How do I prevent black mold growth on garage walls during NB spring and summer?

Mould on garage walls in New Brunswick is almost always a moisture problem, not a cleaning problem — and the fix starts with understanding where that moisture is coming from.

NB's spring and summer bring a particular combination of conditions that make garages vulnerable: warm, humid Maritime air moving into a structure that's still cold from winter, melting snow and rain saturating the soil around the foundation, and vehicles dripping road salt and snowmelt well into April. When warm humid air contacts a cold wall surface, it drops its moisture right there — and mould follows within 24-72 hours on organic materials like wood framing, OSB sheathing, and drywall.

Identify Your Moisture Source First

Before treating mould, you need to know whether you're dealing with condensation (airborne moisture hitting cold surfaces) or infiltration (water physically entering through the foundation, walls, or floor). Press a piece of plastic sheeting against the affected wall and tape all four edges. Leave it for 48 hours. If moisture appears on the room side of the plastic, you have a condensation problem. If it appears on the wall side, water is coming through the structure itself.

Condensation is the more common culprit in NB garages, especially in spring when the garage walls are still at near-freezing temperatures from winter and the outdoor air suddenly warms up and carries significant humidity off the Bay of Fundy and Gulf of St. Lawrence. An uninsulated garage wall in May can easily be 5-8 degrees Celsius while outdoor air is at 18 degrees and 70% relative humidity — that's a recipe for constant surface condensation.

Infiltration typically shows up as efflorescence (white salt deposits) on concrete block walls, damp patches at the base of walls, or water pooling on the slab near the perimeter. This points to grading problems, failed parging on the exterior of the foundation, or cracks in the foundation wall itself.

Ventilation Is Your Primary Weapon

Improving air circulation is the single most effective thing you can do to reduce mould risk in a NB garage. Stagnant, humid air sitting against cold wall surfaces is where mould thrives. Even a simple through-wall exhaust fan ($80-$200 at any NB building supply store) running on a humidistat — a humidity-sensing switch that activates the fan when relative humidity exceeds 60% — makes a dramatic difference. Set it to exhaust air from the lowest point of the wall where cold air pools.

If your garage has windows, open them on warm days to create cross-ventilation. The goal is to keep the interior humidity below 60% relative humidity during the warm months. A basic digital hygrometer costs $15-$25 and takes the guesswork out of monitoring conditions.

Address the Exterior Grade and Drainage

Spring is when NB garages take on the most ground moisture. Snowmelt and April rain saturate the soil, and if the grade around your garage doesn't slope away from the foundation at a minimum of 2% (roughly 1 inch per foot for the first 6 feet), that water is sitting against your foundation walls and wicking through. Walk around your garage after a heavy rain — if you see water pooling within 3-4 feet of the walls, grading is part of your mould problem.

Clean your eavestroughs every fall and make sure downspouts discharge at least 4-6 feet away from the foundation. A $15 downspout extension is one of the cheapest mould-prevention investments you can make.

Insulation as a Long-Term Solution

If condensation is your recurring problem, insulating the garage walls eliminates the cold surface that moisture condenses on. A properly insulated wall with a vapour barrier on the warm side stays close to interior temperature — warm humid air no longer hits a cold surface and drops its moisture. This is the permanent fix for condensation-driven mould in NB garages, though it's a more significant investment ($1,500-$4,000 for a two-car garage depending on method).

If you go this route, closed-cell spray foam is the superior choice for NB garages because it acts as both insulation and vapour barrier in one application, leaving no air gaps where moisture can accumulate. Batt insulation works well too but requires a separate 6-mil poly vapour barrier installed carefully on the warm side with all seams overlapped and taped.

Treating Existing Mould

For surface mould on concrete or painted walls, a solution of 1 part household bleach to 3 parts water scrubbed with a stiff brush and left for 15 minutes before rinsing is effective. Wear gloves, eye protection, and a proper respirator — not just a dust mask. For mould that has penetrated into drywall or wood framing, those materials need to be removed and replaced, not just cleaned. Mould in framing lumber that covers more than roughly 1 square metre warrants a professional assessment.

For persistent or extensive mould, reach out through the New Brunswick Construction Network at newbrunswickconstructionnetwork.com to find local contractors experienced with garage moisture remediation and insulation upgrades.