What causes condensation on the inside surface of Calgary windows?

Interior window condensation (moisture on the room-facing glass surface) is caused by high indoor relative humidity contacting the cold glass surface. In Calgary winters, single-pane windows and poorly insulated window frames get cold enough to condense moisture at indoor humidity levels as low as 25–30% RH. Double-pane windows with a thermally broken frame are less prone to interior condensation because the inner pane stays warmer. Solutions: (1) Reduce indoor humidity with exhaust fans in kitchen and bathrooms and limit humidifier output. (2) Improve air circulation near windows — open blinds and curtains during the day so warmer room air reaches the glass. (3) If the window frame itself is cold and dripping, the frame insulation may be inadequate — caulking the frame-to-wall junction and adding weatherstripping can help. (4) For persistent interior condensation, a heat recovery ventilator (HRV) can reduce indoor humidity while maintaining air quality in a tight Calgary home.

What does condensation between window panes mean in a Calgary home?

Condensation between the panes of a double or triple-pane window (fogging inside the IGU — insulated glass unit) means the hermetic seal of the unit has failed. The space between panes is filled with argon or krypton gas (which provides insulation) and contains a desiccant that absorbs any residual moisture. When the perimeter seal breaks, moist outdoor air enters the cavity. The desiccant saturates and the moisture condenses on the interior glass surfaces. You cannot clean between-pane condensation — it is inside a sealed unit. Options: (1) Replace the IGU (the sealed glass unit) — the frame is reused and only the glass unit is replaced. This is the most cost-effective solution if the frame is in good condition. (2) Replace the entire window — warranted when the frame itself is damaged, rotting, or no longer thermally effective. Calgary's extreme temperature cycling (-40°C to +35°C) stresses window seals more than in moderate climates, which is why IGU seal failure is common in homes built in the 1990s and early 2000s.

Is exterior condensation on Calgary windows normal?

Exterior condensation (moisture on the outside surface of the glass) is normal and is actually a sign that the window is working well. It occurs on cool mornings when outdoor air is humid and the glass surface is colder than the dew point of the outside air. This is exactly the same as condensation forming on the outside of a cold drink glass. It typically evaporates within an hour as the glass warms with the morning sun. In Calgary, exterior condensation is most common in late spring and early fall when overnight temperatures drop significantly but daytime humidity rises. It requires no action — if anything, it indicates the window is thermally effective enough that the outer pane remains cold even as indoor heat is retained.

How much indoor humidity should a Calgary home have in winter to prevent window condensation?

The recommended indoor relative humidity for a Calgary home in winter varies with outdoor temperature: at -30°C outdoor, keep indoor RH at 15–20%. At -20°C outdoor, 20–25% RH. At -10°C outdoor, 25–30% RH. At 0°C outdoor, 30–35% RH. Calgary's natural outdoor air in winter is extremely dry (5–15% RH). A tightly sealed home accumulates moisture from cooking, showers, and occupants — and the balance between adequate comfort humidity and window condensation is narrow. Most Calgary homes with standard double-pane windows can maintain 30–35% RH in mild winter weather (-5 to -15°C) without condensation issues, but must drop to 20–25% RH during deep cold snaps (-25°C and below).

What does window seal failure and IGU replacement cost in Calgary?

IGU (insulated glass unit) replacement costs in Calgary: single IGU replacement (standard bedroom window size, 60cm × 90cm) — from $175–$350 for the glass unit, plus $65–$95 labour to remove and reinstall. Larger windows (150cm × 120cm) — from $350–$650 for the unit. Full window replacement (frame + glass) — from $400–$900 per window installed for a standard vinyl double-pane. A handyman can handle caulking, weatherstripping, and minor frame repairs around windows. IGU replacement requires sourcing the correct unit (measured to within 1mm) — a window company typically supplies and installs. If the condensation is interior (on the room-facing glass surface) rather than between-pane, improved caulking and air sealing around the window is often the fix, which is handyman scope.

Window Condensation Calgary

The three types of window condensation

Where moisture appears	Cause	Action needed
Outside surface of glass	Cold glass below outdoor dew point — normal	None — evaporates with morning sun
Inside surface of glass (room-facing)	Indoor humidity too high for current outdoor temp	Reduce humidity; improve air circulation at window
Between the panes (foggy, cannot wipe off)	IGU perimeter seal has failed — moisture inside unit	IGU replacement or full window replacement

Indoor humidity targets by outdoor temperature — Calgary

Outdoor temp	Max indoor RH (double-pane)	Notes
0°C	30–35%	Most comfortable range for Calgary winters
-10°C	25–30%	Reduce humidifier output
-20°C	20–25%	Minimum practical humidity for comfort
-30°C and below	15–20%	Calgary cold snaps — expect dry conditions indoors

Why IGU seal failure is common in Calgary

Calgary's annual temperature range is one of the most extreme of any major Canadian city — from -40°C in deep winter to +35°C in summer, a swing of 75°C. The perimeter seal of a double-pane IGU is a butyl rubber and silicone compound that expands and contracts with this cycle every year. After 15–25 years, the seal fatigues and allows moist outdoor air into the argon-filled cavity.

Homes built in the 1990s and early 2000s with vinyl double-pane windows are now at or past the typical IGU lifespan. Between-pane fogging on these homes is expected, not exceptional.

Interior condensation fixes — what a handyman can do

Caulk the window frame-to-wall junction (interior) to eliminate cold air channels that chill the frame
Replace failed weatherstripping in operable windows to reduce cold air infiltration
Caulk the exterior window frame perimeter to reduce cold air reaching the inner frame
Advise on blind and curtain positioning (keeping blinds open lets warm room air circulate to the glass)

IGU replacement (between-pane fogging) is a window company scope — the unit must be measured and ordered to within 1mm tolerance.

Window caulking and weatherstripping in Calgary

If interior condensation is dripping at the frame or sill, the window caulking and weatherstripping may be letting cold air in. We inspect, re-caulk, and seal — preventing condensation damage before it reaches the drywall.

View caulking and weatherstripping service →Request a window sealing quote

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