Window sills lie at the crossroads of many things that foster mold growth: condensation, water leakage, dirt, and warm air. So, it's no wonder that mold develops on window sills more frequently than in other areas.
Not only a health hazard, mold on window sills is also unattractive—if anything because sills are so visible. Mold on a window sill can rapidly spread to other areas that are more difficult to access and clean than the sill: inside the wall on studs, window framing, drywall, and insulation. The good news is that mold is easy to remove if you catch it early enough.
Before You Begin
To kill and remove mold from a window sill, you can either scrub it with undiluted white vinegar or with a household bleach solution. It's usually worthwhile to start with the vinegar approach because vinegar is less harsh than bleach.
If using vinegar and mechanically abrading the mold doesn't do it, switch to bleach. Bleach is a biocide, so it can kill mold. The EPA notes that bleach is an effective biocide against mold but it does caution users against using bleach "as a routine practice."
What Is Biocide?
A biocide is any substance that is capable of killing living organisms.
When to Kill Mold on a Window Sill
When you see mold on a window sill, it should be killed immediately. Mold will not die off on its own as long as the conditions that caused it are still in effect.
Always avoid mixing chlorine bleach with ammonia or any cleaning product that contains ammonia. Mixing the two results in a toxic gas that, if inhaled, can be fatal.
Equipment / Tools
- Clean spray bottle
- Nylon scrub brush
- Waterproof gloves
- Safety glasses
- Clean towels
- White vinegar
- Bleach, unscented
Put on waterproof gloves, a mask, and safety glasses.
Dry Off the Window Sill
If the window sill is wet, dry it off by wiping it down with a clean, absorbent cotton or microfiber towel.
Spray the Vinegar
Pour about 1 cup of undiluted white vinegar into the spray bottle. Mist the mold area, thoroughly saturating it. Let the vinegar solution sit for about an hour. Do not rinse it off or touch it in the meantime.
Scrub the Mold
Scrub the vinegar solution with the nylon brush. You'll need to rub vigorously since mold can become embedded in paint.
Let Dry, Then Assess
Let the window sill dry out, then assess it. Every spot of mold must be gone for the window sill to be fully clear of mold. Even one remaining spot of mold will be enough for the mold to quickly reactivate. If the mold isn't gone, then switch to bleach.
Prepare the Bleach Solution
Clean out the spray bottle. Clean off the brush, too. Add 1 cup of bleach and 1 cup of cool water. Replace the sprayer to close up the bottle, then slosh the bottle gently to mix.
Spray the Bleach
Mist the bleach solution on the window sill to saturate the mold.
Scrub the Mold
Similar to earlier steps, vigorously scrub the mold on the window sill with the nylon brush. Work the bleach solution into the mold.
Wash Off the Bleach Solution
Thoroughly wash off the window sill to remove the mold and the bleach solution.
How to Prevent and Control Mold on a Window Sill
- Eliminate window moisture: In cold months, condensation beads up on windows when it's warm and humid inside but cold outside. In warm months, it's the opposite. Use a dehumidifier and encourage airflow to reduce condensation.
- Keep the window sill clean: Dust, dirt, and hair that collect on the window sill provide food for the mold to grow. Regularly wiping down the window sill goes a long way toward preventing mold growth.
- Use mold-resistant paint: Aside from having a tight molecular structure that discourages mold growth, mold- and mildew-resistant paint has fungicides that help kill mold spores.
- Choose glossy paint: Paints that have higher sheens are better at resisting mold because they are less porous and less absorbent. Paint the window sill with semi-gloss or glossy paint.
When to Call a Professional
Call a mold remediation company if the mold on the window sill is part of a larger problem that extends up or down the wall, inside the wall cavity, or on the ceiling. When selling a house that has mold problems, the buyer sometimes requests a paid invoice from a mold remediation company as a condition of sale.