Condensation on Windows

Window Condensation: Causes, Moisture Build-Up, and How Home Ventilation and HRV/ERV Systems Prevent It

What the Problem Is

Condensation on windows appears as water droplets forming on the inside of glass, most commonly in the early morning or during cold weather. It often affects bedrooms, living areas, and south-facing rooms, and can range from light misting to water pooling on sills.

While many homeowners see this as a window problem, condensation is actually a symptom of excess moisture in the air.

Why Window Condensation Exists

Indoor air always contains moisture. Everyday activities such as showering, cooking, breathing, and drying clothes release water vapour into the air.

Warm air can hold more moisture than cold air. When warm, moisture-laden indoor air comes into contact with a cold surface — such as window glass — it cools rapidly and releases moisture in the form of condensation.

In modern homes, this problem is made worse by:

  • Improved airtightness that traps moisture indoors
  • Better insulation that reduces natural air leakage
  • Cold window surfaces, especially at night
  • Limited or inconsistent ventilation

Opening windows occasionally may provide short-term relief, but it does not remove moisture consistently or reliably.

How Ventilation Fixes Window Condensation

Ventilation reduces condensation by lowering indoor humidity levels. By removing moist air and replacing it with drier outdoor air, the air inside the home is less likely to reach dew point when it contacts cold surfaces.

Effective ventilation:

  • Removes moisture generated by daily living
  • Prevents humid air from lingering overnight
  • Reduces the likelihood of condensation forming on glass
  • Helps maintain more stable indoor humidity levels

Recommended Ventilation Systems

1. Home Ventilation Systems

Whole-home ventilation systems continuously introduce fresh air while removing stale, moisture-laden air from the home.

These systems are effective because they:

  • Work automatically and consistently
  • Ventilate all rooms, not just one area
  • Reduce overnight condensation in bedrooms
  • Do not rely on occupants opening windows

Whole-home ventilation is often the most practical solution where condensation occurs throughout the house.

2. Heat Recovery Ventilation (HRV/ERV) Systems – Airtight Homes

Heat Recovery Ventilation systems supply fresh air and extract stale air at the same time, maintaining stable pressure inside the home.

They are particularly well suited to:

  • Modern, airtight homes
  • Homes with recurring window condensation
  • Households seeking better indoor air quality year-round

By controlling both supply and extraction, these systems offer reliable moisture management without excessive heat loss.

3. Subfloor Ventilation Systems

Subfloor ventilation can also play an important supporting role in reducing window condensation.

Damp subfloors can contribute to higher indoor humidity levels as moisture rises into the home through floors, gaps, and wall cavities. Improving subfloor airflow helps reduce this moisture source, which in turn can lower overall indoor humidity.

Important Disclaimer:

While subfloor ventilation can reduce background moisture levels, it is not always a complete solution for window condensation on its own. If condensation on windows is the primary concern, it is worth considering one of the above forms of home ventilation or HRV/ERV systems, as these systems directly control indoor air moisture where condensation occurs.

Other Helpful Fixes (Non-Ventilation)

While ventilation addresses the root cause, additional measures can help:

  • Improve window performance (double glazing or thermally broken frames)
  • Use bathroom and kitchen exhaust fans correctly and for long enough
  • Avoid drying clothes indoors without ventilation
  • Maintain consistent indoor heating to reduce cold surfaces
  • Ensure furniture is not blocking airflow near windows

Key Takeaway

Condensation on windows is not a defect — it is a sign that moisture levels inside the home are too high for the temperature of the glass. Ventilation is the most reliable way to address the underlying cause by controlling indoor humidity, reducing moisture build-up, and maintaining healthier indoor air.

While subfloor ventilation can reduce moisture entering the home from below, addressing condensation at the living level usually requires home ventilation or HRV/ERV systems to deliver lasting results.