How to Find the Cold Spot Behind a Closet Mold Complaint
Why this matters
Mold on the back wall of a bedroom closet, usually high in an outside corner, is one of the most common winter calls a handyman, painter or insulation contractor gets. The usual response is to clean it, paint it with a mold-resistant primer, and leave. It comes back next winter, because cleaning treated the growth and not the reason that patch of wall stayed damp. The reason is usually a cold surface: a spot where missing or wind-washed insulation, or an air leak, lets the drywall run cold enough that the air against it sits near saturation for weeks. Sometimes it is a leak instead, and the fix is completely different. This procedure finds which one, and where, before anyone wipes away the pattern that shows it. Separating condensation from a leak in full belongs to Moisture Source: Condensation vs Leak - A Decision Tree; the physics of why surfaces condense belongs to Dew Point and Condensation, the Reasoning Behind the Fault.
Do not disturb the growth. Everything below is measured without touching visible mold: an infrared thermometer is non-contact, and a pinless moisture meter reads through the surface without breaking it. Do not scrape, wipe, sand or vacuum the growth during the investigation. Clothing and boxes from the closet may carry spores; let the customer move them, or wear gloves and move them yourself, and for anyone on your crew a respirator is worn under a written program per 29 CFR 1910.134. For the attic, walk only on framing, wear eye protection, gloves and a dust mask for fiberglass, and do not disturb pebble-like loose fill that may be vermiculite.
The procedure, with one house worked through it
The house: a two-story in January, outdoor 25 F overnight. The complaint is the closet of a northwest second-floor bedroom, with gray-black spotting along the top of the back wall and heaviest in the upper outside corner. The closet door is kept shut and the closet is packed with clothes.
1. Photograph everything as found
Photograph the growth, the closet contents against the wall, the door position and the room. Note how long the problem has been seen and whether it is seasonal. Why first: the pattern of growth is evidence, and it is the first thing a cleaning job erases. In this house: the owner says it appears every winter and fades in summer, which already leans toward condensation over a leak, but does not settle it.
2. Measure the room's air and its dew point
At the center of the bedroom, read temperature and RH and convert to dew point (most hygrometers display it). Then open the closet door just enough to read the closet air, and close it again. Why: the dew point tells you how much moisture the air carries. If the closet's dew point matches the room's, it is the same air and the closet adds no moisture of its own; if it is higher, there is a source inside the closet (damp laundry, a leak) and that changes the investigation. In this house: room 68 F and 45 percent RH, dew point 45.9 F. Closet air 62 F; at the room's dew point that reads about 55 percent RH, and the hygrometer showed 55 percent. Same air, cooler. No closet source.
3. Empty an arm's width of the wall and measure surfaces immediately
Move enough contents to expose about 3 ft of the back wall from floor to ceiling, including the outside corner. Within the first two or three minutes, before room air warms the wall, read surface temperature with an infrared thermometer (set to the emissivity for painted drywall, typically around 0.9, and held close enough that its spot is smaller than the area you are reading) at four places: mid-wall, the upper outside corner where wall meets ceiling, the corner at mid-height, and the floor line. Why the hurry: the clothes were insulating the wall from the room. The surface temperatures right after you move them are the ones the wall has lived at all winter. In this house:
| Point | Surface temp |
|---|---|
| Mid-wall, back | 58 F |
| Upper outside corner | 50 F |
| Outside corner, mid-height | 53 F |
| Floor line | 55 F |
4. Turn each surface temperature into a surface RH
The air touching a surface has the room's dew point but the surface's temperature, so its RH is higher than the room's. Read it off a psychrometric chart or calculator with the dew point from step 2 and each surface temperature. Mold does not need liquid water; it needs the air at the surface damp for long enough. ASHRAE Standard 160, in substance (check the current edition for the exact criteria), treats a 30-day running average surface RH above 80 percent at moderate temperatures as the condition for mold growth risk. Use 80 percent as the line. In this house, at a 45.9 F dew point:
| Point | Surface temp | Surface RH |
|---|---|---|
| Mid-wall | 58 F | 64 percent |
| Upper corner | 50 F | 86 percent |
| Corner, mid-height | 53 F | 77 percent |
| Floor line | 55 F | 71 percent |
Only the upper corner is over 80 percent, and it is exactly where the growth is heaviest. The mid-height corner at 77 percent is close and the growth thins there. No surface reached the 45.9 F dew point, so there was no visible condensation, which is why the owner never saw water. That combination (growth matching the coldest surfaces, nothing ever wet to the eye) is the signature of a cold-surface problem.
