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Quick answer

The usual signs of mold behind drywall are a musty smell with no visible source, stains, discoloration or wet spots, peeling paint, drywall that sags or bows, condensation on the wall, and a past leak in or above that wall. [4] [8] Find and stop the water first, then check the wall with a moisture meter and by looking at baseboards, behind furniture and the other side of the wall. EPA says that in most cases you can handle a moldy area under about 10 square feet yourself, and that if you believe you have a hidden mold problem, you should consider hiring an experienced professional. [4]

Do this first: find and stop the water

Mold inside a wall is a moisture problem before it is a mold problem. EPA says the way to control indoor mold growth is to control moisture. [1] FEMA adds that mold will keep growing until the source of moisture is eliminated and the mold problem is dealt with. [2] Cleaning the surface of the wall does nothing if water is still feeding the back of it.

Stop if water is near power

If the wet wall is near outlets, switches, cords or the electrical panel, do not touch them. Turn power off at the panel only if you can reach it from a dry spot; otherwise call your utility or an electrician. FEMA says to turn off main power if wiring is wet or moldy and to have an electrician check the system before turning power back on. [2] Stay out from under any ceiling that is sagging.

  1. Think about what sits on the other side of or above the wall: a shower, toilet, sink, washing machine, water heater, window, roof line or an outside wall that takes rain.
  2. Shut off the water to a leaking fixture, or close the main valve if you cannot find the source.
  3. With every faucet and water appliance off, watch your water meter. If it keeps moving, water is still flowing somewhere, possibly inside a wall.
  4. Photograph the stain, the wall and any damaged items, with dates. FEMA advises taking photos of discarded items for insurance claims. [3]
  5. Start drying the room with a dehumidifier. Keep indoor humidity below 60 percent, ideally 30 to 50 percent. [4]
  6. Do not paint or caulk over a moldy surface. EPA says paint applied over moldy surfaces is likely to peel. [4]

Wet drywall is on a short clock. EPA says that if wet or damp materials are dried within 24 to 48 hours of a leak or spill, in most cases mold will not grow. [4] EPA's remediation guide says wallboard may be dried in place if there is no obvious swelling and the seams are intact; if not, remove and replace it. [5]

What are the signs of mold behind drywall?

Mold grows where there is moisture, and it can grow on drywall, paint, wallpaper, insulation and wood. [6] EPA lists the back side of drywall, areas inside walls around leaking or condensing pipes, and the surface of walls behind furniture among the places mold hides. [4] Because you usually cannot see the back of the drywall, look for these clues on the front.

Common signs of hidden mold in a wall, and what to check next
SignWhat it can meanWhat to check next
Musty or earthy smell with no visible moldEPA says a building that smells moldy when you cannot see the source is a reason to suspect hidden mold.Smell along baseboards and inside closets and cabinets on that wall.
Stains, wet spots or discolorationWater is reaching the face of the drywall, or mold is growing on it. Mold can look like spots of many colors.Pencil the edge of the stain and look again in a few days to see if it spreads.
Peeling paint or discolorationPossible moisture. University of Georgia Extension's checklist says peeling paint, imperfections and discoloration may indicate moisture.Feel for dampness and take a moisture meter reading.
Sagging, bowed or swollen drywallWater damage. Georgia Extension says to inspect walls for sagging, bowing or wet spots. EPA says wet wallboard with obvious swelling, or seams that are not intact, should be removed and replaced.Look for loose tape or open seams. Press gently and note any spot that gives or crumbles.
Condensation that keeps coming backHigh indoor humidity. EPA says condensation can be a sign of high humidity, and that mold is frequently found on walls in cold corners behind furniture where condensation forms.Check room humidity and the wall behind beds, dressers and couches.
A past leak in or above that wallThe framing or insulation may never have dried fully.Ask whether the wall was dried within 24 to 48 hours, and check it with a meter.

Table sources: EPA, [4] [7] [5] CDC [6] and University of Georgia Extension's mold and moisture checklist. [8]

One more clue is health. EPA says you may suspect hidden mold if you know there has been water damage and the people living there are reporting health problems. [4] That is a reason to look at the wall, and a reason to talk with a doctor, not a diagnosis.

How to check inside a wall without tearing it open

Start with the least damaging checks. Most of these take a few minutes and a flashlight.

