Top 5 Mold Hotspots in New York City Apartments You’re Probably Overlooking

Mold in New York City is more common than what you see in the bathroom or the black stuff on shower grout. The brick walls, steam heat, PTAC units in the walls, small rooms, and the damp air near the coast all help moisture stay in the same hidden spots in many buildings. The problem shows up again and again.

This guide will show you the main places where mold can be found in NYC apartments. You can use it to check your own house, even if you do not have any special tools.

Key Highlights

• In NYC, you will often find mold near PTACs, radiators, windows, corners, and closets. Mold is not usually found in the middle of a wall. Mold grows where cold surfaces and moist air come together.

• Leaks and water build-up from building systems, PTAC units, steam radiators, pipe chases, and window openings cause a lot of mold problems. 

• Basement and garden units often show white, powder spots on stone or brick walls. This is not mold, but it shows that water moves through the wall. It also means that paint, drywall, or things stored nearby can be at a higher risk.

• Under Local Law 55, people who own buildings that are covered by the law must get rid of mold and the dampness that leads to it. They need to use better ways to do this and not just scrape and repaint.

Why NYC Apartments Grow Mold in Odd Places

How Older NYC Buildings Trap Hidden Moisture

Many old brick buildings built before the war now have drywall, panels, and closets put right up against the old brick. This leaves a small, wet space where mold can grow, and you may not see it. Water gets into the brick from rain, snow, or wet dirt and slowly moves inside. If there is no space or insulation between the new wall and the brick, the inside wall can stay cool and wet for a long time, even after the weather gets better.

How High‑Rises Behave

In white-brick towers and tall slab buildings, the concrete floors, steel beams, and PTAC openings work like cold gaps in the wall. Moist air hits the cold area and turns into water. That is why you may notice spots or bubbling paint below windows or near PTACs, instead of on whole walls. If these spots never get fully dry and are not fixed the right way, they can turn into places where mold will grow for a long time.

Extra Moisture and Weak Ventilation

Weak fans, tight replacement windows, steam radiators, and PTACs all add moisture that stays on the coldest surfaces. Interior bathrooms that do not have good airflow trap steam at the ceiling. PTAC units make water when they cool the air. Steam radiators can leak a bit at valves. When this extra moisture cannot escape, it ends up in corners, behind furniture, inside wall spaces, and on cold glass and sills.

Hotspot 1: PTAC Units and Wall Sleeves

In many high-rises, PTAC units sit in strong frames set into the outside wall. When the unit cools the air, it takes water from the air and lets it drip into a tray at the bottom. If this tray does not drain right, water stays there. Then, mold can grow on the coils, inside the tray, and along the path where air blows into the room.

Warm indoor air can leak around the unit. It can touch the cold outside cover and become water. This water can get the drywall and insulation near it wet. After some time, this can make a weak or soft spot in the wall right under the unit. The rest of the wall can still look fine. A bad, musty smell when the PTAC starts up after being off a while is another clear sign there is a problem.

What to Check?

First, turn the unit off. Next, open or take off the front part. You can use a flashlight to look into the pan at the bottom. It is not a good sign if you see standing water, slime, or thick dust on the surfaces. You should also look at the wall below and around the sleeve. Gently press on any areas with stains. If the wall feels soft, hollow, or breaks apart, that is not good. Be sure to take clear photos. Ask the owner to fix the problem and do this in writing, so you have proof.

Hotspot 2: Steam Radiators, Covers, and Pipe Chases

Steam radiators may drip or spit out a little water at the valves and vents. A slow drip that goes on for months can make the floor, dust, and baseboard near the radiator feel a bit wet. If the radiator is inside a wooden cover, the heat and moisture can get trapped in the box. Mold may start to grow on the floors, baseboards, and inside the cover. People might not see the mold until they take off the cover.

Pipe chases that run up and down connect one floor to another. They carry steam, hot water, or drain lines. These chases can also move warm and damp air from places like basements or laundry rooms. If the pipe chases are open at the bottom or are not sealed at each floor, moisture can go up inside the wall. On cooler drywall, this moisture can gather. You may see stains or mold in thin strips that line up from one floor to the next.

What to Check?

Look at the floor and the baseboard under and behind radiators. This is important, especially if there is a cover. If you see boards that are dark, cupped, or feel soft, it is not just because they are old. This often means that there has been moisture there for some time. When it is heating season, run your hand along the riser walls. See if you get a musty smell. If you notice a strip of paint bubbling or peeling from the floor to the ceiling, or see the same problem on several floors, it probably means there is building-system moisture. A landlord must fix this in buildings.

Hotspot 3: Exterior Walls, Corners, and Packed Closets

Outside walls are often colder than the room air. This is true at corners, along floor lines, and where slabs or beams touch the wall. If drywall or paneling is placed close to these walls, moisture can get trapped in a small space. This happens when water comes through the wall or forms on the inside. Over time, you may see paint bubble, peel, or change color. This can happen even if you do not notice any clear leaks.

Closets and big furniture against outside walls can stop air from moving around and hide early signs. A wardrobe, bed, dresser, or crowded closet pressed close to an outside wall keeps that part cold. It also makes it hard for this area to dry out. The wall behind can stay cold and wet. This can make mold show up on paint and things touching the wall, such as shoes, bags, or hanging clothes.

What to Check?

