Window Condensation Cure: Why a Dehumidifier Is the Only Real Fix
Too Long; Didn't Read
- Interior condensation = high indoor humidity, not bad windows. Even brand-new triple pane windows will sweat if indoor humidity exceeds 40% when it's -10°C outside.
- Target humidity: 30-35% in winter for Toronto homes. Buy a $15 hygrometer to check.
- Dehumidifiers work. A 30-50 pint unit running in the basement pulls moisture from the whole house via natural air circulation.
- Other fixes: Run bathroom fans 20 minutes after showers, vent the dryer outside (never inside), and crack a window for 10 minutes daily.
- Condensation between panes is different — that's a seal failure, not a humidity issue.
Answer First: Interior window condensation is a humidity problem, not a glass problem. The fix is reducing indoor humidity to 30-35% in winter — use a dehumidifier, run exhaust fans, and ensure proper ventilation. No amount of glass upgrading eliminates condensation if the indoor air holds too much moisture. A $200-$400 dehumidifier solves what a $15,000 window replacement can't.
The Physics: Why Windows Sweat
Dew point — the temperature at which air can no longer hold its moisture, and water condenses on the nearest cold surface. Your windows are the coldest surface in the room. When the glass surface temperature drops below the dew point of the indoor air, water forms on the glass.
This is identical to what happens when you pour an ice-cold beer on a summer day — the glass sweats because the cold surface chills the surrounding air below its dew point.
In a Toronto winter, the outdoor temperature drops to -10°C to -20°C. The interior surface of your window glass might be 5-15°C (depending on glass spec). If your indoor air at 21°C has a relative humidity above 35-40%, the dew point of that air is right around 5-8°C — exactly the temperature of the glass.
Result: condensation.
The Condensation Chart
| Outdoor Temp | Max Indoor Humidity (Double Pane) | Max Indoor Humidity (Triple Pane) |
|---|---|---|
| 0°C | 45-50% | 55-60% |
| -5°C | 40-45% | 50-55% |
| -10°C | 35-40% | 45-50% |
| -15°C | 30-35% | 40-45% |
| -20°C | 25-30% | 35-40% |
| -25°C | 20-25% | 30-35% |
Triple pane tolerates higher humidity because its interior glass surface stays warmer — the extra glass layer and gas cavity keep more cold out. But even triple pane has a limit. At -20°C with 50% indoor humidity, triple pane will sweat.
Where the Moisture Comes From
A family of four in a Toronto home generates 10-15 liters of moisture per day through:
| Source | Moisture Output |
|---|---|
| Breathing (4 people, overnight) | 1-2 liters |
| Cooking (gas stove) | 1-2 liters |
| Showers (4 per day) | 2-4 liters |
| Laundry drying (1 load) | 2-4 liters |
| Plants, aquariums, pets | 0.5-1 liter |
| Basement moisture (through concrete) | 1-3 liters |
In old, drafty houses, this moisture escaped through air leaks around windows, doors, and soffits. The house was inadvertently ventilated — uncomfortable and energy-wasteful, but dry.
New construction and renovated homes are sealed tight. The moisture has nowhere to go. It saturates the indoor air, and the windows are the first surface to show it.
The Fixes (Ranked by Effectiveness)
1. Dehumidifier (Best Single Fix)
A portable dehumidifier in the basement removes moisture from the entire house. Warm, humid air naturally sinks to the basement (the coolest part of the house), where the dehumidifier extracts it.
Recommended specs:
- Capacity: 30-50 pints/day for a typical Toronto home (1,500-2,500 sq ft)
- Placement: Basement, near a floor drain for continuous draining
- Setting: Target 30-35% relative humidity
- Run time: 8-12 hours daily during winter months
- Cost: $200-$400 for a quality unit (Frigidaire, Danby, or hOmeLabs)
- Operating cost: $15-$30/month in electricity
2. HRV (Heat Recovery Ventilator)
An HRV exchanges stale indoor air for fresh outdoor air while recovering 70-85% of the heat from the outgoing air. It's the permanent, whole-house solution for humidity control.
