How to Remove Mold From Inside Walls

Mold Inside Walls

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Project Overview
  • Working Time: 8 - 10 hrs
  • Total Time: 1 - 2 wks
  • Yield: 10 feet by 10 feet
  • Skill Level: Intermediate
  • Estimated Cost: $200 to $500

Mold inside your walls is a sure sign of moisture intrusion. Mold also leads to the deterioration of insulation, studs, and drywall. Some types of mold can be hazardous to your health. And for anyone thinking of selling their home, mold in the walls will undoubtedly show up on the inspection report. 

Mold remediation in walls is a relatively simple, though messy, project that can stretch for a week or two. Because of this and because of safety concerns, many do-it-yourselfers decide to hire a professional mold remediation company.

7 Common Signs Mold Is Inside Your Walls

Musty Smell

Mold has a musty smell often compared to wet cardboard, wet pets, or an old house's basement.

Visible Mold

Mold inside walls often stays within the walls. But when it does migrate to the drywall paper, wallpaper, paint, or baseboards, it appears in patchy clusters of small spots. Mold can be black, white, gray-green, or gray-brown.

Dampness

Walls might be temporarily damp from the use of a shower or from cooking. But if they are permanently damp and there is no apparent moisture source from within the room, it's likely that the drywall, insulation, or studs inside are wet.

Soft Drywall

If the drywall is mushy, flakes away, or indents under your touch, it is probably wet. If the drywall is wet, it will most likely be moldy, too.

Stains

Dark stains on walls usually indicate previous or current dampness within the walls.

Previous Flooding

If flooded water has come into contact with the walls, it's almost certain that mold is growing in the walls.

Poor Health

Not all mold is toxic and not all people are susceptible to the effects of mold spores, though they can lead to various negative health effects.

How to Check for Mold Inside Your Walls

  1. Locate the center of the area most likely to have mold inside.
  2. Shut off power to the area at the electric service panel.
  3. With a pencil and straightedge, lightly mark a square approximately 6 inches by 6 inches.
  4. Cut along the outline with a drywall jab saw.
  5. Remove the cut-out and inspect its back for mold.
  6. If there is no insulation in the wall, look at the back wall. Also, hold a small mirror inside and shine a flashlight on the mirror to inspect the back of the drywall.
  7. If there is insulation, any mold on the drywall or studs will have spread to the insulation. So, the presence of mold on insulation usually means mold on other building materials. 

When to Remove Mold From Inside Walls

Unlike some other types of deterioration around the home, there is no acceptable level of mold in your walls and no waiting period. Once you discover mold, it's best to open up the walls and remove the mold as soon as possible.

Mold can spread and affect other areas of the walls, insulation, ceiling, flooring, studs, and joists. The longer you wait to remove the mold, the longer mold has to grow.

Tip

Identifying and fixing the source of moisture in the walls is a necessary component of removing mold. Common sources include leaking ceilings, elevated indoor humidity, indoor condensation, and outdoor leakage from gutters or drainpipes.

Before You Begin

Removing mold from inside walls is a four-step process: removing moldy drywall and other materials; killing mold; encapsulating remaining mold; and rebuilding part or all of the wall with new drywall, insulation, and other building materials.

  • Removal/Disposal: Moldy drywall and insulation must be removed. None can be reused.
  • Kill Mold: Spray the mold with a liquid biocide to kill it. Alternatively, exposing the mold to air and light for long enough will kill it. Do not use household bleach.
  • Encapsulate Mold: Cover the mold with a fungicidal mold encapsulant. Choose a coating that contains the active ingredient calcium hydroxide. Do not use ordinary house paint.
  • Rebuild: The area must be rebuilt with new building materials. New insulation is added to exterior walls (interior walls do not require this). Drywall is hung, then painted. For areas with persistent mold, you may even choose mold-resistant drywall.

Tip

Citing environmental concerns, the EPA's position on mold biocides is that they are permitted but not recommended. The EPA makes an exception for areas where immune-compromised individuals live. The EPA does not ban biocides. Instead, it recommends that the mold be killed with ventilation and light.

Safety Considerations

Wear NIOSH-approved breathing protection, disposable gloves, and coveralls or old clothing that you can wash at the end of each day. Double-bag the moldy debris in large plastic contractor clean-up bags. Avoid carrying the bags through the house; drop them out of windows if possible. If your area regulates the disposal of moldy debris, do not place the bags in the regular municipal trash run. Instead, check with your city for transfer stations designated for this type of trash.

