If you have ever woken up in Toronto or Vaughan to a window that looks like it got iced like a donut, you are not alone.
Interior window ice is one of the most common cold weather complaints in the GTA, and it usually has less to do with bad glass
and more to do with moisture, airflow, and how your home handles winter physics.
Here is the big idea. Warm indoor air holds moisture. When that moist air touches a cold window surface, it can drop water,
then freeze into frost or ice. The colder it gets outside, the easier it is for that inside surface to hit freezing temperatures.
Toronto winters are more than cold enough for this to show up. Environment and Climate Change Canada climate normal (1991 to 2020) list January daily averages around -3.5°C, with daily minimums near -6.7°C. (Climate Data)
That is plenty cold to drive interior glass temperatures down when humidity is high or drafts are present.
Below is a field tested, homeowner friendly guide with real thresholds to stop window icing,
protect your frames, and keep your home comfortable.
What Interior Window Ice Is Really Telling You
Think of your windows as the check engine light for indoor humidity.
Health Canada recommends keeping indoor humidity between 30 percent and 50 percent.
Save on Energy Ontario adds a practical note that 30 percent to 40 percent is typically ideal in winter
to stay comfortable without leaving condensation on windows. (saveonenergy.ca)
When your humidity climbs above what your cold glass can handle, moisture condenses and then freezes.
The Quick Science
- Relative humidity is how full the air is with water vapor at a given temperature
- Warm air can hold more moisture
- When warm moist air hits a cold surface like glass, it cools down
- If the surface temperature drops below the dew point, water forms
- If the surface is at or below 0°C, that water becomes frost or ice
Where the Ice Shows Up and What It Means
| What You See | Most Likely Cause | What It Typically Means |
|---|---|---|
| Ice or frost on room side of glass, often bottom corners | High indoor humidity, poor airflow, cold snaps | Humidity and airflow problem first |
| Condensation that turns to ice overnight | Same as above | Indoor RH is above what the glass can tolerate |
| Fog or moisture between panes | Failed insulated glass seal | Window unit failure, not a humidity issue |
| Ice on frames or surrounding drywall | Air leaks, missing insulation, thermal bridging | Installation or air sealing issue |
| Condensation on outside of glass | High efficiency glass | Usually normal, not a defect |
The Number One Driver: Indoor Humidity
Many GTA homes run humidifiers in winter, sometimes tied directly into the furnace.
That is not automatically bad. The issue is running a set and forget humidity level during cold snaps.
CMHC guidance is clear. Indoor relative humidity should generally be between 30 percent and 50 percent,
but when outdoor temperatures drop below -10°C, indoor humidity should be closer to 30 percent,
and sometimes as low as 25 percent to prevent condensation on cold surfaces like windows.
Practical Winter Humidity Targets
| Outdoor Temperature | Suggested Indoor RH | Why |
|---|---|---|
| 0°C to -5°C | 35 percent to 40 percent | Comfortable and usually avoids window sweating |
| -5°C to -10°C | 30 percent to 35 percent | Less moisture so glass stays clearer |
| Below -10°C | Around 30 percent or lower if needed | Prevents condensation during extreme cold |
Pro tip. Buy a basic hygrometer. Guessing humidity is like guessing your phone battery percentage without looking.
Hidden Moisture Sources That Spike Humidity Overnight
- Long hot showers without using the bathroom fan
- Cooking and boiling water without ventilation
- Drying laundry indoors
- Damp basements
- Firewood stored indoors
- Humidifiers left on constant settings
Ventilation Matters Even in Winter
Health Canada recommends using kitchen and bathroom exhaust fans and ensuring they vent outdoors. (Health Canada)
If your home has an HRV or ERV system, CMHC notes these systems help lower indoor humidity
by exhausting moist air and reducing condensation risk.
Fast Ventilation Wins
- Run bathroom fans during and after showers (Canada)
- Use range hoods when cooking
- Confirm fans vent outdoors, not into the attic
- Maintain HRV or ERV filters and winter settings
Airflow at the Window Matters More Than You Think
Even with proper humidity, stagnant air at the glass can still freeze.
- Heavy curtains pressed tightly against windows
- Blinds kept closed during cold snaps
- Furniture blocking heat registers
- No warm air reaching the glass surface
Quick Fix Checklist
- Open blinds slightly during extreme cold
- Keep curtains from sealing window bays
- Ensure vents are not blocked
- Investigate rooms that stay consistently colder
When Drafts and Insulation Are the Real Problem
Ice on frames or drywall usually points to air leakage or insulation gaps.
CMHC notes that cold surfaces paired with humidity can cause condensation and moisture damage.
The Homeowner Draft Test
- On a windy day, hold a tissue near window edges
- If it flutters, air leakage is present
- Check weatherstripping, locks, and caulking
When New Windows Are the Right Solution
| Your Problem | What Helps | Window Types to Consider |
|---|---|---|
| Drafts near sash | Better air tight seals | Casement Windows, Tilt and Turn Windows |
| Cold fixed openings | Warmer interior glass | Fixed Windows, Custom Shaped Windows |
| Need airflow without snow | Controlled ventilation | Awning Windows, Tilt and Turn Windows |
| Wide openings | Durable sliding systems | Sliding Windows |
| Traditional layouts | Proper sealing and hardware | Hung Windows |
| Cold bay areas | Insulation and correct install | Bay and Bow Windows |
FAQ: Window Ice in the GTA
Is interior window ice normal?
It is common, but it signals a humidity or airflow imbalance that should be addressed.
Should I increase humidity because air feels dry?
Not blindly. If condensation is present, humidity is already too high for current conditions.
My humidity is 40 percent and I still get ice. Why?
Either the window surface is extremely cold or airflow to the glass is blocked.
Do HRVs and ERVs help?
Yes. They exhaust moist indoor air and reduce condensation risk.
When should a window be replaced?
Persistent drafts, frame icing, moisture damage, or fog between panes all signal replacement time.
Stop Window Icing the Right Way
If your windows are icing up anywhere in the Greater Toronto Area or surrounding communities, AZTech Doors & Windows can help identify the cause and fix it properly.
Call (416) 946-1061 to book a free estimate or fill out a contact form request and we will reach out you.
