Window Condensation in Calgary: The Truth Most Companies Won't Tell You

Why Condensation Happens — The Science

Condensation forms when warm, moisture-laden air meets a cold surface. That's the simplified version. The technical version involves something called the dew point—the temperature at which air becomes saturated and water vapor turns to liquid.

Every volume of air can hold a certain amount of moisture. When air is warm, it can hold a lot. When it cools down, it loses that capacity. The moment air temperature drops below its dew point, moisture condenses into visible droplets. In Calgary winters, this happens on windows constantly because glass is one of the coldest surfaces in your home.

Here's why Calgary specifically amplifies this problem: our winters are brutally cold. In January and February, exterior temperatures regularly drop to -20°C or lower. A single-pane window's interior surface will be nearly as cold as the outside air. Even a double-pane window experiences significant temperature drop. This means interior air—which in winter contains a decent amount of humidity from showering, cooking, and breathing—hits that cold glass and condenses almost instantly.

Condensation Temperature Chart

This chart shows at what interior temperatures condensation begins to form at different humidity levels:

Example: If your home is at 21°C with 50% humidity, your dew point is 9°C. Any glass surface cooler than 9°C will form condensation.

The Three Types of Window Condensation

1. Interior Condensation (Most Common)

This forms on the inside surface of your window—the side facing your living room. You can wipe it away. It's most common on cold mornings when interior humidity is high and exterior temperatures plummeted overnight. This usually means your home is too humid for winter conditions. It's not necessarily a window problem; it's a humidity problem.

2. Exterior Condensation (Actually Good News)

You see moisture on the outside of the window, and it happens in spring or fall when nights are cold but days are warm. This actually indicates your windows are performing exceptionally well. Your window is so well-insulated that the exterior glass surface stays cold, while the sun warms the air around it. This creates condensation on the outside. High-performance triple-pane windows see this regularly. It's not a failure—it's a sign of excellent insulation.

3. Between-Pane Condensation (Seal Failure)

Fog or moisture permanently trapped between the glass layers is a red flag. This means the seal separating the panes has failed, allowing air and moisture to enter the space that should be sealed and filled with inert gas (argon or krypton). This window cannot be repaired and must be replaced. There's no home remedy. Between-pane condensation indicates the window is no longer performing its insulation function.

What Most Companies Won't Tell You

New Windows Can Actually Trap More Humidity

This sounds counterintuitive, but it's building science fact: when you replace old, leaky windows with new, airtight windows, you might see more interior condensation initially. Why? Old windows leaked air constantly. That leakage (called infiltration) dried out your home. New windows seal that gap. Now your home's humidity stays trapped inside. Until you adjust your ventilation habits, you'll notice condensation that didn't appear before. The window isn't failing—your home is now too humid for winter.

The "It's Your Humidity" Deflection

Yes, some companies use this as a catch-all dismissal. But here's the nuance: sometimes it IS valid. Interior humidity above 40% in winter is high. Sometimes it's not valid—especially if only one or two windows show condensation while others don't. Localized condensation points to weak insulation or poor installation, not home humidity. A reputable company will distinguish between the two.

Cheap Windows Condense First

Budget windows with aluminum spacers (the frame separating the panes) are thermal bridges. Heat escapes through aluminum rapidly, making the interior edge of those windows much colder than center glass. Condensation forms at edges first. Warm-edge spacers (foam or fiberglass) reduce this dramatically. This is one key difference between $200 windows and $600 windows—and it directly affects condensation performance.

Installation Quality Matters as Much as Window Quality

A premium triple-pane window installed poorly—with gaps in the air seal, insufficient caulking, or misaligned frames—will condense heavily. The installation creates cold spots. Conversely, a decent mid-range window installed professionally, with continuous air sealing, may outperform it. Window companies rarely admit that their installation practices are the weak link.

Building Science: Air Sealing and Vapor Barriers

Modern building science recommends homes be airtight (to prevent drafts and infiltration) but breathable (to allow moisture vapor to escape). Most older Calgary homes are neither. When you air-seal without adding proper vapor management, moisture gets trapped. When you install windows without addressing home humidity control, condensation results. Windows don't work in isolation—they work within your home's entire moisture and ventilation ecosystem.

How to Prevent and Manage Condensation in Calgary

Target Humidity Levels

The ideal indoor humidity varies by season. In Calgary winters:

• December–January: 25–30% humidity (when outside air is very dry)
• February–March: 30–35% humidity (as outside air warms slightly)
• April–May: 35–40% humidity (spring brings external moisture)

If your home is above 40% humidity in winter, condensation risk rises sharply.

Exhaust Fans Are Your First Defense

Run bathroom and kitchen exhaust fans during and for 20 minutes after showers and cooking. A 30-minute shower pumps massive moisture into your home. Proper ventilation prevents it from settling on windows. Range hoods and bathroom fans that vent outside (not into the attic!) are essential.

Monitor with a Hygrometer

A hygrometer costs $20–50 and shows you real-time humidity. Place one in your main living area. When you see spikes above 50%, you know ventilation needs improvement. This objective data helps identify the real problem instead of guessing.

Heat Recovery Ventilator (HRV): The Real Solution

An HRV continuously exchanges stale indoor air with fresh outside air while recovering 70–80% of heat energy. As it brings in cold outside air, it warms it with outgoing air. This provides continuous ventilation without losing precious heat—the gold standard for Calgary homes. Cost: $3,000–6,000 installed. Benefit: eliminates condensation permanently and improves air quality.

Don't Block Window Airflow

Heavy curtains hung flush against windows trap cold air and humid air between the curtain and glass, accelerating condensation. Use thermal curtains but leave 4–6 inches of airflow behind them, or use motorized blinds that don't seal the window.

Interior Window Films: Limited Help

Temporary anti-fog or insulating films can reduce condensation slightly by raising interior glass temperature, but they're a Band-Aid. They also reduce view quality and require replacement. They work best as temporary solutions while you address underlying humidity issues.

When Condensation Means You Need New Windows

Not all condensation requires replacement. These signs, however, indicate your window has failed and replacement is necessary:

• → Between-pane fog that won't disappear: The seal has failed. No fix except replacement.
• → Heavy, persistent condensation only at edges: Indicates poor spacer technology or installation gaps. These windows aren't insulating properly.
• → Ice formation on interior glass: Your window's R-value is too low for Calgary. Upgrade to higher-performance windows.
• → Wood rot or mold on frames: Chronic condensation has caused water damage. The window is compromised and must be replaced.

Best Window Specs to Prevent Condensation in Calgary

If you're buying new windows, these specifications minimize condensation risk in Calgary's harsh climate:

• ✓ Triple-pane glass: Three layers provide superior insulation. Interior glass surface stays warmer, above dew point more often.
• ✓ Warm-edge spacers: Foam or fiberglass spacers, not aluminum. Prevents the thermal bridge that causes edge condensation.
• ✓ Dual Low-E coatings: Multiple low-emissivity coatings reflect heat back into your home while letting light through.
• ✓ Argon gas fill: Argon between panes insulates better than air. Some windows use krypton for even better performance.
• ✓ Proper installation with continuous air seal: Caulking, flashing, and sealing create an unbroken barrier. Installation quality is as important as window quality.

Windows meeting these specs will have interior surface temperatures that stay above dew point even in Calgary winters, dramatically reducing interior condensation.

Frequently Asked Questions