How do I prevent condensation and moisture in my garage during NB spring thaw?

Condensation during New Brunswick's spring thaw is caused by warm, moisture-laden air contacting cold garage surfaces — and preventing it requires a combination of ventilation, temperature management, and moisture source control. This is one of the most common garage problems across the province, typically peaking from late March through May when daytime temperatures climb above freezing but garage floors, walls, and concrete slabs remain cold from months of winter.

During the spring thaw, the ground releases enormous amounts of moisture as frost retreats from NB's 4-to-5-foot frost depth. That moisture migrates upward through concrete slabs that lack a proper vapour barrier (6-mil poly) beneath them — a common deficiency in older NB garages. Simultaneously, warmer outside air entering the garage carries more moisture than cold winter air, and when that warm, humid air contacts your still-cold concrete floor and walls, the moisture condenses into visible water droplets. The result is dripping walls, wet floors, foggy windows, and the musty smell of mould starting to grow on stored items, drywall, and wood framing.

Ventilation is the most effective solution. At minimum, install a through-wall exhaust fan on the wall opposite your overhead door — a 12-inch fan rated for 800-1,200 CFM costs $150-$400 and can be wired to a humidistat that activates automatically when relative humidity exceeds 60%. Running this fan for even 30-60 minutes per day during the spring thaw period dramatically reduces condensation by exchanging humid interior air for drier exterior air. If your garage has windows, opening them on mild days creates natural cross-ventilation that achieves the same effect at no cost.

For insulated and heated garages, the condensation dynamic is different but equally problematic. When you park a vehicle covered in snow, slush, and road salt in a warm garage, the snow melts and releases moisture into the enclosed space. A single snow-covered vehicle can release several litres of water into the garage air. In this scenario, you need both ventilation and active dehumidification. A portable dehumidifier rated for 50-70 pints per day costs $300-$500 and will keep humidity below the condensation threshold. Position it away from the overhead door and empty or drain it regularly.

Addressing moisture at the source is equally important. If your concrete slab sweats — water appearing on the surface even when the air feels dry — you likely have moisture wicking up through the concrete from below. A penetrating concrete sealer (silicate-based, not a topical coating) can reduce this migration by up to 80% and costs $50-$150 for a two-car garage as a DIY application. For slabs without a sub-slab vapour barrier, this is the most cost-effective retrofit available.

Check your exterior grading and drainage as well. Spring thaw combined with rain can direct water toward your garage foundation if the grade slopes inward. The ground around your garage should slope away at a minimum of 2% grade (1 inch per foot for the first 6 feet). Clean your gutters and ensure downspouts discharge at least 4-6 feet from the foundation. In low-lying areas, a French drain around the garage perimeter may be necessary — a professional installation runs $2,000-$5,000 depending on the perimeter length and soil conditions.

Inside the garage, reduce stored moisture sources during the spring thaw period. Cardboard boxes absorb and release moisture — switch to plastic bins with lids. Keep firewood outside, not in the garage. Ensure any floor drains are clear and functional, and mop up standing water from snow melt rather than letting it evaporate into the garage air.

If condensation has been a persistent problem and you are finding mould on walls, ceiling, or stored items, the underlying cause may be an insulation or vapour barrier deficiency that requires professional assessment. Mould remediation in a garage typically costs $500-$2,000 depending on the extent, and addressing the root cause prevents recurrence. Browse contractors in the New Brunswick Construction Network directory at newbrunswickconstructionnetwork.com for professionals experienced with garage moisture issues in our Maritime climate.