Guide

Does Bleach Kill Mold?

The chemistry, in one paragraph

Household bleach is roughly 94 percent water with sodium hypochlorite as the active ingredient. On a hard surface the hypochlorite contacts the growth directly and oxidises it, which works. On a porous surface the hypochlorite ion does not travel into the material well, while the water carries straight in. So the visible surface bleaches white and looks clean, the growth below it survives, and the material is now wetter than it was before you started.

That is the entire story. It is not that bleach is useless. It is that it is the wrong tool for the majority of the surfaces this actually happens on.

What the EPA says

The EPA's mold guidance for homes does not list bleach as a recommended remediation method. Its instruction for hard surfaces is to scrub with detergent and water and dry completely, and its instruction for absorbent or porous materials with mold growth is that they may have to be thrown away, because the growth fills in the spaces and crevices and cannot be completely removed.

Note the framing: remove or discard, not disinfect. The goal of remediation is physical removal of the growth, not sterilisation of it. Dead mold still causes the same reactions, which is why "kill it" is the wrong objective in the first place.

Where bleach is genuinely fine

• Tiled shower walls and floors, on the tile face itself
• Glass and glazed surfaces
• Sealed or laminate countertops
• Porcelain and enamel fixtures
• Sealed concrete, on the surface only
• Plastic, vinyl and fibreglass surrounds

All of those are non-porous. Small surface growth on any of them is a cleaning job, not a remediation job, and the EPA's own threshold for a homeowner handling mold themselves is around 10 square feet.

Where it will let you down

• Drywall. Paper facing is both porous and food. Bleach whitens the paper and wets the gypsum behind it
• Wood framing, subfloor and trim. Growth penetrates the grain
• Grout and unsealed masonry. Porous, and the water carries in
• Caulk. Once growth is under the surface of a silicone bead, the bead gets replaced
• Insulation, carpet padding, ceiling tile, particleboard. These are removed, not cleaned
• Fabric and upholstery. Bleach damages them and does not reliably remove growth

Two things that make it worse rather than neutral

The water. You are applying a mostly-water solution to a surface with a moisture problem. In a damp basement, bleaching a wall every spring is a small contribution to the conditions that produced the growth.

The false resolution. A bleached wall looks clean. It reads as handled, so nobody opens it, and the growth continues in the cavity for another year or two until somebody smells it or renovates. The most expensive mold jobs we see are not the worst growth, they are the ones that were treated cosmetically for years.

Never mix it with ammonia

Bleach and ammonia produce chloramine gas, which is genuinely dangerous in an enclosed space. Plenty of household cleaners contain ammonia without saying so prominently, and a bathroom or a basement corner is exactly the kind of poorly ventilated space where this goes wrong. Use one product, ventilate, and do not improvise a mixture.

What to do instead

1. Find and fix the moisture source first. Nothing else holds without this
2. For small surface growth on non-porous material, scrub with detergent and water, then dry thoroughly
3. For porous material with real growth, remove it rather than treating it
4. HEPA vacuum the area and surrounding surfaces afterwards
5. Dry the space to a moisture content that will not support regrowth, and check it with a meter rather than by hand
6. Keep relative humidity between 30 and 50 percent so it does not restart

More on the sequence in our process, on the moisture side in humidity and mold, and on when a surface problem is actually a structural one in mold behind drywall.

Common questions

So bleach never works?

It works on non-porous surfaces. A tiled shower wall, a glass shower door, a sealed countertop, a sink. The problem is that most of the material mold actually grows on in a house is porous, and that is where it fails.

What should I use on drywall or wood instead?

Detergent and water with physical scrubbing, then thorough drying, then HEPA vacuuming. The mechanical removal is what does the work. If the material is saturated or the growth is extensive, the material comes out rather than getting cleaned.

Why does the mold come back after I bleach it?

Because the moisture that caused it is still there, and on a porous surface so is part of the growth. If you are cleaning the same wall every spring, you have a water problem, not a cleaning problem.