  1. Use a moisture meter. EPA says moisture meters may be helpful for measuring the moisture content of building materials and can be used to monitor drying; the thin probe is inserted into the material or pressed against its surface. [9] Take a reading on the suspect spot and on a dry wall in another room, then compare. A reading that stays high days after the leak was fixed points to a wall that is not drying.
  2. Measure the air. A humidity gauge tells you if the room is above 60 percent. [4] EPA says models that monitor both temperature and humidity are generally available for less than $50. [7]
  3. Look at the baseboards. Check for swelling, stains, gaps or a damp feel at the floor line. Pulling off a short section of baseboard lets you see the bottom edge of the drywall; FEMA's flood guidance removes baseboards to help walls drain. [2]
  4. Move furniture away from outside walls and look at the wall behind it, where EPA says condensation and mold often form. [7]
  5. Inspect the other side. Open the cabinet under a sink on that wall, look in the closet or room behind it, and check the ceiling below any bathroom. Outside, look at siding and paint for peeling, chipping or bubbling. [8]

Careful opening walls

Do not cut into a wall to hunt for mold without protection. EPA says investigating hidden mold requires caution when it means disturbing places where mold may be growing. For example, removing wallpaper can lead to a massive release of spores if mold is growing on its underside. If you believe you have a hidden mold problem, EPA says to consider hiring an experienced professional. [9] [4] Walls also hide wires and pipes.

Do you need a mold test?

Usually not to decide on cleanup. EPA says that in most cases, if visible mold growth is present, sampling is unnecessary, and that no EPA or other federal limits have been set for mold or mold spores. [10] CDC does not recommend mold testing. It says tests cannot reliably predict whether someone will get sick, can be costly, and there are no set standards for acceptable mold levels. [6] CDC also says you do not need to know the type of mold, because all molds are treated the same for health risk and removal. [11]

Testing has a narrower use. EPA says surface sampling may be useful to check whether an area has been adequately cleaned, and that sampling should be done by professionals with experience designing sampling protocols and interpreting results. [10] For a wall you suspect, the questions that decide the job are where the moisture is coming from and how much material is wet or moldy.

Can you clean it up yourself?

Sometimes. EPA says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases you can handle the job yourself. [4] That fits mold on the face of a wall from a fixed leak or condensation, where the drywall is still firm.

Safe to do yourself

Mold on the wall surface, under about 10 square feet, from clean water, with the leak fixed and no one in the home with health concerns. Wear an N-95 respirator, gloves and goggles. [4] [3] Open windows and doors. CDC says mold can be removed from hard surfaces with household products, soap and water, or a bleach solution of no more than 1 cup of household laundry bleach in 1 gallon of water, and never to mix bleach with ammonia or other cleaners. [6] EPA does not recommend bleach or other biocides as a routine step in mold cleanup. [4]

Drywall is porous, so scrubbing has limits. EPA says absorbent or porous materials may have to be thrown away if they become moldy. [4] Massachusetts guidance says to cut out and discard wallboard that has not dried enough within 48 hours, [12] and FEMA's flood guidance says soaked wallboard should be removed to a foot above the water mark and the inside of the wall checked for hidden mold. [2]

When a mold remediation company makes sense

Call a mold remediation company that serves your area when any of these apply:

  • More than about 10 square feet. EPA says you can usually handle a smaller area yourself; when mold covers more than 10 square feet, or there has been a lot of water damage, it points to its remediation guide for schools and commercial buildings, which it says applies to other building types too. [4] That guide recommends limited containment for 10 to 100 square feet of mold and full containment above 100 square feet, work most homeowners are not set up to do. [9]
  • Mold inside the wall cavity, or a hidden source. If you believe you have a hidden mold problem, EPA says to consider hiring an experienced professional. [4]
  • Mold near the heating and cooling system. EPA says do not run the HVAC system if you know or suspect it is contaminated with mold. [4]
  • Sewage or other contaminated water. EPA says to call in a professional with experience in buildings damaged by contaminated water. [4] Treat it as Category 3, grossly unsanitary water that can carry pathogens. [13] Wear gloves, boots and eye protection, and keep children and pets away.
  • Drywall still wet after 24 to 48 hours, or swollen with failed seams. [4] [5]
  • Health concerns. CDC says people with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. [6] EPA says to consult a health professional before starting cleanup, [4] and FEMA says to consider hiring a professional if you feel your health is affected. [2] Take health questions to a doctor.

