Mold is usually treated as a summer problem, so a musty smell in January catches people off guard. Nothing leaked, and the house has been closed up for weeks.
The water causing it did not come from outside. It came from the air inside the house, and it condensed on the first cold surface it reached.
Here is what homeowners across the Great Lakes and the Upper Midwest should know about winter condensation mold, where it tends to grow, and how remediation handles it.
Why Mold Grows in Winter Even Without a Leak
Warm air holds more moisture than cold air. When warm indoor air touches a cold surface, it cools and releases that moisture as condensation.
A normal household generates a steady supply of it. Showers, cooking, laundry, dishwashers, houseplants and simply breathing all add moisture to the air.
In summer that moisture leaves through open windows. Once the house is sealed for heating season it has nowhere to go, so interior humidity climbs and settles on whatever is coldest.
Where Condensation Collects in a Cold-Climate Home
Attics and Roof Sheathing
Warm moist air rises and leaks into the attic through ceiling penetrations, recessed lights, bath fans and the attic hatch. It then hits roof sheathing that is close to outdoor temperature.
Frost forms on the underside of the roof deck, melts during a thaw, and drips onto the insulation below. Attic mold found in spring usually started this way.
Bath fans vented into the attic rather than through the roof are a frequent cause, and that detail is common in older housing stock across Michigan, Ohio and Wisconsin.
Exterior Walls, Window Frames and Closets
Window sills and frames are the visible version. Condensation beads on the glass, runs down and sits in the sill, and the paint or caulk starts to darken.
Closets on exterior walls are the hidden version. They stay closed, air does not circulate, and the wall behind hanging clothes is colder than the rest of the room.
Furniture pushed tight against an exterior wall creates the same dead air pocket.
Basements and Crawl Spaces
Basement walls stay cool year round, and an unconditioned crawl space can hold high humidity through the winter.
Storage boxes on a basement floor, carpet over a slab and insulation in a crawl space all absorb moisture and hold it long enough for growth to start.
Signs of Winter Mold in the House
- A musty smell that is strongest when the furnace first kicks on
- Condensation or frost on the inside of windows most mornings
- Dark spotting on window sills, frames or the corners of exterior-wall rooms
- Peeling paint or soft drywall on an outside wall
- Damp or compressed attic insulation, or water stains on the ceiling below the attic
- Clothing or shoes in a closet that smell musty while the rest of the room does not
- Frost on nails poking through the roof sheathing, visible from inside the attic
What to Do When You Find Mold in Winter
- Leave it alone rather than scrubbing it. Disturbing growth releases spores into the air and spreads them to other rooms.
- Find the moisture source. Treating the mold without fixing the condensation means it returns within the same season.
- Lower the indoor humidity. Running bath and kitchen exhaust fans during and after use removes a surprising amount of it.
- Open up the cold spots. Closet doors left ajar and furniture pulled a few inches off exterior walls let air reach those surfaces.
- Photograph what you find before any cleanup, including the surrounding area.
- Call for an inspection when growth covers a sizable area, keeps coming back, or sits in an attic, crawl space or wall cavity.
How Mold Remediation Works
- Inspection and moisture readings. The crew identifies the affected areas and the moisture source feeding them, using meters rather than visual inspection alone.
- Containment. The work area is sealed and put under negative air pressure so spores do not travel into clean parts of the house.
- Air filtration. HEPA air scrubbers run throughout the work.
- Removal. Porous materials with growth, such as drywall, insulation and carpet pad, are cut out and bagged inside the containment.
- Cleaning and treatment. Remaining surfaces are HEPA vacuumed and treated with antimicrobial products.
- Drying and moisture correction. The area is dried, and the underlying condensation problem is identified so the growth does not return.
- Repairs. Removed materials are rebuilt once the area reads dry.
911 Restoration provides mold remediation, along with the reconstruction needed to put the space back together.
Why Winter Mold Keeps Coming Back
Cleaning visible growth off a window sill without addressing the humidity behind it is the most common reason a homeowner sees the same spots the following winter.
Bath fans that vent into the attic, blocked soffit vents and an attic hatch with no weatherstripping are all routine culprits, and none of them are obvious from inside the living space.
A crawl space with exposed soil and no vapor barrier releases moisture into the house all winter, which raises humidity everywhere above it.
How to Reduce Condensation Before Mold Starts
Keeping indoor relative humidity in a moderate range through the heating season does more than anything else. A simple hygrometer shows whether the house is running high.
Running bathroom and kitchen exhaust fans every time, and leaving them on afterward, pulls out the moisture at the point it is created.
Confirming that bath fans vent outside rather than into the attic is worth checking once, and it often turns up a problem.
In a crawl space, a vapor barrier over exposed soil cuts the moisture entering the house from below.
911 Restoration: 24/7 Mold Remediation in Cold-Climate Homes
911 Restoration handles mold remediation across the Great Lakes region, including Grand Rapids and West Michigan, Metro Detroit, Milwaukee and southeast Wisconsin, Columbus in central Ohio, and Buffalo and Rochester in western New York.
Crews are available 24 hours a day, with a typical arrival goal of 45 minutes.
Inspection, containment, removal, sanitizing and repairs are handled by one team, so there is a single point of contact from the first call through to a finished home.
Contact 911 Restoration for 24/7 emergency service.
