Windows condensation inside your home forms when warm, moist indoor air meets a cold glass surface and the water vapour turns to liquid. In most cases it means your indoor humidity is too high, your ventilation is poor, or your insulated glass unit (IGU) has lost its seal. The good news is that most causes are diagnosable and fixable without a full window replacement — though sometimes replacement is the smarter call.
What Actually Causes Condensation on the Inside of Windows?
Condensation follows a simple rule of physics: when moist air hits a surface cooler than its dew point, moisture drops out of the air and clings to that surface. In a typical Toronto or GTA home during winter, the interior glass temperature can drop well below the dew point, especially with older single-glazed or low-performance double-glazed units.
Common causes include:
- High relative humidity indoors (above 50% in winter)
- Inadequate exhaust ventilation in kitchens and bathrooms
- Cooking, showering, and drying laundry without proper extraction
- Poor air circulation near window surfaces (heavy drapes, furniture blocking airflow)
- Cold window frames that act as thermal bridges
- Failed or degraded IGU seals allowing gas to escape
Understanding which of these applies to your home is the first step to solving the problem rather than just mopping it up.
Interior Condensation vs. Between-Pane Condensation: Know the Difference
These two problems look similar but mean very different things.
| Type | Location | Cause | Fix |
| Interior surface condensation | On the room-facing pane | High indoor humidity | Ventilation, dehumidifier, airflow |
| Between-pane condensation | Trapped inside the IGU | Failed window seal | IGU replacement or full window replacement |
| Exterior condensation | Outside surface | Normal in humid weather | No action needed |
Interior surface condensation is a humidity problem. Condensation between glass panes is a glass-unit failure. Exterior condensation — the kind that appears early on a summer morning — is actually a sign that your window is thermally efficient and is completely normal.
If you can wipe the moisture off, it’s interior condensation. If it’s fogged inside the glazing and won’t wipe away, you have a failed seal.
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The Role of Indoor Humidity in Condensation
Relative humidity (RH) is the single biggest driver of interior condensation. Health Canada recommends keeping indoor RH between 30% and 55%. During a Canadian winter, running the RH above 40–45% in an older or less-efficient home almost guarantees condensation on windows.
Humidity sources add up quickly:
- A family of four generates roughly 10–15 litres of moisture per day through breathing, cooking, bathing, and laundry
- Houseplants release moisture through transpiration
- Unvented gas appliances add combustion moisture
- Wet basements contribute through stack effect air movement
A basic hygrometer (around $15–$30 CAD) is worth having in any room that frequently shows condensation. Track the RH over a week before spending money on fixes.
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Why Window Performance Matters — A Lot
Higher-performing windows keep the interior glass surface warmer, which means the glass temperature stays above the dew point more often. This is measured through the U-factor: the lower the U-factor, the less heat escapes through the glazing assembly.
| Glazing type | Typical U-factor (W/m²K) | Interior glass temp at –15°C outside |
| Single-glazed | 5.0–6.0 | Very cold — high condensation risk |
| Double-glazed (standard) | 1.6–1.9 | Moderate risk in humid conditions |
| Triple-glazed (Low-E + argon) | 0.8–1.3 | Significantly warmer — low risk |
Triple-glazed units with Low-E 272 or Low-E 366 coatings and warm-edge spacers keep the interior glass surface considerably warmer. This directly reduces the frequency and severity of condensation — one reason energy-efficient window replacement tends to solve persistent condensation problems that ventilation alone can’t fix.
Brendan Kowalczyk, RRO, Building Science Consultant, Toronto: “Most homeowners focus on fixing humidity levels, and that’s correct as a first step. But when I’m called in on a persistent condensation complaint, I almost always find that the window’s centre-of-glass temperature is too low. A high-performance IGU with warm-edge spacers changes that equation. You can run a reasonably comfortable indoor humidity level and still not get condensation — that’s the goal.”
Practical Fixes for Interior Condensation
If your windows are in reasonable condition and you’re dealing with surface condensation, work through these steps before considering replacement:
- Improve ventilation. Run bathroom and kitchen exhaust fans during and 20 minutes after cooking or showering. Make sure the fans actually vent outside and not into the attic.
- Use a whole-home or room dehumidifier. Dial indoor RH down to 35–40% during winter months.