What You'll Need

Equipment / Tools

  • One-gallon pump sprayer
  • Shop vacuum with HEPA filter
  • Fans
  • Work light
  • Flashlight
  • Pencil
  • Stud finder
  • Prybar
  • Paint brush
  • Paint roller and covers
  • Latex gloves
  • N95 respirator
  • Cordless drill

Materials

  • Mold encapsulant
  • Mold biocide
  • Contractor clean-up bags
  • Plastic sheeting
  • Painter's tape
  • Drywall
  • Drywall screws
  • Drywall tape
  • Drywall compound (mud)
  • Insulation

Instructions

  1. Contain Work Area

    Use plastic sheeting and painter's tape to block off doorways leading to other areas. Tape plastic sheeting on the floor in front of the wall.

  2. Shut off Electricity and Water

    At the electric service panel, shut off circuit breakers to any suspected circuits running through the wall. If water lines run through the wall, shut off the water at its main shut-off valve.

  3. Identify Area of Mold

    Mark off the general area of the mold either with a pencil or with painter's tape. Use the stud finder to identify the positions of wall studs in the area.

  4. Remove Drywall

    With the prybar, gently chip into soft, moldy drywall and pull it off. Immediately place all moldy drywall into large contractor's bags. Keep expanding the area until you reach dry, solid drywall that has no mold on it.

  5. Remove Insulation

    Carefully pull the insulation away from the staples that attach it to the wall studs. Roll up moldy insulation and place it in the bags. Continue until you reach insulation that has no mold on it.

  6. Assess Mold

    With moldy drywall and insulation bagged and removed, shine a bright work light inside the wall. Mold will often remain on hard surfaces: studs, sill plates, headers, and the back of the siding. Unless these materials are severely damaged, they can be cleaned and the mold can be encapsulated.

  7. Dry out Area

    If the area is wet, leave it open to dry it out. Directing fans set to low will speed up the drying.

  8. Vacuum Mold

    With the HEPA-filtered shop vacuum, thoroughly vacuum all of the mold. Vacuum the rest of the wall cavity, too, picking up dust, spider webs, and rodent droppings.

  9. Disinfect Mold

    With the one-gallon garden pump sprayer containing the mold biocide, completely spray down all of the moldy areas. Fully soak the materials but not so much that the fluid drips onto the floor.

  10. Dry out Area

    Let the area dry out. No moisture should remain on any of the surfaces.

  11. Encapsulate Mold

    With a brush or roller, paint the area with the fungicidal mold encapsulant. After the first coat has dried, apply a second coat of mold encapsulant.

  12. Close up Walls

    Insulate the wall, if necessary. Hang drywall. Tape the seams, apply drywall compound (mud), and sand smooth. Apply drywall primer, then paint with interior latex paint.

    Tip

    Fiberglass insulation is not recommended for moisture-prone areas like exterior basement walls. Slab foam or spray foam insulation are effective alternatives to fiberglass in these areas.

Troubleshooting Removing Mold From Inside Walls

After cleaning, the mold still looks like it's there.

Mold can be very difficult to completely scrub away. But as long as the mold has been properly disinfected, there is no need to remove all visible mold inside of the walls. The walls will eventually be closed up with drywall. Also, the encapsulating coating will cover up much of the visible mold.

Chemicals are not preferred.

Killing mold naturally is just as effective as killing mold with powerful chemicals—it just takes longer. Ventilation and light are enough to kill mold. You'll need to have the wall open long enough for this method to take effect.

Fungicidal coating did not fully kill the mold.

Fungicidal mold encapsulants are meant to aid, not replace, the core method of killing the mold. They also help with some limited ongoing mold problems. Do not use only a fungicidal mold encapsulant on the assumption that it will both kill and encapsulate the mold.

When to Call a Professional

How Extensive Is the Mold?

Mold damage up to 10 square feet is manageable by most do-it-yourselfers. As the size increases, the job becomes exponentially unwieldy. The EPA recommends that mold patches exceeding 10 square feet be removed by professionals.

What Is Your Schedule?

Mold remediation companies often can remove mold in a matter of days, not weeks. Unless you can afford to spend a week or two on the project, have the pros do it.

What Time of Year Is It?

Opening up drywall and removing insulation in the winter is enough to challenge even the most robust HVAC system. By doing the job rapidly, professionals can limit your home's exposure to the elements.

What Is the Source of the Moisture?

If the source of the moisture that caused the mold growth has not yet been determined and repaired, hire a professional to diagnose and fix the water issue.

Is This Part of a Home Sale?

Homebuyers who request that the seller remediate the mold prior to purchasing the home usually want to see professional work. Equally importantly, they want to see an invoice reflecting that the work has been done. If you do the work yourself and it doesn't pass the second inspection, the home sale comes to a halt until the work is complete.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Olson, Wanda et al. "Dealing With and Preventing Mold in Your Home." University Of Minnesota Extension, 2018.

  2. "Should I use bleach to clean up mold?" U.S. Environmental Protection Agency.

  3. "Mold Cleanup in Your Home." U.S. Environmental Protection Agency.