Mold Under Sinks and Around Windows: Where It Comes From and What to Do

A dark bloom in a window corner, a musty cabinet under the kitchen sink: mold needs water and a surface, and indoors the water nearly always comes from condensation on something cold or from a leak. Unless the moisture goes, the mold comes back — which is also what to look for when you view a place to rent.

What is happening

Condensation on cold surfaces. When room air touches a surface colder than itself, the humidity at that surface rises. The WHO's example: air at 20 °C and 58% humidity, cooled to about 15 °C, reaches about 75%; at about 11 °C it hits 100% and water settles out. Window corners and reveals, metal frames, poorly insulated external walls, the wall behind a wardrobe and cold-water pipes under a sink are those cold spots.

It is the surface, not the room. Most building materials stay clean while humidity at their surface is below roughly 75–80%, depending on the material (WHO). A room can read fine on a hygrometer while the corner behind the sofa grows mold.

Leaks are the other source. Under a sink it is usually a dripping trap or connection. Materials dried within 24–48 hours of a leak usually do not grow mold (EPA, US); a stain that keeps spreading is covered in ceiling stains.

Why it matters. Per the WHO's 2009 guidelines, people in damp or moldy buildings have more respiratory symptoms, respiratory infections and asthma attacks, and damp is linked to developing allergic rhinitis and asthma; the few intervention studies show that fixing the damp reduces these problems. The species does not change the plan: there is no accepted "safe" level, the CDC (US) does not recommend testing, and the EPA calls sampling visible mold usually unnecessary.

What to do

Fix the moisture first. Repair leaks, insulate cold pipes (EPA). Run extractor fans or open a window while cooking and showering, keep kitchen and bathroom doors shut, put lids on pans, dry laundry outdoors when you can and leave a gap between furniture and external walls (UK guidance). More on airing out your home; if a humidifier is running, turn it down. What each device does for damp: ventilation, purifier or humidifier.
A small patch you can clean yourself. The EPA (US) draws the line at about 10 square feet (roughly 0.9 m²). Larger areas, extensive water damage, or mold after sewage or other contaminated water whatever its size, call for professionals. If you suspect the ventilation or air-conditioning system is contaminated, do not run it (EPA).
Detergent and water, then dry completely. Wear an N95 respirator, long gloves and goggles without vent holes (EPA). Moldy porous materials such as ceiling tiles or carpet may have to go.
Bleach is optional, not the plan. The EPA does not recommend biocides as routine: dead mold is still allergenic and has to be removed, not just killed. If you use a chlorine product (the EPA and CDC in the US allow it; UK guidance prefers dedicated mold products), use it alone, with a window open, as the label says. Never mix it with ammonia or other cleaners: why.
Do not paint over it. Paint or sealant over mold hides it and peels; clean and dry first (EPA).

If you rent

At a viewing, open the cabinet under the sink and check the back panel and base; look at window corners, the bathroom ceiling and behind furniture against external walls. A musty smell counts even when nothing is visible, and fresh paint in one corner is a question to ask. See also never sign a lease without visiting and who pays for minor repairs.

After moving in, responsibility is shared: the WHO puts the building and its upkeep on the owner, and use of water, heating and ventilation on the occupants. UK government guidance calls blaming damp on tenants' "lifestyle" completely unreasonable: cooking, washing and drying clothes inevitably produce moisture. In the UK, report broken ventilation, leaks or poor heating or insulation to the landlord, then to the local council if nothing happens (NICE). In England, after two-year-old Awaab Ishak died in 2020 from prolonged mold exposure at home, Awaab's Law has since October 2025 required social landlords to investigate significant damp and mold within 10 working days and deal with emergency hazards within 24 hours — social housing only.

When this is not the case

Some people should keep away from mold and not clean it themselves: babies and children, older people, pregnant women, people with asthma, allergies, eczema, COPD, cystic fibrosis or other lung or heart disease, and anyone with weakened immunity, including during chemotherapy. In vulnerable people mold can cause a fungal lung infection, aspergillosis (UK guidance, NHS inform).
Symptoms, by urgency. Emergency number (911 in the US, 999 in the UK, 112 in the EU; in Russia 103 or 112): severe breathlessness, unable to finish a sentence, blue lips, confusion. A doctor the same day: a new or worsening cough, wheeze, breathlessness or fever — sooner with weak immunity or lung disease. A routine appointment: stuffy nose, cough, wheeze, asthma or eczema that flare at home and ease away from it; mention the damp.

Sources

Checked: 24 September 2026