the one who pays for the sometimes extreme measures that are suggested.

Consultants who offer OPM-based advice simply playing it safe and they are not earning their fee.

Our photograph (above left) shows extensive demolition performed to remove problem mold on floor framing and subflooring in an older home.

In this case the combination of multiple layers of moldy subflooring and flooring combined with a cost comparison of alternative approaches indicated that it was actually less costly to remove the subfloor and clean the joists than to leave the subfloor in place. In other buildings and circumstances it is reasonable and more economical to leave the subflooring in place.

Reader Questions About Mold "Trapped" Between Wooden Building Material Surfaces - not readily accessible for cleaning without costly measures

The following questions were sent to and suggested this article: TRAPPED MOLD BETWEEN WOOD SURFACES

  • Occupant sensitivity to mold or other allergens: I have been building a house in the mountains of New Mexico for quite awhile and have undergone a lot of difficulties with the process. I have an allergic and chemically sensitive condition and was attempting to build a healthful house. I have put everything I have into this house and my family and friends have contributed all they could too. I want to do the best thing, but not over-react.
  • Building leak history and hidden mold risk: While the house was being framed, it rained heavily. Water would sit up against the sill plates even though I would try to sweep the water out every day. The framing process was slow so this went on for too long. The framing of the house developed some molded areas, so I had most of it sanded off at fairly high expense. Recently, I had a couple of plates cut out that went across doorways and found that the bottoms of the plates were blackened. I dropped a section of the plate off to someone who deals with mold issues and he said it was mold on the bottoms of the plates.
  • Current building conditions affecting mold risk: he house has been dried-in for over a year now and the exterior is well waterproofed. I have not done the insulation or interior walls yet.
  • Other risks due to hidden trapped mold between wood surfaces? The person who deals with mold who saw the plate said that he did not think I had molds that would compromise the structure, but there could conceivably be some toxic molds. He said the more I pursue it however the more I would need to disclose if I sell the place and just ignoring it was a course to consider.
  • Opinions of "mold experts" about the mold risk in the building: When I had talked to him earlier, he did not express much concern about mold being on the bottom of the sill plates and said that I could just run a bead of caulk around it. He said that it was just important that the wall stay dry.

    After looking at the wood, he sounded more concerned. He said that it might not be a problem, but sounded far from certain. I have talked to builders and gotten differing views on how bad and/or abnormal the situation is.

    The project manager who was over-seeing the building of my house at the time said most of the time when he takes up sill plates in doorways they are blackened on the bottom. Another builder said that is not usual.
  • Goofy mold killing proposals? we found some information on the web about companies that "pasteurize" mold by heating up the building (to 150 degrees). I talked to a representative from a company that does that process and he said that mold typically develops during framing if it rains and he did not see mold under the plates as a big problem and suggested I do nothing.

    If this is a relatively normal thing, I may choose to do that. The person who deals with mold said it is not usual to have mold under the plates and that someone would normally tent a house if there was rain while framing. I have never seen that done in my area and it would be very difficult with the winds here if it could be done at all.

    It was very unfortunate for me that the rains were so heavy at the time of the framing and I believe there were inadequacies in the framing process.
  • What do you think about the process of heating the house up to 150 degrees for enough time to at least reduce the amount of viable mold and then sealing around the inner edges of the plates to keep spores from entering the living space?
    The companies that do this are not located in New Mexico, but one representative said I could do it myself with construction heaters and that he would advise me on how to do it.

    I have been wondering if there is some way to just heat up the sill plates since I have sanded most of the mold that was on the rest of the framing.

    I have seen ads for something called a dry vapor steam cleaner that can kill mold, but I don't know if it could penetrate under the plates enough. The water managed to penetrate however (even though there are beads of caulk under there). I have also considered trying to force some borate solution under the plate. we are wondering if you think Concrobium would be helpful to keep the mold contained under the plate.
  • Simple trapped mold remedy? It has been suggested to me that a bead of caulk on the interior of the sill plate (as suggested by the mold consultant) would just hold in any moisture that might develop under the plate and make the situation worse. But exterior to the plate and exterior sheathing there is 3 inches of 2 lb. foam and I have been told that the high density foam keeps the dew point away from the inside of the wall so there may not be appreciable condensation in the wall.

    The exterior sheathing has two layers of Tyvek, one of which is stucco wrap which is supposed to keep condensation from building up under the stucco. I don't know if condensation is likely to occur between the slab and the plate. There is a vapor barrier under the slab. Most of the house is fairly high up from grade.

Inspecting Multiple Layers of Flooring & Subfloors or Wall Studs & Sheathing for Hidden Mold

Test cuts for mold between flooring layers (C) Daniel Friedman Test cuts for mold between flooring layers (C) Daniel Friedman

Our two photographs just above demonstrate a procedure we use to inspect for evidence of high levels of mold trapped between layers of flooring and subflooring in a building.

Detailed suggestions for deciding when such inspections are appropriate and how to conduct hidden mold in flooring are located in a separate article: HIDDEN MOLD in FLOOR / SUB-FLOOR.

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.

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.

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.

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.



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