Can an inspector test for mold in walls without cutting them open? Partly. Without cutting, air samples compare the mold spores inside with the air outside, and a thermal camera and a moisture meter show where a wall is wet. To find out whether mold is growing inside the wall, the inspector drills into it and takes swab and air samples from inside. We do both, with the homeowner's or landlord's approval before any drilling.
How do inspectors test for mold in walls without opening them?
Without opening the wall, a non-invasive mold inspection uses a thermal camera and a moisture meter to find where the wall is wet, and air samples to compare the mold spores indoors with the air outside. None of these, on its own, proves that mold is growing inside the wall.
Hidden mold can grow on the back of drywall, wallpaper or paneling, under carpets and pads, and inside walls around leaking or condensing pipes, as EPA's guide to mold, moisture and your home explains.
For suspected hidden mold, we recommend one inspection with four parts. Three work without opening the wall; the fourth, destructive testing, is covered below.
- Air samples: more spores inside than outside, or different kinds, point to a mold source indoors.
- Thermal imaging reads temperature differences on wall and ceiling surfaces. A wet area often shows as a cooler patch, which tells the inspector where to measure and sample.
- Moisture testing measures how wet the wall material is.
A pinless meter reads without breaking the surface and shows roughly where moisture is; a pin meter reads only the spot its pins touch and confirms it, as Protimeter's guide to how moisture meters work explains. Both tools find moisture, not mold, and a wet spot needs a lab test to confirm mold.
Mold you can see on drywall is a different case: see our guide to black mold on drywall. Not sure anything is wrong? Start with the signs of hidden mold.
Can a thermal camera see mold behind walls?
No. A thermal camera cannot see through a wall, and it cannot detect mold. It reads the heat of the wall's surface, not what is behind it.
Something inside the wall shows up only if it changes the surface temperature enough, as FLIR's guide to whether thermal cameras see through walls puts it. Hidden moisture can do that, though the temperature differences are small. The Infraspection Institute's note on infrared imaging and mold adds that moisture often goes with mold. Normal parts show up too: studs can show as lines colder than the insulation around them.
A pattern that looks like water does not prove there is water, which is why FLIR's guide to finding water leaks with thermal imaging and InterNACHI's guide to infrared moisture inspections both pair the camera with a moisture meter. Weather and indoor humidity change what the camera can pick up, and a clean thermal scan does not rule out mold.
What can each mold sample tell you, and what can't it?
An air sample compares the spore types in your indoor air with the air outside, at one moment, and a tape sample lets a lab tell whether a substance on a surface is mold or only looks like it. On its own, an air sample cannot show whether mold is growing or how much.
An inspection and a lab test answer different questions; our guide to mold inspection vs mold testing explains which you need.
Air samples. Our air and surface samples go to an AIHA-accredited lab. AIHA's guidance on spore trap air sampling limits the result to one question: are the spore types indoors clearly different from those outdoors or in a comparison room? The lab reads spore types under a microscope; naming the exact mold takes a culture or a DNA test. Counts in one room can change up to 10,000 times over a day, so one sample does not represent an area. An air sample also cannot tell you the health risk to the people in the home, and there are no federal limits for mold spores.
Surface samples. Sampling may also help tell mold apart from soot or dirt. How air and surface samples compare covers when each fits.
Dust tests. ERMI and HERTSMI-2 read the DNA of mold in settled dust: 36 molds for ERMI and 5 for HERTSMI-2 in our lab's reports (see how ERMI compares with air sampling). EPA's fact sheet on ERMI calls it a research tool and does not recommend its routine use in homes, because it has not been validated outside research. HERTSMI-2 rests on its makers' own study, with no independent study we could find, and neither test is one of the four parts we recommend for hidden mold.
If the air sample is clean, is the wall clean?
Not necessarily: an inspector who tests only the room air can miss hidden mold inside a wall. Mold inside a wall is not always in direct contact with the room air, so a clean room-air result cannot rule it out.
In a 2002 study of mold inside the insulated walls of 88 Finnish flats, mold in the wall insulation had no significant effect on the room air. Few walls had heavy mold, which made an effect hard to see. Other climates, wall designs or ventilation may behave differently, and the study does not show how often this happens in US homes.
Mold inside a wall may or may not reach the room air, and a few short air samples can miss what is in the air over time.
How do inspectors test for mold inside a wall?
To test inside a wall, the inspector drills into it and takes swab and air samples from the wall cavity; this is called destructive testing, and we do it when you ask for it, with the homeowner's or landlord's approval.
