Maintenance
Condensation on Original Wood Windows in Canada: Limits, Compromises and Repair Signals
A cold-climate guide to condensation on original wood windows in Canada: humidity ceilings, storm and panel effects, compromises, and when repair is the answer.
What to take away
- Condensation on original single glazing is a temperature problem first. You can warm the glass or dry the air, but not ignore both.
- Illustrative ceilingskeep indoor relative humidity near 30% when overnight lows sit at -20°C, and nearer 40% when they sit close to 0°C.
- Exterior storms and interior panels beat a dehumidifier, because they warm the glass instead of drying the whole room.
- Air sealing without added ventilation raises indoor moisture. Add exhaust in the same season.
- Stop patching when a bottom rail has been spliced twice in five years or a small dehumidifier fills daily.
The winter limit in numbers
An old window is a cold surface with warm air on one side. In Winnipeg, Saskatoon and Montreal, lows stay below -10°C for roughly four months and often drop past -20°C.
Glass in a single-glazed sash tracks outdoor temperature, so its inner face can sit near -15°C on a January night. Room air at 21°C then meets a surface far below its dew point, and water appears on the pane.
The constraint is measurable. These ceilings are illustrative, for single glazing with no storm and no panel.
| Overnight low | Humidity ceiling indoors | What residents notice |
|---|---|---|
| 0°C | 40% | Comfortable for most |
| -10°C | 35% | Dry throat, cracked lips |
| -20°C | 30% | Static shocks at night |
| -30°C | 25% or lower | Below most humidifier settings |
Values move with glazing, air movement at the glass, curtain use and how well the sash seals. Read the table as a starting point for a hygrometer.
What a cold climate rules out
Set a whole-house humidifier to 45% in January in Winnipeg and the water goes somewhere. It lands on the glass, runs to the sill, then into the bottom rail and the wall below. On unimproved single glazing below -20°C, a comfortable 45% is not available.
That limit also rules out keeping a piano or a wide-plank floor in a room held at 45% behind single glazing all winter. Moisture leaves through the coldest surface, and the window is it. Failures of this kind often run under the paint and appear in spring; the common ones are collected in the mistakes that quietly damage old houses.
What still works
Work in this order and stop when the glass stays clear.
Order of Work to Stop Condensation
- Read each floor with hygrometer and thermometer
- Weatherstrip and adjust sash to latch tight
- Add exterior storm window or interior panel
- Vent moisture at the source
- Keep curtains and blinds off glass at night
Paint that fails from the inside on a lower rail is usually room moisture, and the paint problems brief shows how to read those patterns before repainting.
A pane that clears by noon each day is a nuisance. A pane that pools water in the sill through February is a repair.
Plaster answers to the same swing. Very dry air in January and humid air in July work on old repairs, which is the pattern described in the fixes that shift with freeze-thaw winters.
The compromises worth making
Lower the indoor target and accept the side effects: static shocks, dry skin, one humidified bedroom instead of a humidified house. Close that bedroom door and add a panel to its window. Run a small dehumidifier in the bathroom after showers rather than in the living room all night.
Where plaster was patched after an earlier moisture problem, the guidance on keeping repaired plaster sound explains why a repaired wall should not be re-wetted by the same dry-wet cycle that cracked it.
The compromises that are not worth it
- Shrink film taped to the casing and left for months, trapping water against the sill.
- A sash cracked open all winter to let moisture escape, which cools the room and risks a frozen pipe.
- Draining indoor air to 15% relative humidity, which shrinks floorboards and cracks plaster.
- Wiping the sill each morning and calling the problem handled.
- Antifreeze poured into the sill channel, which stains wood and does not stop the water.
When the workaround stops paying
The workaround has failed when a 20-litre dehumidifier fills more than once a day, the same bottom rail is spliced twice in five years, or the sill stays damp after the glass clears.
When to Stop Patching
20-litre dehumidifier fills more than once a day?
assembly needs money, not the room
continue workaround and monitor
Then the assembly needs money, not the room. Where several windows are at that stage, a first-year checklist for restoring a historic home will sequence the work better than chasing leaks room by room.
Sash repair disturbs paint, and on a pre-1978 window that sets the lead-safe rules. Containment and cleanup follow the RRP program, alongside provincial rules in Canada.