Insurance, renters and cost

Coverage depends on the cause. Washington's insurance commissioner says homeowner insurance usually covers sudden leaks, such as leaking plumbing, but usually does not cover gradual leaks or water through cracks in walls or gaps between building materials. It usually does not cover mold unless the mold comes from water damage the policy covers, and some insurers offer limited mold coverage. [14] Check your own policy and talk with your insurer.

Renters: report the smell, stain or leak to your landlord or property manager in writing and keep a copy. EPA tells renters to report all plumbing leaks and moisture problems immediately to the building owner, manager or superintendent. [4]

For budgeting, HomeAdvisor's June 2026 cost guide puts mold remediation at an average of $2,368, with a typical range of $1,223 to $3,757, or about $10 to $25 per square foot. [15]

Common questions

Can you smell mold behind drywall?

Often, yes. CDC says mold can smell musty, and EPA says you may suspect hidden mold if a building smells moldy but you cannot see the source. [6] [4] A smell tells you something is there, not how big it is, so follow it with a moisture meter and a look at the baseboards and the other side of the wall.

How do you check for mold behind drywall without removing it?

Use a moisture meter on the suspect spot and compare it with a dry wall, look at baseboards for swelling or stains, move furniture off outside walls, and inspect the closet, cabinet or room on the other side. [9] [7] Do not cut into the wall without protection. EPA says investigating hidden mold requires caution, notes that removing moldy wallpaper can release a massive amount of spores, and says to consider hiring an experienced professional if you believe you have hidden mold. [4]

Does drywall with mold behind it have to be replaced?

Often, yes. EPA says porous materials may have to be thrown away if they become moldy, [4] and Massachusetts guidance says to remove and discard wallboard showing signs of mold growth. [12] Wet drywall that is not moldy may be dried in place if there is no obvious swelling and the seams are intact. [5] Never paint over mold, because EPA says the paint is likely to peel. [4]

Should I get a mold test before opening the wall?

It is generally not needed to decide on cleanup. CDC does not recommend mold testing, and EPA says no federal limits exist for mold or mold spores. [6] [10] EPA says surface sampling may be useful to check whether an area was adequately cleaned, and that sampling should be done by experienced professionals. [10]

Is mold inside walls dangerous to my health?

CDC says damp and moldy environments may cause a variety of health effects, or none at all. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. [6] CDC says all molds are treated the same for removal. [11] If you have symptoms or health concerns, talk with a doctor.

Sources

  1. Ten Things You Should Know about Mold U.S. Environmental Protection Agency, accessed October 12, 2026
  2. Dealing With Mold and Mildew in Your Flood Damaged Home FEMA, accessed October 12, 2026
  3. Prevent Mold from Becoming a Secondary Disaster FEMA, accessed October 12, 2026
  4. A Brief Guide to Mold, Moisture and Your Home U.S. Environmental Protection Agency, accessed October 12, 2026
  5. Mold Remediation in Schools and Commercial Buildings Guide: Chapter 4 (Table 1: Water Damage, Cleanup and Mold Prevention) U.S. Environmental Protection Agency, accessed October 12, 2026
  6. About Mold and Your Health Centers for Disease Control and Prevention, accessed October 12, 2026
  7. Mold Course Chapter 2 U.S. Environmental Protection Agency, accessed October 12, 2026
  8. Mold and Moisture Home Inspection Checklist (Circular 1083, reviewed March 1, 2023) University of Georgia Cooperative Extension, accessed October 12, 2026
  9. Mold Remediation in Schools and Commercial Buildings Guide: Chapter 3 U.S. Environmental Protection Agency, accessed October 12, 2026
  10. Mold Testing or Sampling U.S. Environmental Protection Agency, accessed October 12, 2026
  11. Mold facts and stats Centers for Disease Control and Prevention, accessed October 12, 2026
  12. Quick reference: water damage response in buildings Commonwealth of Massachusetts, accessed October 12, 2026
  13. Water damage guidelines (follows IICRC S500) University of California, Riverside, Environmental Health & Safety, accessed October 12, 2026
  14. Leaks, water damage and mold Washington State Office of the Insurance Commissioner, accessed October 12, 2026
  15. Mold remediation cost (updated June 19, 2026) HomeAdvisor, accessed October 12, 2026