Pull large furniture a bit away from outside walls. Look at the paint in the corners and near the ceiling. Try to see if there are any blotches, small spots, or paint that seems to come off easy or feel like chalk. If you have closets on outside walls, pick one part and take everything out. Use a light to look at all of the back wall and along the ceiling line. If you find paint with blotches, thin mold on the surface, or paint that seems to peel or feel wet, these all show the wall does not dry the right way.

Hotspot 4: Windows, Sills, and Below‑Window Walls

Windows are places where the warm air inside your home touches cold glass and frames. In winter, water can stay on window sills, frames, and places where the frame meets the wall. This water can help mold grow on paint, sealant, and on the wood trim around windows. The NYC Health Department says that this water showing up every day is a big reason for mold in apartments.

Below windows, when sealant is loose, old caulk is worn, or brick is cracked, rain can get into the wall. Water will sometimes move along metal parts or slab edges. It can show as soft plaster, stains, or bubbling paint below the window, even if there is no wet spot that you see. At or below the ground level, you may notice white powder on bare walls. This means water goes through the wall, dries on the outside, and leaves salts on the surface.

What to Check? 

Watch where water forms on cold days. See how long it stays there. Wet spots that show up again and again in the same place are more of a risk than fog that goes away fast. Take white, salty marks on walls as a sign of water problems. Try not to put boxes, wood furniture, or cloth things close to that area. If you see the paint under your windows keep bubbling, cracking, or peeling in the same places, let someone know. Keep photos so you can show it comes back more than one time.

Hotspot 5: Interior Bathrooms and Under‑Sink Areas

Many bathrooms in NYC don’t have a window and the fan in there can be too weak or loud. Steam from showers stays on the ceiling and top walls, and can’t get out. If these areas never get dry before the next shower, the paint and joint filler will slowly wear out. Mold can show up on top of the paint and just underneath it.

Kitchens and baths have a lot of pipes for water, drains, and vents. Small leaks where the pipes meet or at valves can drip water into the spaces under the sink and behind walls for a long time. Many times, the first thing you see is a puffy spot at the bottom of the sink area, dark and wet boards, or a bad smell when you open the doors. This happens before you ever see mold on the outside wall.

What to Check? 

Notice how long the walls and ceiling stay wet after you take a shower. If they are still wet an hour later, even with the fan on, then the ventilation is not good and the surfaces might have water problems. Sometimes, take everything out from under each sink. Feel the floor and the back inside part. If you feel that these parts are soft, puffy, or show some stains and you also notice a musty or sour smell, there could be a leak that needs more than just a quick wipe.

Quick Mold Check Routine for NYC Apartments

Every month, and after big storms or a stretch of hot and wet weather, take about ten minutes to walk around your apartment. Keep an eye out for mold as you go. You do not need any tools for this. You just need light, your hands, and your nose.

• Open PTAC fronts and look at the pan and wall under the sleeve for standing water, slime, or new stains.

• Look under and behind radiators and along riser walls for dark spots, boards that bend or feel soft, or bubbling paint.

• Look at outside corners, closet backs, and ceiling lines on outside walls for blotches, tiny spots, or peeling.

• Look at windows, sills, and under‑sink cabinets for stains, things that feel puffy, or musty smells that were not there before.

Take photos with dates showing anything that is getting worse or spreading. A photo log with some notes about when things get better or worse will help you see patterns. This makes it much easier to show these problems to the person in charge, HPD worker, or mold checker in the future.

When To Call a Pro Instead of Just Scrubbing

In buildings under Local Law 55, landlords have to look for mold and damp spots. They also need to fix leaks and get rid of mold safely by using set work rules. If you see more than a few small spots, if mold comes back after you clean it, or if you see clear damage from heaters, risers, PTACs, or the outside wall, write down your request. Be sure to keep a copy for yourself.

If it takes a long time to get a reply, if the workers just scrape and repaint, or if the problem comes back soon after they fix it, you can call 311 and ask for HPD’s indoor allergen team. Someone from the team can look at the place, take photos, and give out violations. This means the owner has to fix both the water problem and any mold you can see.

It can be a good idea to hire a licensed mold assessor when you think there is hidden mold in your walls, ceilings, or floors. You should also think about it if many rooms or stacked apartments are having problems, if someone in the place has asthma or bad allergies, or if there is any argument about how bad the damage really is. A good assessor spends most of the time looking for where water is getting in and how the building is made. They only use tests if the results will change what needs to be done next. If you want a company that only does mold checks and does not also do the clean-up, Fast Mold Testing New York City does careful mold checks and gives you clear reports you can give to your landlord, HPD, or another clean-up company.

Frequently Asked Questions

Is a little mold on my windowsill in winter normal in NYC?

Light mold often shows up on sills when it's cold out. This is seen more with old or metal windows. The NYC Health Department says you can clean small spots on hard things if you also keep moisture down. You should wipe up any wetness and let air in. If you keep finding mold or if the wood feels soft, the wall or trim might still be wet.

How can I tell the difference between efflorescence and mold?

Efflorescence is a white, powdery salt that shows up when water goes through brick or stone and dries. It looks like a chalky film. You can brush it off or wash it away with clean water. Mold is different. Mold is darker, may appear fuzzy or smeared, and doesn't wash away in the same way. Efflorescence matters because it shows that moisture moves through there, and that can let mold grow close by.

Do I always need a mold test?

For most homes, you do not need lab work to know if you should do something. If you see mold or smell a strong musty smell inside, you should get rid of the mold and fix where the water is coming from. Most tests are saved for cases that are special, like when doctors need answers, when there are fights in court, or when you want to make sure a big cleanup did what it should.

Learn more about mold in our recent feature in NY Weekly.