Does bleach kill mold on drywall?

Why drywall is hard to disinfect

A sheet of drywall is a gypsum core sandwiched between two layers of paper. That paper is cellulose, which is food for mold, and it holds water well. The back layer faces the inside of the wall, where it’s dark, still and often the first place water reaches during a leak.

So mold on drywall tends to grow in three places: on the painted face you can see, into the paper facing, and on the back of the board inside the cavity. Many times the growth on the back is larger than anything visible in the room.

What actually happens when you spray bleach on it

Household bleach is mostly water. On a hard surface like tile, it can kill surface mold on contact. On drywall, the paper and gypsum soak up the water quickly, while the active chlorine reacts at the surface and breaks down. The result is predictable:

• The stain lightens or disappears, because bleach removes color. That looks like success.
• Growth in the paper and on the back side isn’t reached.
• The wall is now wetter than it was, which is the condition mold needs.
• Dead mold stays in place, since bleach doesn’t physically remove anything.

The EPA’s guidance on mold cleanup says that using a biocide such as chlorine bleach is not recommended as a routine practice during remediation, and that the goal is to remove mold, not only kill it. Bleach also isn’t designed to break down mycotoxins, which is covered in our answer on whether bleach kills mycotoxins.

Surface spots vs. growth in the board

Not every mark on drywall means the board is lost. What matters is whether the mold is on the surface or in the material.

What you see	What it usually means
Small spots on glossy or semi-gloss paint, in a bathroom, that wipe off	Surface growth on the paint film. Detergent, water and better ventilation often handle it.
Staining that comes back through the paint	Growth in the paper or a leak behind it
Soft, bubbled, warped or crumbling drywall	The board has been wet for a while. It will need to come out.
Mold near the floor after a leak or flood	Water likely wicked up inside the wall. The back side needs checking.
A musty smell strongest at one wall	Possible growth on the back of the board or in the cavity

When the growth is in the paper or on the back side, no spray will fix it. Our answer on whether you have to remove drywall with mold explains how that decision gets made.

What to do instead

First, stop the water. Then decide whether it’s a cleaning job or a removal job. For a small surface spot on painted drywall, wipe it with a damp cloth and detergent, dry it, and fix whatever made that spot damp. Don’t sand it dry, and don’t scrub hard enough to break the paper.

If you suspect the growth goes deeper, leave it alone and get it assessed. A moisture meter shows whether the wall is still wet, and a wall cavity air sample can show whether mold is growing inside without opening the wall. Our guide to mold inside walls covers how it’s located and removed under containment. And one safety rule applies no matter what you try: never mix bleach with ammonia, vinegar or other cleaners.