Can Mold Grow Behind Drywall? How to Find It and When It Comes Out
Yes. The paper on the back of drywall is mold food, and the inside of a wall is dark and still, so a wall that stays wet from a leak, condensation or a flood can grow on the back of the board long before anything shows on the front. It gets found with a moisture meter, a thermal camera and a small inspection hole for a borescope. Drywall that mold has grown through gets cut out and replaced, not cleaned or painted, and the water gets fixed.
The front of a wall can look almost fine while the back of the board is covered. That’s the trouble with drywall: the side mold likes best is the side you never see. The good news is you don’t have to tear out a whole wall to find out what’s going on back there.
The signs from the outside
- A musty smell in one room or one corner, strongest near the wall
- A stain, often brown or yellow, that slowly grows
- Paint bubbling, peeling or discoloring in one area
- Drywall that feels soft or spongy when you press it
- Baseboards pulling away or swelling
- Wallpaper lifting near corners
- “We had a leak months ago, and now the wall smells”
Any one of those can be something else. Two or three together, near a water source, is worth a look inside.
How it gets found without tearing everything out
Moisture readings first. A meter swept across the wall shows whether it’s wet behind the paint and how far the wet area spreads. Read it next to a dry spot on the same wall. How to read one is in moisture meter readings explained.
A thermal camera can point to cool, damp patches worth metering, though it can’t see mold or see through the wall. More in what a thermal camera shows about hidden moisture.
A small inspection hole. If the readings say the cavity is wet, a borescope goes in through a hole about the size of a pencil. You can see the back of the drywall, the insulation and the studs. If it’s clean and dry, you patch a small hole. If it’s growing, you know exactly where to cut.
Every hole you open should have a reason behind it.
Why grown-through drywall comes out
You can clean mold off a hard surface. You can’t clean it out of drywall that it’s grown into. The paper face and the gypsum core are porous, and once growth goes into them, scrubbing or spraying just treats the surface. A dehumidifier won’t save it either. Once the growth is in the sheetrock, drying the air doesn’t fix it. That’s why moldy drywall gets cut out and replaced, and the EPA’s cleanup guidance says not to paint or caulk over moldy surfaces.
How the cut-out is done
- Containment first. Plastic around the work area and an air scrubber with a HEPA filter, so the dust stays put.
- Cut above the damage. The cut line comes from the meter, not the stain, and it’s set high enough to clear anything growing on the back of the board.
- Bag it right there. Pieces go straight into bags inside the containment.
- Insulation comes out if it’s wet or moldy.
- Studs get cleaned, HEPA vacuumed and scrubbed, and checked for soundness.
- Dry it until the framing reads dry, then check again before new drywall goes up.
Find the water before you hang new drywall
The mold behind the wall is never the whole story. A slow drip from a shower valve, a leaking supply line, water coming through a foundation or condensation on a cold outside wall put it there, and it’ll do it again to the new board. Following the water back is covered in tracking down the hidden leak.
Don’t wait on a soft wall
A small spot on a wall often means more behind it, and it doesn’t stay small. A wall that’s soft today was wet for a while, and the longer it stays wet, the more of it comes out. Reading the wall before opening it is the first thing I do on any mold inspection and remediation visit. How the whole job runs is in the overview of mold remediation in Gettysburg, PA.
Questions about mold behind drywall
Can I just spray mold killer into the wall?
It won't fix it. Spraying a product through a hole doesn't remove the growth on the back of the drywall or on the studs, and dead mold left in place is still debris in the wall. The EPA's approach is physical removal and cleaning, plus fixing the moisture. If the cavity has active growth, the affected drywall comes out, the studs are cleaned and dried, and whatever wet the wall gets repaired.
How big a hole do you need to check inside a wall?
Often a small one. A borescope fits through a hole about the size of a pencil and lets you see the back of the drywall, the insulation and the studs. If the readings and the camera say the cavity is clean and dry, you patch a small hole. If they show growth, you know where to cut and how big the job is before anything bigger comes out.
Is it safe to cut out moldy drywall myself?
For a small area, with care, many people do. The EPA's rule of thumb is about 10 square feet for a homeowner cleanup. Cutting moldy drywall makes dust, so seal off the area, mist the surface lightly first, bag the pieces right there and use a HEPA vacuum, not a household one. The EPA's mold cleanup guide lists the gloves, goggles and N-95 respirator to wear. Anything larger, anything inside an HVAC system or anything you're unsure about is worth a professional look.
John will look before anything gets torn out
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