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

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

How to find hidden mold in buildings: trapped mold between building surfaces or materials.

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.

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 Friedman

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]

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:

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?

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:

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.


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





Suggested citation for this web page

HIDDEN MOLD in FRAMING / SHEATHING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Questions & answers or comments about mold traps: mold that may be hidden between layers of building materials.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman