Why is there frost on my Toronto window frames in winter?

Frost on your window frames is almost always a sign of excess indoor humidity combined with cold exterior temperatures — your warm, moist indoor air is hitting a cold surface and freezing.

This is one of the most common winter complaints we hear from Toronto homeowners, and it's worth understanding because persistent frost and condensation can lead to real damage: rotting wood frames, mould growth, and deteriorating drywall around the window opening. The good news is that in most cases, the fix is straightforward once you identify the cause.

Why it happens comes down to a concept called the dew point. When warm, humid indoor air contacts a surface cold enough to drop below that dew point, moisture condenses — and if the surface is cold enough (which Toronto window frames certainly can be in January and February), that condensation freezes into frost. Older single-pane or even older double-pane windows are especially vulnerable because the frames and glass conduct cold much more readily than modern triple-pane units. Aluminum frames are particularly bad for this — they're highly conductive and will frost up even in moderately cold weather.

Indoor humidity is usually the root cause. Toronto homes in winter ideally sit between 30–40% relative humidity. Many homes, especially well-sealed newer builds or homes with humidifiers running too high, creep up to 50–60% — and that's when you start seeing frost. Common culprits include whole-home humidifiers set too high, poor kitchen and bathroom exhaust ventilation, drying laundry indoors, or simply a lot of people and plants in a smaller space. A basic hygrometer (available at any hardware store for $15–$30) will tell you exactly where your humidity sits.

Toronto-specific context matters here. When outdoor temps drop to -15°C or colder — which happens regularly in January and February in the GTA — even decent windows will struggle if humidity is elevated. Homes in older neighbourhoods like The Annex, Cabbagetown, or Leslieville often have original wood frames or older aluminum replacements that simply weren't designed for today's tighter, more humid homes.

What you can do yourself: Start by turning down your humidifier if you have one, running bathroom and kitchen exhaust fans longer after showers and cooking, and ensuring your HRV (Heat Recovery Ventilator) is running properly if your home has one. These steps alone often solve the problem within days.

If you've addressed humidity and frost persists, the issue may be inadequate window insulation value (low R-value or U-factor), failed window seals, or air leakage around the frame. At that point, it's worth having a contractor assess whether weatherstripping, caulking, or window replacement makes sense. Window replacement in Toronto typically runs $400–$900 per window for standard double-pane units installed, or $700–$1,400+ for triple-pane, depending on size and frame material.

Your next step is to grab a hygrometer, check your humidity levels, and adjust your humidifier or ventilation first — it costs almost nothing and solves the problem in the majority of cases. If frost continues after getting humidity under 40%, browse window and weatherproofing contractors at the Toronto Construction Network to get a few opinions on whether your frames need upgrading.