- Increase air circulation near windows. Move heavy curtains or furniture that blocks warm air from reaching the glass surface.
- Check window weatherstripping. Cold drafts along frames draw warm moist air toward glass edges and cause edge-bead condensation.
- Open windows briefly each day. Even 5–10 minutes of air exchange dramatically lowers indoor moisture levels. This is especially relevant in tightly sealed newer homes.
- Add a heat recovery ventilator (HRV). An HRV is one of the most effective long-term solutions for controlling indoor humidity without losing heat — common in new Ontario construction and worth retrofitting in older homes.
If you’ve done all of the above and condensation persists, the window’s thermal performance is likely the limiting factor. This is particularly common with older windows that have common issues like degraded glazing, cracked seals, and aluminium spacers that create cold edges.
When Should You Replace the Windows?
There’s a point where fixing the environment around a poor-performing window is less cost-effective than replacing the window itself. Replacement makes the most sense when:
- The IGU seal has failed (fogging between panes that won’t clear)
- The frame is visibly deformed, cracked, or allowing cold air infiltration
- The window is single-glazed or an early double-glazed unit with aluminium spacers
- Condensation appears repeatedly on multiple windows despite humidity control
- You’re already planning an upgrade for other reasons (aesthetics, insulated frames, security)
Replacement windows and frames with multi-chamber uPVC profiles and warm-edge spacers can reduce edge-zone condensation by up to 25% compared with aluminium-spaced units, and the centre-of-glass temperature improvement from triple glazing is dramatic enough to change the outcome even at normal winter humidity levels.
Patricia Hanes, C.E.T., Building Envelope Specialist, Mississauga: “A failed IGU seal is a one-way door. Once argon gas has leaked out and moisture has entered the cavity, the unit’s thermal performance drops significantly and you’ll never restore it by cleaning or adjusting. The only correct fix is replacing the glazing unit or the entire window. Homeowners sometimes try workarounds — drilling small holes, desiccants — and none of them restore the original U-factor.”
ENERGY STAR™ Certification and Condensation Resistance
ENERGY STAR™ certified windows for the Canadian Northern climate zone must meet strict U-factor and ER (Energy Rating) thresholds. These requirements also push manufacturers toward glazing assemblies and spacer systems that substantially raise interior glass temperatures.
When you’re selecting replacement windows, ask for the Condensation Resistance Factor (CRF) or the window’s certification documentation. ENERGY STAR™ certified products have been independently tested and verified — not just rated by the manufacturer. If you’re considering who makes the best replacement windows for the money, ENERGY STAR™ certification is a useful objective filter.
Is It Worth Replacing Windows Just to Stop Condensation?
Condensation alone may not justify full window replacement — but it’s rarely the only symptom in a home with older windows. If your windows are 20+ years old, how often windows should be replaced is worth thinking through. Most vinyl windows have a service life of 20–30 years, and performance declines gradually over that period.
A replacement project done for energy efficiency, comfort, and condensation control also improves sound attenuation, security, and property value. Is it worth getting windows replaced? — for most GTA homeowners dealing with chronic condensation, the answer tends to be yes once the windows pass the 15-year mark.
FAQ
Why do my windows have condensation on the inside in winter?
Condensation forms on the interior glass surface when indoor air is more humid than the glass can tolerate at its current temperature. Lower your indoor humidity to 35–40% RH and improve ventilation — that usually resolves it.
Is condensation on the inside of windows a sign they need replacing?
Not always. Surface condensation is a humidity issue. However, if the glass is fogged between the panes and won’t wipe clear, the IGU seal has failed and the glass unit or full window should be replaced.
How do I stop condensation on windows overnight?
Keep indoor RH below 40% in winter, run an HRV or exhaust fan before bed, open bedroom doors to improve air circulation, and avoid drying laundry indoors. These steps reduce overnight moisture buildup significantly.
What is a normal humidity level to prevent window condensation in Canada?
Health Canada recommends 30–55% RH year-round. In winter, aim for 35–40% RH to keep interior glass surfaces above the dew point, especially in homes with double-glazed or older windows.
Can new windows completely eliminate condensation?
High-performance triple-glazed windows with warm-edge spacers and Low-E coatings dramatically reduce condensation risk by keeping interior glass warmer. However, if indoor humidity is very high, some condensation can still occur even on premium windows.
