Mildew shows up looking harmless — a powdery gray film on grout, a black speckle pattern on a shower curtain, a musty edge along a basement baseboard. Ignore it for a season and the same spores produce stubborn root systems that bleach a wood subfloor and damage drywall paper. Knowing what kills mildew on contact, versus what only fades the surface stain, separates a 20-minute cleanup from a $400 remediation. Here’s what actually works, with the dwell times and concentrations that matter.
Why Mildew Is Different from Mold
Mildew is a surface fungus, typically in the Aspergillus or Penicillium families, and it stays on top of materials rather than penetrating deeply. That makes it easier to kill than the black mold (Stachybotrys) that colonizes wet drywall behind the scenes. If the patch wipes away with light pressure and leaves only a stain, it’s mildew. If it returns within days or smells strongly earthy from inside the wall, you’re dealing with mold and should call a remediation pro before scrubbing.
Chlorine Bleach: The Heavyweight
A 10 percent solution of household chlorine bleach (1 part bleach to 9 parts water) kills mildew spores in 10 to 15 minutes of contact time. The EPA registers sodium hypochlorite as a fungicide at this dilution. Use it on nonporous surfaces — ceramic tile, fiberglass tubs, vinyl shower curtains, and painted concrete.
Skip bleach on grout you want to preserve. Repeated applications break down the polymer binders and accelerate cracking. Never mix it with ammonia, vinegar, or rubbing alcohol — the resulting chloramine or chlorine gas can land you in the ER.
Hydrogen Peroxide: The Surface Specialist
Three percent hydrogen peroxide straight from the drugstore brown bottle kills mildew in about 10 minutes and lifts the staining as it bubbles. Spray it on, wait, then scrub with a stiff nylon brush. It’s safe on grout, natural stone (test first on marble), fabric, and most painted surfaces. The only catch is light sensitivity, so store the bottle in the dark and use within 6 months of opening.
For tougher patches, a paste of hydrogen peroxide and baking soda sits on vertical surfaces without dripping and gives you 20 minutes of dwell time before rinsing.
White Vinegar: The Gentle Option
Distilled white vinegar at 5 percent acetic acid kills about 82 percent of mildew species, according to USDA research. It won’t match bleach for kill rate, but it’s safer around kids, pets, and food-prep surfaces. Spray full strength, let it dwell for 60 minutes (yes, an hour — short contact times are why people think it doesn’t work), then scrub and rinse.
Vinegar shines on shower doors, refrigerator gaskets, washing machine drums, and laundered fabrics. It does etch natural stone and unsealed grout, so keep it off marble, travertine, and limestone.
Commercial Mildew Removers
When DIY mixes fall short, jump to a registered fungicide. Three reliable options:
- Concrobium Mold Control — $19 per quart, no bleach or ammonia, kills on contact and leaves an antimicrobial film for ongoing protection
- RMR-86 Instant Mold Stain Remover — $20 per quart, contains sodium hypochlorite, lifts stains visibly within 15 seconds, ideal for grout and caulk
- Tilex Mold and Mildew Remover — $5 per spray bottle, bleach-based, the workhorse for shower walls and tubs
Read the label every time — some products specify a 10-minute dwell, others demand 30 minutes for full kill.
Killing Mildew by Surface Type
Tile and Grout
Hydrogen peroxide paste for sealed grout, RMR-86 for badly stained colored grout. Reseal with a penetrating silane-siloxane sealer after the surface dries for 48 hours.
Drywall and Paint
If mildew sits on top of latex paint and the wall is dry, a vinegar spray followed by 60 minutes of dwell removes it. If you see soft, bubbled, or stained drywall, cut out the panel — surface cleaning won’t save it.
Fabric and Upholstery
Brush dry mildew outdoors first, then launder with hot water plus 1 cup of borax. For upholstery, work in a hydrogen peroxide solution with a microfiber cloth and dry the piece in direct sun.
Wood and Subflooring
A 1:1 vinegar and water solution lifts mildew without damaging the finish. For raw wood, sand lightly after drying and apply a stain-blocking primer before refinishing.
Prevention Beats Cleanup
Mildew needs three things: a food source (dust, soap scum, organic debris), humidity above 60 percent, and stagnant air. Cut any one and growth stops. A bathroom exhaust fan rated for at least 80 CFM running for 20 minutes after a shower, a dehumidifier holding basement humidity at 45 to 50 percent, and a monthly wipedown of shower walls with a squeegee will eliminate 90 percent of household mildew problems before they start.
Cheap analog hygrometers ($10 to $15) in basements, bathrooms, and laundry rooms give you a daily readout of humidity. Anything above 55 percent calls for action — open a window, run the fan, or move the dehumidifier into the room. Replace caulk lines around tubs and showers every 4 to 6 years because old caulk develops microscopic cracks that hold moisture and feed new colonies.
When to Call a Professional
Surface mildew almost never requires a remediation contractor. Mold inside walls, ceilings stained over 10 square feet, or persistent musty smells with no visible source do. Industrial hygienists charge $300 to $600 for an air sample test that confirms species and spore counts. Full remediation on a contained area runs $1,500 to $4,500, far cheaper than ripping out moldy framing two years later.
Final Thoughts
Knowing what kills mildew is half the battle — applying it long enough is the other half. Most homeowners spray, wait 30 seconds, and wipe. That’s why mildew comes back. Give bleach 10 minutes, peroxide 10 to 20, vinegar a full hour, and let the chemistry do the work. Then fix the moisture source so you’re not repeating the job next month.
