The three things mould needs, and how to remove them
Mould grows when three conditions are present at the same time: moisture, warmth, and organic material to feed on (like wood, drywall, or dust). Stop any one of these and mould stops growing. In most homes, moisture is the easiest to control, which is why reducing humidity and fixing leaks stops mould before it starts.
The target is to keep indoor humidity below 50 percent. Above that level, mould spores that are already floating in the air will begin to settle and grow on surfaces. You cannot eliminate mould spores entirely—they are everywhere outdoors and come inside on clothes and through open windows—but you can make your home inhospitable to them.
Key Takeaways
- Keep indoor humidity between 30 and 50 percent by running exhaust fans during and after showers, fixing leaks promptly, and ensuring gutters drain away from your foundation.
- Improve air circulation in damp areas like bathrooms and basements by opening windows when weather permits and running fans to move stale air out.
- Insulate cold pipes and seal air leaks around windows and doors to prevent condensation, which is a major source of moisture mould feeds on.
- Clean and dry surfaces within 24 to 48 hours of water exposure, because mould can begin growing on damp materials in as little as one to two days.
- Check your HVAC system's condensate drain line annually to may support it is not blocked, as a clogged drain can cause water to back up inside your home.
Run exhaust fans to move moisture out of your home
Bathrooms and kitchens produce the most moisture in a typical home. When you shower or cook, that steam needs somewhere to go. If it stays inside, it condenses on walls, mirrors, and tiles—creating the damp surfaces mould loves.
Run your bathroom exhaust fan during your shower and for at least 20 to 30 minutes after you finish. If your fan is noisy or slow, it may not be moving air effectively; a fan that sounds weak often means the ductwork is blocked or disconnected. Kitchen exhaust fans should run while you cook and for 10 minutes after. Both should vent to the outside, not into your attic or crawlspace, because that just moves the moisture problem elsewhere.
If you do not have an exhaust fan, opening a window while the shower runs is a temporary solution, though less effective in cold or humid weather. Installing a fan is a straightforward job for a handyperson with basic electrical knowledge, or you can hire a contractor to do it.
Fix leaks and may support water drains away from your home
Any water that sits inside your home will eventually grow mould. This includes obvious leaks—dripping pipes, roof leaks, foundation cracks—and hidden ones: slow seeps in basement walls, water pooling under your house, or condensation building up inside walls.
Check under sinks monthly for soft spots or discoloration on the cabinet floor, which signals a slow leak. Look at your basement or crawlspace walls after heavy rain; water stains or wet patches mean water is entering from outside. Roof leaks often show up as water marks on ceilings or attic rafters, sometimes weeks after rain stops.
Outside your home, make sure gutters are clear and downspouts extend at least 4 to 6 feet away from your foundation. Water that pools against your foundation wall will seep in and create damp conditions in basements and crawlspaces. If you have a sump pump, check that it is running and that the discharge line carries water away from the house, not back toward it.
Insulate pipes and seal air leaks to prevent condensation
Cold water pipes sweat in warm, humid air—the same way a cold glass of water beads up on a hot day. That condensation drips onto framing, insulation, and flooring, creating ideal conditions for mould. Wrapping cold water pipes with foam pipe insulation stops this. The insulation is inexpensive and takes minutes to install; you straightforward slide it onto the pipe and seal the seams with tape.
Air leaks around windows, doors, and foundation rim joists also allow warm, humid outdoor air to enter and condense on cold surfaces inside. Caulk gaps around window frames and door frames, and seal the rim joist (the band of wood where your foundation meets your walls) with spray foam or caulk. These are low-cost fixes that also improve heating and cooling efficiency.
Improve air circulation in damp areas
Stagnant air holds moisture. Moving air helps it evaporate. In bathrooms, basements, and other naturally damp areas, use ceiling fans or portable fans to keep air moving, especially after moisture-producing activities like showers or laundry.
Open windows on dry days to exchange indoor air with outdoor air, even in winter if humidity inside is high. In basements, a dehumidifier can help if the space is not actively wet; if water is entering, fix the water source first, because a dehumidifier alone will not solve a leak or drainage problem.
Clean and dry surfaces quickly after water exposure
Mould can begin growing on damp materials within one to two days. If you have a spill, a leak, or water damage from a storm, dry the affected area as soon as possible. Use towels, a wet vacuum, or fans to remove standing water and speed evaporation.
If drywall or insulation stays wet for more than 48 hours, it often needs to be removed and replaced, because mould can grow deep inside the material where you cannot see it. Carpet that gets wet should be dried within 24 hours or removed; padding underneath absorbs water and holds it, making it a perfect breeding ground for mould even if the carpet surface looks dry.
Check your HVAC system's condensate drain
Air conditioning and furnace systems produce condensation as they cool or dehumidify air. This water drains through a small line (usually PVC pipe) that runs from your indoor unit to a floor drain or outside. If this drain line clogs, water backs up inside your home, often into the basement or crawlspace.
Once a year, locate your condensate drain line and pour a cup of white vinegar down it to clear algae buildup that can block the line. If water is pooling around your furnace or air handler, the drain is likely clogged; you can try flushing it with vinegar and hot water, or call an HVAC technician to clear it. A clogged drain is one of the most common causes of basement moisture and mould.
Frequently Asked Questions
What humidity level should I aim for?
Keep humidity between 30 and 50 percent. You can measure it with an inexpensive humidity meter (hygrometer) from any hardware store. If humidity stays above 50 percent even with fans running and windows open, you may need a dehumidifier or to address a moisture source like a leak or poor drainage.
Can I use a dehumidifier instead of fixing leaks?
No. A dehumidifier removes moisture from the air, but it does not stop water from entering your home. If water is actively seeping in, a dehumidifier will run constantly and your electric bill will rise, but the mould problem will continue. Fix the leak or drainage first, then use a dehumidifier if humidity is still high.
How often should I run my exhaust fan?
Run bathroom fans during and for 20 to 30 minutes after showers. Run kitchen fans while cooking and for 10 minutes after. If you have a humidity sensor on your fan, it will run automatically when moisture rises. If not, running it on a timer ensures it stays on long enough to remove moisture.
Is mould growing if I see condensation on windows?
Condensation on windows is a sign humidity is too high, but mould is not yet growing on the glass itself (it is too smooth). However, condensation on window frames, sills, and walls nearby can support mould growth. Wipe condensation dry and address the humidity source—usually poor ventilation or a leak.
What should I do if mould is already growing?
Small areas (less than 10 square feet) can be cleaned with a brush and a solution of detergent and water, or a commercial mould cleaner. Dry the area completely afterward. Larger areas or mould inside walls, insulation, or ductwork should be assessed by a professional, because you may need to remove and replace affected materials.