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How to Test for Mold Inside Walls

How to Test for Mold Inside Walls

If your home has suffered water damage, it might also experience mold growth. If the water damage is serious, mold can grow on your property within twenty-four hours after the water has soaked through to reach the drywall. 

Additionally, leaking pipes in your home can trap moisture in the walls resulting in water damage and possibly mold growth. 

Testing for mold inside walls can help you deal with the issue early enough. Here’s how to test for mold inside walls:

Top Signs That You Have Mold Behind Drywall

Sometimes, figuring out if you have mold inside your wall might take a lot of work. If you want more in-depth exploration, find professional mold testing services. 

However, before finding professional services, several clues can tell you that mold is growing in your walls. Here are some signs that you have mold behind your drywall:


If your home has suffered severe water damage, mold will grow inside your walls if the water isn’t removed within 24 to 48 hours. If the water reaches the drywall, cutting it one foot above the water line is best. 

Leaking pipes might also trap moisture inside your walls. Even when the leaks are visible, moisture can get inside walls and provide mold a place to grow. 

You can tell that drywalls are moist if:

  • First, there are visible water stains.
  • You can see dark rings.
  • There’s discoloration
  • There’s deterioration, such as peeling, bubbling, or cracking of the paint or wallpaper.

Additionally, if the walls are warped, bulging, or bowed, they most likely contain moisture. You can also tell if the walls’ surface feels wet. 

Visible Signs of Mold

If the wall is rotting, it likely has mold underneath it. However, in less severe cases, mold might not be that noticeable. It’s best to inspect the walls and check for mold on walls behind furniture and along baseboards.

Mold can also have different colors and textures; therefore, be sure to recognize them. For example, mold can be black, gray, green, brown, or white. It can also appear pink, orange, or purple when it grows behind vinyl wallpaper. 

You might also note the discoloration of walls even if it has been painted over. If the water damage inside the wall persists, mold will show signs on the surface. 

Musty Smell

If you don’t see signs of mold but can smell it, it might be hidden in your walls. Mold smell often feels earthy, like rotting leaves or decaying wood in a dense, damp forest. 

If you suspect mold in your walls, it might be better to get on your hands and knees and smell the electrical outlets. Doing so might help you sense if mold is growing within walls because outlets have better access to the area behind the walls. Therefore, smelling them might help you identify the mold problem. 

How to Check for Mold Inside Your Walls

It’s best to know if you have mold inside your walls early enough to deal with it appropriately. 

Here’s how to tell if you have mold inside your walls:

  • First, locate the center of the area most likely to have mold inside.
  • Next, shut off power to the area at the electric service panel.
  • Next, lightly mark a square approximately 6 inches by 6 inches with a pencil and straightedge.
  • Cut along the outline with a drywall jab saw.
  • Remove the cut-out and inspect its back for mold.
  • If the wall doesn’t have insulation, look at the back wall. Also, hold a small mirror and shine a flashlight on the mirror to inspect the back of the drywall.
  • If there’s insulation, any mold on the drywall or studs will have spread to the insulation. Therefore, mold on insulation usually means mold on other building materials.

How to Remove Mold From Inside Walls

There’s no acceptable level of mold in your home and no waiting period. Once you discover mold, it’s best to open up the walls and remove it as soon as possible. 

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

Remember that finding hidden mold in your house and fixing the source of moisture in the walls is a crucial and necessary step in removing mold. Common sources include elevated indoor humidity, leaking ceilings, indoor condensation, and outdoor leakage from gutters or drainpipes. 

Before You Begin

Before you begin removing mold from inside walls, it’s better to understand that you’ll have a four-step process as below:

  • Removing moldy drywall and other materials
  • Killing mold
  • Encapsulating remaining mold
  • Rebuilding part or the entire wall with new drywall.


Removing moldy drywall and insulation is best because none can be reused.

Kill Mold

It’s best to kill the mold with a liquid biocide. Alternatively, you can expose mold in the air and light long enough to kill it. Don’t use household bleach to kill mold. 

