Interior-Window-Condensation-Is-a-Ventilation-Problem-First (Home Systems)

Interior Window Condensation in Coastal BC Is a Humidity Problem — Fix the Ventilation First

idea

Claim: when condensation forms on the interior surface of window glass (the surface you can touch and wipe from inside), it is caused by indoor humidity exceeding the dew point at the glass temperature — not by a window seal failure. In coastal BC where outdoor air is already humid, this problem is endemic in tight, under-ventilated homes and appears on even high-performance windows. Replacing the window will not solve it; managing indoor humidity and improving ventilation will.

Mechanism

The interior glass surface of a window is always cooler than the room air, especially in winter. When indoor relative humidity is too high, the glass surface is at or below the dew point for that air — and water vapour condenses on it, just as it would on a cold drink glass on a summer day.

Why coastal BC makes this worse:

• Outdoor air is already humid year-round in Metro Vancouver (average relative humidity 75–85% in winter)
• Modern construction is tighter (less natural air infiltration) than older buildings, so moisture generated indoors accumulates
• Common moisture sources in a typical occupied unit: cooking, showers, breathing, houseplants, and drying laundry indoors

The recommended indoor humidity range for a typical home is 30–50%. Above 55%, condensation on windows becomes frequent. Above 60%, mould growth on sills and surrounding drywall is likely within weeks.

Why a better window only partly helps: A higher-performance window (lower U-factor, triple pane) has a warmer interior glass surface because it insulates better — so condensation is less likely on the glass. But if indoor humidity is 70%, even a triple-pane window will show condensation when outdoor temperatures drop below about 5°C. The fundamental fix is lowering indoor humidity.

The effective interventions, in priority order:

1. Run the bathroom exhaust fan during and 20–30 minutes after every shower — showers are the single largest single-event moisture source
2. Use the range hood exhaust when cooking — vent to exterior (not recirculating)
3. Run an HRV (heat recovery ventilator) continuously or on a humidity-sensing setting — the code-correct solution for tight buildings; exchanges humid interior air for fresh exterior air while recovering heat
4. Reduce indoor plants and avoid drying laundry indoors if condensation is severe
5. Open a window briefly for cross-ventilation — effective but sacrifices heat in winter
6. Use a dehumidifier as a stopgap — treats the symptom while the root cause (ventilation shortfall) is addressed

What condensation is NOT

This rule covers condensation on the interior glass surface that can be wiped away from inside. It is a completely different problem from:

• Fogging or haziness trapped between the panes (not wipeable from inside) — that is IGU seal failure; see Failed-Sealed-Unit-Is-IGU-Replacement-Not-Full-Window (Home Systems)
• Water at the sill after rain — that is water ingress from outside; see Window-Perimeter-Leak-Is-a-Wall-Problem-Not-a-Window-Problem (Home Systems)

Mould on sills and frames

Persistent interior condensation saturates the sill material, paint, and caulk, creating ideal conditions for mould (typically Cladosporium or Penicillium — common but potentially problematic for occupants with respiratory sensitivities). Wiping with a 10:1 water-to-bleach solution treats visible mould; stopping the condensation prevents recurrence. If mould has penetrated drywall, a remediation contractor is needed.

Sources

• Centra Windows / Magic Window, window industry guides — interior condensation forms when warm humid air meets cool glass; recommended humidity range 30–50%; above 55% condensation becomes frequent; HRV recovers 60–80% of heat while circulating fresh air — https://www.centrawindows.com/blog/why-is-there-condensation-on-windows
• Green Building Advisor — tight high-performance homes can have worse condensation issues than older leaky homes if ventilation isn’t sized correctly; the solution is always ventilation, not weaker windows — https://www.greenbuildingadvisor.com/question/tight-houses-and-mold-condensation-high-performance-windows

Idea Compass

North: Where this comes from

• windows (Home Systems) — the parent component note; interior condensation is the third commonly misdiagnosed window problem
• ventilation (Home Systems) — the system where the actual fix lives; window condensation is a symptom of a ventilation shortfall

East: Tensions / failure

• Failed-Sealed-Unit-Is-IGU-Replacement-Not-Full-Window (Home Systems) — the misdiagnosis this note prevents: replacing the IGU or window when humidity is the cause
• Tight building syndrome: modern energy-efficient buildings are more airtight, which trades air quality for energy efficiency — condensation is the early symptom

South: Where this leads

• ventilation (Home Systems) — HRV specification and maintenance; the long-term fix for chronic interior condensation
• Mould remediation if condensation has persisted long enough to saturate drywall

West: What’s similar

• Window-Perimeter-Leak-Is-a-Wall-Problem-Not-a-Window-Problem (Home Systems) — different mechanism, same diagnostic trap: water near the window that is not caused by the window itself