Specs:
- Cost: $2,000-$5,000 installed
- Efficiency: Recovers 70-85% of heat from exhausted air
- Capacity: 100-200 CFM (sized to the house)
- Control: Humidistat-controlled — it ramps up when humidity rises
Most new Ontario homes built after 2012 have an HRV installed. If yours is in the basement collecting dust, turn it on. Set the humidistat to 35%.
3. Exhaust Fans
Run the bathroom exhaust fan for 20 minutes after every shower — not during, after. Most people turn it off when they leave the bathroom, but the peak moisture lingers.
Run the kitchen range hood while cooking and for 10 minutes after. Gas stoves produce more moisture than electric (combustion releases water vapor).
4. Ventilation Habits
- Crack a window for 10 minutes daily. Even in winter. The burst of dry, cold outdoor air exchanges with the humid indoor air and drops the humidity noticeably.
- Don't dry laundry indoors. A single load of wet laundry releases 2-4 liters of water into your home. Use the dryer (vented outside) or dry clothes in a room with an open window.
- Keep interior doors open. Closed doors trap moisture in individual rooms. Open bedroom doors during the day to let air circulate.
5. Window Upgrades (Last Resort)
If you've addressed humidity and still see condensation, the windows themselves may be underperforming:
- Single pane: Interior glass surface is extremely cold. Upgrade to double or triple pane.
- Failed seals: Foggy glass between panes means the insulating gas is gone, reducing the glass surface temperature.
- Aluminum frames: Metal frames conduct cold aggressively, creating condensation on the frame itself even when the glass is dry. Vinyl or fiberglass frames solve this.
Condensation Between Panes vs. Interior Surface
These are completely different problems:
| Location | Cause | Fix |
|---|---|---|
| On the interior glass surface | High indoor humidity | Dehumidifier, ventilation, exhaust fans |
| Between the two glass panes | Broken seal on IGU | Replace the glass unit |
| On the exterior glass surface | Low-E coating working well | No fix needed — it's a good sign |
If you see fog or moisture between the panes, that's a seal failure — the insulating gas has leaked out and humid air has entered the cavity. A dehumidifier won't fix that. The IGU needs replacing.
The Mold Risk
Chronic condensation on windows creates the perfect environment for mold:
- Persistent moisture on wood sills, drywall, and caulking
- Organic material (wood, paint, dust) as a food source
- Poor air circulation at window edges
Black mold can establish within 48-72 hours on a wet surface. Once it's in the drywall or window frame, cleaning the surface isn't enough — the material may need to be cut out and replaced.
Prevention: Wipe condensation daily with a dry cloth during cold snaps. Improve air circulation at window sills (don't block them with curtains that trap moisture against the glass). And address the humidity source — that's the permanent fix.
Frequently Asked Questions
Is condensation on the inside of windows dangerous?
Chronic condensation leads to mold growth within 48-72 hours. Black mold on window frames and drywall is a health hazard, especially for children and people with respiratory conditions.
Why do my new windows have more condensation than my old ones?
Old windows leaked air, passively dehumidifying the house. New windows seal tightly, trapping moisture inside. Better ventilation — not worse windows — is the solution.
What indoor humidity level causes window condensation in Toronto?
At -10°C, condensation starts on double-pane windows above 35-40% humidity. At -20°C, the threshold drops to 25-30%. Triple pane tolerates higher humidity.
Should I run a dehumidifier in winter?
If your windows sweat, yes. Set it to 30-35% relative humidity. A basement unit is most effective. Running it 8-10 hours overnight is usually sufficient.
Does exterior condensation on windows mean anything?
It means your windows are working well — the outer pane is cold enough to condensate, meaning the Low-E coating is keeping heat inside. It evaporates as the glass warms.
Tired of wiping windows every morning? We can assess whether it's a humidity issue or a glass/seal issue — and recommend the right fix for your situation. Get in touch and we'll help you solve it permanently.