5. Repeat the surface readings with the door open
Leave the door open and the section empty for 30 to 60 minutes and read the same points. Why: it shows how much of the cold is the closet itself. In this house: the upper corner rose to 53 F, which is a surface RH of 77 percent at the same 45.9 F dew point. That alone puts it just under the line, which tells the customer that keeping the closet door open and contents off the exterior wall helps, and also that it will not fix the corner by itself: 77 percent is too close to 80 to rely on, and nobody keeps a closet door open all winter.
6. Rule out a leak before you look for the cold
With the pinless moisture meter, scan the corner, the wall and the floor line, and compare with the same wall outside the closet. Look for tide lines or stain rings, staining that radiates from one point, a correlation with rain or with a plumbing fixture on the other side of the wall, and bubbling paint. Why: a leak and a cold surface can produce mold in the same corner, and the fixes share nothing. In this house: the meter read slightly elevated at the corner surface and normal a few inches away, there were no tide lines, no plumbing in the wall, and the owner reports no change with rain. A leak would show a wetter, deeper pattern spreading from its entry point. Everything here fits surface dampness from cold. If the signs pointed the other way, stop here and follow the leak branch of the decision tree instead.
7. Find why that corner is cold, from the attic and the outside
Go to the attic above the closet. The upper outside corner sits under the eave, where the roof slope leaves the least depth for insulation over the top plate and where soffit air enters. Look at the insulation over that top plate: depth, contact, whether it has been pushed back, and whether there is a baffle and a dam at the eave. Check the top plate itself for gaps or wire holes. Then look at the outside corner from the ground for missing trim, a gap at the corner boards or gutter overflow staining. Why: the surface temperature told you where; this tells you why. In this house: over the closet corner the batts had been pulled back about 18 in from the eave, apparently when a cable was run to an exterior light, leaving the top plate bare. There was no baffle, so soffit air washed across the plate and the ceiling drywall edge. A wire hole through the top plate was open to the wall cavity below. See related: Insulation Contact and Wind Washing.
8. Report the cause before anyone cleans
Write down the room dew point, each surface temperature and its surface RH, the leak checks, and what you found in the attic, with photos. Why last, and why before cleaning: the report is what keeps the next contractor from cleaning and repainting. The remediation of the growth itself follows EPA guidance (a small area under about 10 sq ft is often a homeowner or general cleanup job; larger areas call for a professional), and it comes after the cause is fixed, not before. In this house: the work order says seal the top-plate wire hole, install a baffle and dam at the eave over that bay, restore insulation to full depth and contact over the plate, then clean the corner. A recheck next January at similar conditions should find the upper corner well above the 50 F as-found reading, and under 80 percent surface RH with the closet packed and the door shut.
What changes the answer
- The closet is on the first floor of a two-story house. There is no attic over it. A cold floor line points to an uninsulated or leaky rim joist, checked from the basement; a cold mid-wall points to a void in the wall insulation, which an infrared camera from inside on a cold morning shows better than spot readings. See related: Thermal Bridging in Ordinary Houses.
- The room's dew point is high. At a 52 F dew point, the 58 F mid-wall would sit at about 80 percent surface RH too. Then the whole wall is at risk and the first fix is lowering indoor humidity (ventilation, a bath fan that actually runs, no humidifier set high), with the cold corner second. See related: How to Measure Indoor Humidity and Dew Point Properly.
- The closet's dew point was higher than the room's. Something in the closet is wet. Find it first; it may be a leak behind the wall, or wet laundry and boots stored inside.
References
- ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings (surface RH criterion for mold), current edition
- US EPA, Mold Remediation in Schools and Commercial Buildings, and A Brief Guide to Mold, Moisture and Your Home (area thresholds for cleanup)
- 29 CFR 1910.134, Respiratory Protection, for employees wearing respirators
- See related: Moisture Source: Condensation vs Leak - A Decision Tree; Dew Point and Condensation, the Reasoning Behind the Fault
- See related: The Cold Surfaces Where a House Condenses First; The Tightened House That Grew Mold in the Closets