Window Condensation in Ontario: Inside the Room, Between the Panes, or on the Outside, Three Different Problems, and the Humidity Chart That Settles the First One

The numbers

Inside the room: the house

Warm air holds water; cold surfaces squeeze it out. In January the inside face of your glass is the coldest thing in the room, and when the air at the glass drops below its dew point the water appears, first at the bottom corners where the spacer is, then up the pane. The window is doing nothing wrong. The question is how much water is in the air. Showers, cooking, drying clothes indoors, a crawlspace with a dirt floor, forty houseplants, a humidifier set to 45% by somebody who read it was good for the furniture, and a new house full of framing lumber and concrete still drying out for its first two winters.

Rough guide, at 21 C indoors, for a good Low-E argon unit with a warm-edge spacer. Old aluminum-spacer double glazing fogs 5% sooner; single glazing fogs at almost any humidity. The numbers are also where mould starts to grow on cold wall corners, so they are worth hitting regardless of the windows.

Fixing the house

1. Measure. A $20 hygrometer in the living room. Over the numbers above: the air, not the glass.
2. Run the bathroom fans, 20 minutes after every shower, on a timer switch so nobody has to remember. If the fan vents into the attic, that is a different and worse problem: the attic page.
3. Range hood on when cooking, vented outside, not a recirculating one.
4. Humidifier off or down. Furnace humidifiers set and forgotten are the commonest cause in Ontario.
5. Open the blinds. A closed blind or heavy drape makes the glass colder by cutting it off from room heat; the fog behind a blind is the first fog of the season.
6. Crawlspace and basement. Dirt floors, damp block walls, a sump with no lid. Poly on the dirt, a lid on the sump, a dehumidifier.
7. An HRV, if the house is tight. A new house or a well air-sealed old one needs mechanical ventilation to carry the moisture out; the HRV’s winter setting is the control. $2,500 to $4,500 installed, and required in new Ontario houses.

Between the panes: the window

Fog, streaks or a milky film between the two sheets of glass that you cannot wipe from either side is a failed sealed unit. The seal has let go, the argon is gone, and moist air is in the gap. The frame is fine. A glazier makes a new sealed unit to size and swaps it into the sash: $250 to $450 depending on size and whether it is tempered, twenty minutes on site. On a window with a sound frame this is the whole repair; the repair page has when it is not.

On the outside: nothing

Dew or frost on the outside of the glass on a cool, still, clear morning in spring or fall, gone by nine. It means the outer pane is cold because the window is not letting the house’s heat through to warm it, which is the point of the window. It happens more on triple glazing and on the best double glazing, and more on north and east faces. The only fix is a worse window.

When the window actually is the cause

• Single glazing, or 1960s aluminum frames with no thermal break: the frame itself ices. Replacement is the fix, and it costs this.
• An aluminum box spacer: the band of fog along the bottom edge at moderate humidity. A warm-edge spacer in a new sealed unit or a new window moves the fog line up by 5 to 10% RH.
• No foam at the perimeter: cold air leaking in around the frame chills the jamb and the glass edge and the fog is at one side only. Pull the casing and foam.
• A window set too far out in a thick wall, with a deep cold reveal inside, in a house that is otherwise fine. Rare; the detail on an ICF or exterior-insulated wall.

Sources and further reading

• Addressing moisture and mould in your home: Health Canada. Humidity, condensation and what mould on a window or wall means.
• Keeping the Heat In: Natural Resources Canada. The free guide to windows, doors, air sealing and insulation written for Canadian houses.