When hidden mold is suspected, the inspection we recommend includes it, because the camera reads only the wall's surface and mold inside a wall may not show up in the room air. It is not part of every inspection, and neither the InterNACHI home inspection standard nor the IAC2 mold inspection standard requires opening anything.
If you rent, get the landlord's or owner's approval first. A tenant who is not the owner may be liable for damage from this kind of inspection.
Some investigators look inside a wall with a borescope instead: an optical probe pushed through a small drilled hole, which shows a small part of the wall without much damage. EPA's mold course on finding mold and moisture lists borescopes and moisture meters among the equipment that may help find hidden mold.
Opening a place where mold may be growing needs care, because it can release spores; peeling moldy wallpaper, for example, can release a great many. EPA's mold remediation guide for schools and commercial buildings names invasive work in a wall cavity among the actions likely to stir up mold, and calls for care and protective equipment.
Does a home inspection find hidden mold?
Not by design. A home inspection is a non-invasive, visual look at the accessible parts of a home, and it is not meant to find concealed defects.
Under InterNACHI's home inspection standards of practice, the home inspector is not required to determine whether mold is present, use a moisture meter, or open or uncover anything.
The IAC2 mold inspection standards of practice go further: moisture readings and a surface sample in each area of concern, and indoor air samples with outdoor controls. Even that standard puts hidden mold outside its scope and lets the inspector decline anything that could damage the property. Looking inside a closed wall takes an extra step the customer asks for, such as our destructive testing.
How do you test for mold in your HVAC and air ducts?
Under the IAC2 mold inspection standard, the inspector looks at the air handler, fan, filter, condensate pump, visible ductwork and registers, and takes at least one air sample at a supply register, the vent that blows air into a room. The standard does not require a look inside the ducts, and many parts of a system cannot be reached anyway. A lab may be needed to confirm that what looks like mold is mold.
Where does mold grow in an HVAC system?
Where moisture collects: inside sheet-metal ducts and in registers, cooling coils, drain pans, the fan and the air handler housing, according to EPA's guide to air duct cleaning.
What are the signs of mold in an HVAC system?
Condensation or high humidity in the system is a warning sign. Cooling coils can be a major source of that moisture, so the condensate pan has to drain. Suspect the system when it is part of a known moisture problem or there is mold near its air intake.
What if your HVAC has mold?
Don't run it: a system you know or suspect has mold can spread mold through the building. Don't count on duct cleaning either; it has never been shown to prevent health problems.
Do you check the HVAC during a mold inspection?
Yes. We look at the HVAC and swab it as part of our mold inspection; it is not a separate service. What to do next is set out below.
What should you do if you suspect hidden mold?
Skip the DIY kit, get one mold inspection with all four parts, and call a remediator only once mold is confirmed.
- No DIY kit. Our guide on how to test for black mold compares DIY kits with professional testing.
- One inspection, four parts. For hidden mold, EPA also suggests an experienced professional.
- Renters ask first. Get the landlord's or owner's approval before any drilling.
- Remediator last. Calling one before mold is confirmed can add fees you did not need to pay. We do not offer remediation services.
Lab results come within 1–2 business days of the inspection; weekends, holidays and extra services such as ERMI or HERTSMI-2 extend that.
When mold is visible, EPA says sampling is usually unnecessary. Hidden mold is the other case: sampling may be worth considering when the source is unclear, with a plan built on a theory the samples can confirm or rule out.
Frequently Asked Questions
- How do I know if there is mold behind my walls?
- Suspect hidden mold when your home smells moldy but you cannot see where the smell comes from, EPA advises, or after water damage when people in the home report health problems. Mold is one possible cause of those problems, and testing can help rule it in or out.
- Can mold inside a wall get into the air you breathe?
- It can: in lab tests, mold spores passed through a test wall and through wall outlets when air pressure pushed them, and how much got through depended on the pressure difference. In the same lab wall, larger particles did not get through at all unless there was a direct air path.
- Does duct cleaning get rid of mold in an HVAC system?
- Not on its own: mold grows back if the moisture that caused it is not fixed, and EPA says wet or moldy duct insulation and fiberglass duct liner should be replaced, not cleaned. Controlling moisture is the most effective way to keep mold out of ducts.
- Can I test for hidden mold myself?
- Not with a DIY kit: it cannot find mold hidden inside a wall or under a floor. Mold growing on a settling plate, a petri dish left open in a room, does not show that your ducts have mold either, because some growth on a settling plate is normal.