Why am I getting condensation and moisture on the walls and ceiling of my heated garage in Ottawa?

Condensation in a heated Ottawa garage is one of the most common complaints homeowners run into, and it can range from a mild nuisance with foggy windows to a serious problem with water dripping from the ceiling, rust forming on tools and vehicles, and eventually mould growing on walls and stored items. Understanding why it happens is the key to fixing it, and the explanation comes down to basic physics combined with some garage-specific factors.

When you heat a garage, you raise the air temperature, and warm air can hold more moisture than cold air. The problem is that garages have enormous sources of moisture that living spaces do not. In winter, your vehicle brings in snow and ice on the undercarriage, wheel wells, and body panels every time you pull in. A typical car can carry twenty to thirty litres of snow and slush into the garage on a bad Ottawa winter day. As the garage warms up, all of that melts and evaporates into the air. That moisture-laden warm air then contacts cold surfaces like uninsulated walls, the garage door, window glass, or the concrete floor, and the water vapour condenses back into liquid water on those surfaces. This is exactly the same process that puts water droplets on a cold glass of water in summer, just happening on a larger scale across your garage.

The first and most important step to controlling condensation is managing the moisture at its source. If you park vehicles inside, consider letting them drip dry outside for a few minutes before pulling into the garage, or installing a floor drain so meltwater can leave rather than evaporate. Some Ottawa homeowners use containment mats under their vehicles that collect the meltwater and can be drained or mopped periodically.

Ventilation is the second essential piece. A heated garage needs controlled air exchange to remove moisture-laden air and replace it with drier outside air. In winter, cold outside air is actually very dry once you account for relative humidity at those temperatures. A small exhaust fan, even a bathroom-style fan rated at 80 to 100 CFM, running on a timer or humidistat can make a dramatic difference. Mount it high on the wall where warm moist air accumulates, and provide a makeup air inlet low on the opposite wall. A humidistat-controlled fan is ideal because it only runs when the moisture level actually warrants it.

Insulation plays a critical role too, but it has to be done correctly or it can make condensation worse. When you insulate the walls and ceiling, you raise the surface temperature of those materials so they stay above the dew point and condensation does not form on them. However, if you insulate without installing a proper vapour barrier on the warm side of the insulation, moisture from the garage air migrates into the insulation and condenses inside the wall cavity where you cannot see it. In Ottawa's climate, a six-mil polyethylene vapour barrier on the garage interior side of the wall insulation is standard practice, and all seams should be overlapped and taped.

The garage door itself is usually the biggest condensation surface because it is the largest uninsulated area in the space. Even with an insulated garage door, the R-value is modest compared to the walls, and the metal surface temperature drops quickly. There is no perfect solution for garage door condensation other than raising the overall insulation level of the door and ensuring the garage is not excessively humid. Running a dehumidifier is an option but adds to your electricity costs, and most people find that addressing the moisture sources and adding ventilation solves the problem without needing a dehumidifier running all winter.

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