Why closets on outside walls get damp
A closet with its door shut gets less of the house's heat, so its outside wall runs colder than the room next to it. EPA specifically calls out closets as spaces that can be colder than the rooms around them, and notes that circulation is what carries heat to cold surfaces. Coats and boxes packed against that wall cut circulation further. When a surface drops below the dewpoint of the air touching it, water condenses on it.
The same thing can happen inside the wall. The Building America Solution Center, a Department of Energy building-science resource, explains that indoor air leaking into a cold wall cavity condenses on the inside of the sheathing. So a closet wall that looks dry on the painted face can still be wet behind it.
How to tell condensation from a leak
Condensation is the likely suspect when the smell and dampness show up in cold weather, sit in the outside corner or behind packed belongings, and you also see condensation on windows elsewhere in the house (our guide to window condensation covers what that means).
A leak is the likely suspect when you see a water stain, a localized wet spot, or paint that's buckling and peeling. EPA's mold house tour uses exactly that picture, buckled paint on a bedroom corner, to show a roof leak seeping into a wall. Note whether it changes after rain or after plumbing on the other side of the wall gets used.
Also notice where the smell is strongest: on clothes and stored items, at the baseboard, or up near the ceiling. EPA notes that high humidity alone can grow mold on clothing and furnishings, not just walls.
What to try yourself, and what not to touch
Buy a humidity meter. EPA says they cost about $10 to $50. Compare the closet with the room next to it, door closed and door open. EPA's target is indoor relative humidity below 60 percent, ideally 30 to 50 percent. If the house is well above that, fix the humidity first: run bathroom and kitchen exhaust fans vented outdoors, and don't open windows when it's humid outside.
If you don't see mold, pull clean, dry belongings a few inches off the outside wall and leave the closet door open for a couple of weeks. EPA recommends opening closet doors and moving furniture out of corners for exactly this reason. If the smell fades, condensation was the likely cause. Our guide to rooms that stay cold explains why some spots in a house run colder.
Don't scrape, peel wallpaper or open up the wall to look. EPA warns that pulling off wallpaper with mold behind it can release a large burst of spores, and recommends hiring an experienced professional when you suspect hidden mold. Don't paint over a wet or moldy wall either; EPA says paint over mold is likely to peel and the problem comes back. If you see white, crusty deposits on a brick or block wall, that may be salt rather than mold; this guide tells them apart.
When to call, and which kind of pro
Call promptly if the wall is actively wet, a stain keeps coming back, you can see suspected mold, or the smell persists with no visible source. EPA says wet materials dried within 24 to 48 hours usually won't grow mold, so don't wait on a fresh leak.
Match the pro to what you saw. A stain that shows up near a bathroom or kitchen wall points to a plumber. A stain that follows rain points to a roofer or someone who works on walls, windows and flashing. If you can't tell, look for an investigator experienced in both moisture problems and mold in buildings. EPA says investigators use tools like moisture meters and borescopes to find hidden wetting.
Questions worth asking: What did you find that tells you it's a leak and not condensation? How far does the wetting go, and did you measure it? What gets dried, what gets removed, and what just gets refinished?
Be cautious about a quote built around mold testing. EPA says routine sampling isn't recommended and that when mold is visible, sampling usually isn't needed. Ask how a test result would change the repair. The job is to find and stop the water, then clean or replace what's damaged.
For condensation, be careful with insulation as a quick fix. The Building America Solution Center notes that adding cavity insulation to an older wall reduces its ability to dry and can make condensation worse unless air leakage into the wall is also addressed. Ask what the plan does about air leaks, not just R-value.
If you rent in Philadelphia
The City of Philadelphia has no mold regulations, and the Department of Public Health doesn't inspect for mold. What the City will act on is the water: a leaking roof, cracked wall, missing windows or plumbing problems can be reported to Philly 311, or (215) 686-8686 from outside the city, according to the Health Department's home page on mold. Tell your landlord in writing first and keep photos and dates.