Encapsulate Mold

Cover the mold with a fungicidal mold encapsulant. It’s best to go for a coating that contains the active ingredient, calcium hydroxide. Do not use ordinary house paint. 


It would help if you rebuilt the area with new building materials. For example, add new insulation to exterior walls, hang drywall, and paint it. 

You might have to choose mold-resistant drywall if you’re dealing with areas with persistent mold. 

What You’ll Need to Remove Mold From Inside Walls

You will need the following materials, equipment, and tools to remove mold from inside walls:


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


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


  • Use plastic sheeting and painter’s tape to contain the work area by blocking off doorways leading to other areas. For example, you can tape plastic sheeting on the floor in front of the wall.
  • Shut off electricity at the service panel and 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. 
  • 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.
  • Remove drywall: Using the prybar, gently chip into soft, moldy drywall and pull it off. 
  • Immediately place all moldy drywall into large contractors’ bags. Then, keep expanding the area until you reach dry, solid drywall with no mold. 
  • Carefully pull the installation away from the staples attached to the wall studs. Next, roll up moldy insulation and place it in the bags. Continue until you reach insulation that doesn’t have mold on it.
  • Assess the mold situation by shining a bright work light inside the wall. Mold will always remain on hard surfaces such as studs, headers, sill plates, and the back of the siding. You can clean these materials and encapsulate the mold unless they are severely damaged.
  • Leave the area to dry it out. You can speed up the drying by directing fans set to low. 
  • Thoroughly vacuum the entire mold using the HEPA-filtered vacuum. Vacuum the rest of the wall cavity, too, picking up dust, spider webs, and rodent droppings.
  • Use the one-gallon pump sprayer containing the mold biocide to spray down all moldy areas and disinfect them completely. Fully soak the materials but not so much that the fluid drips onto the floor. 
  • Let the area dry out. Ensure that no moisture remains on any of the surfaces.
  • Paint the area with the fungicidal mold encapsulant using a brush or roller. After the first coat has dried, apply a second coat of mold encapsulant. 
  • Insulate the wall if necessary, hang drywall, tape the seams, apply drywall compound (mud), and sand smooth.
  • Apply drywall primer and then paint with interior latex paint. 

When Should You Call a Professional to Remove Mold Inside Walls?

You might need professional mold testing and removal under the following circumstances:

If the Mold Damage is Extensive

If you’re a DIY enthusiast, you can manage mold damage of up to ten square feet, especially if you have a device to detect mold in walls.

However, as the size increases, the job becomes exponentially difficult; therefore, the EPA recommends that professionals remove mold patches exceeding ten square feet. 

If You Have a Tight Schedule

Professional mold remediation companies often remove mold in a matter of days. So, let the pros handle it unless you can afford to spend a week or two on the project. 

Check this too: How to Get Rid of Dog Smells in the House

The Source of the Moisture

If you still need to determine and repair the source of the moisture that caused the mold growth, it’s best to hire a professional to diagnose and fix the issue. 

Time of the Year

Opening up drywall and removing insulation in the winter can be challenging. However, professionals can do the job rapidly and limit your home’s exposure to the elements. 

Can Mold Inside Walls Make You Sick?

Mold inside your walls can be more dangerous than visible mold because it can go undetected for a long time, grow, and compromise multiple areas of your home. 

Extensive mold inside your walls can become dangerous because some mold species produce mycotoxins. Mycotoxins are associated with headaches, breathing problems, and even neurological system damage. 

Additionally, removing mold from your walls might be dangerous if you suffer from asthma, allergies, skin irritation, itchiness, or breathing problems. Removing mold yourself might aggravate your symptoms and any pre-existing conditions unnecessarily. 

Preventing mold from growing inside your walls might be the best way to deal with the issue. You can prevent mold from growing by maintaining your plumbing, keeping your home well-ventilated, and waterproofing windows and doors.

You can also opt for mold prevention paints in the market that have antimicrobial defense inside and have a long-term, lasting effect.