When you think of mold, you probably think of a damp, humid summer. But for facility managers, winter is an equally dangerous, and more deceptive, season for mold growth.
The reason? Condensation.
As we heat our buildings to combat the cold, we create a microclimate where warm, moist indoor air is constantly seeking a cold surface to condense upon. When this happens, liquid water forms, and that water, combined with any organic material (like drywall paper, dust, or wood), is all mold needs to thrive.
Preventing winter mold growth isn’t about chemicals; it’s about controlling moisture and airflow.
Where is the Winter Mold Danger Zone?
Mold won’t grow in the middle of a warm room. It grows in the hidden cold spots where condensation collects. Your winter inspection list should focus on:
- Exterior Walls: Especially behind furniture (sofas, filing cabinets) and in corners where airflow is poor. The drywall here can be significantly colder than the room’s air.
- Slab-on-Ground Perimeters: The edge of your concrete slab, where it meets the exterior wall, is in direct contact with the frozen ground. This is a primary condensation point, often hidden under carpet or baseboards.
- Windows and Vents: Any poorly insulated window frame or unsealed HVAC penetration is a cold bridge to the outside.
- Basements and Crawl Spaces: These below-grade areas are naturally cold and are a massive risk for condensation as warm air from the building above infiltrates them.
Your 4-Step Winter Moisture Control Strategy
1. Control the Source (Humidity)
In a commercial building, the indoor relative humidity (RH) should be kept between 30-50%. If your HVAC system is over-humidifying the air, you are creating a mold problem. Use a hygrometer to take readings in various zones. If humidity is high, adjust your HVAC settings.
2. Stop the Condensation (Temperature)
The goal is to keep all interior surfaces “above the dew point.”
- Insulate: Ensure all exterior walls, attics, and rim joists are properly insulated.
- Find Leaks: Use a thermal (IR) camera to scan your exterior walls. It will instantly show you cold spots where insulation is missing or wet, or where cold air is leaking in.
3. Move the Air (Circulation)
Stagnant air is mold’s best friend. It allows a cold pocket of air to form against a surface, letting condensation happen.
- Ensure Vents are Clear: Don’t let furniture or holiday displays block HVAC supply or return vents.
- Use Fans: In problem areas like basements or known damp corners, simply running a small fan to keep the air moving can be enough to stop condensation from forming.
4. Test Your Slabs (The Hidden Source)
A concrete slab, especially below-grade, can be a constant source of moisture vapor, even in winter. If your building’s heat and negative pressure (the “stack effect”) are pulling moisture up from the ground, that moisture can get trapped under your flooring.
Before any winter flooring project, an ASTM F2170 (in-situ RH) test is the only way to know if your slab is a hidden moisture source that will contribute to a mold problem.