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Windows sweating / condensation inside

A misty window on a cold Canadian morning is common and rarely means anything is broken. Most of the time it's a humidity balance to adjust, not a window to replace.

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In Canadian homes, check your indoor humidity first. A $10–20 hygrometer from any hardware store tells you in minutes. In winter, aim for roughly 30–50% relative humidity, and drop toward 30% or below during a deep cold snap — the colder it is outside, the drier the house needs to be. If you're reading high, the fix is ventilation and habits, not new windows.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. Get a digital hygrometer ($10–20 at any Canadian hardware store) and set it in the main living area — away from the kitchen, bathroom, or a sunny sill, which all read high or low.
  2. Read it on a cold morning, when condensation is at its worst. Note the outdoor temperature at the same time.
  3. Compare against the guide printed on most Canadian humidistats: around 0 °C outside, keep indoor humidity at or below about 50%; around −10 °C, about 40%; around −20 °C, about 30%; below −25 °C, 25% or less. Colder outside means drier inside. These targets assume reasonably efficient double- or triple-glazed windows; with older or single-pane glass and cold corners, aim lower — if the glass still fogs at these levels, bring the humidity down further.
  4. Reading above the guide for that day's temperature points to indoor humidity as the cause to work on — start with the fixes below and re-check in a few days.
buildhygrometer schedule5 min

handymanHow to fix it

Move the moisture out and stop adding it: run fans longer, turn the furnace humidifier down in cold weather, ventilate more, and re-check the hygrometer after a few days.

  1. Run the bath fan during every shower and for 30 minutes after; run the range hood whenever you cook. Lids on pots help more than you'd think.
  2. If your furnace has a humidifier, turn it down — or off — during cold snaps. A setting that's comfortable at 0 °C oversupplies badly at −25 °C.
  3. If you have an HRV/ERV, set it to run continuously on low in winter rather than off-and-on.
  4. Cut the big hidden sources where you can: dry firewood indoors, unvented clothes drying, aquariums without covers, a jungle of houseplants in one room.
  5. Give it two or three days, then read the hygrometer on the next cold morning. Lower number and drier glass means you're on the right track — keep going until the windows stay clear.
schedule30 min savings$0–30
Digital hygrometerAny small digital model with humidity and temperature; $10–20 at Canadian hardware stores
warning If mould keeps coming backA little black spotting on a sill or frame can be washed off with water and dish soap once the humidity is under control. But mould that keeps returning, spreads onto walls or ceilings, or covers a large area is a health matter, not a cleaning chore — follow Health Canada and CMHC mould guidance, and bring in professional remediation for anything beyond small patches. Fix the moisture first; mould always comes back to a wet surface.

visibilityHow to check

  1. Find the unit: a suitcase-sized metal box, usually hung near the furnace or in the utility room, with four ducts and often a small wall control upstairs labelled with humidity percentages or fan modes.
  2. Confirm it's running: a faint fan hum and gentle airflow at its grilles. If the wall control is on "recirculate" or "off", winter humidity has nowhere to go — set it to a continuous ventilate mode.
  3. If the control has a dehumidistat dial, winter settings typically sit near 30–40%; a dial parked at 60%+ effectively tells the unit not to bother.
  4. Unlatch the front panel and slide out the filters. A grey, felted filter — or a core furred with dust — chokes airflow and is the most common reason a working HRV stops helping.
buildNo tools schedule10 min

handymanHow to fix it

warning Unplug it before you open itThe fans inside spin fast enough to hurt fingers, and some units restart on their own. Unplug the unit — not just the wall control — before opening the panel. If yours is hard-wired with no plug, switch it off at the breaker, or leave the cleaning to an HVAC visit; it's a quick, inexpensive service call.

Unplug the unit, wash the filters (and rinse the core if accessible), set the control to continuous low ventilation with the dehumidistat around 30–40% for winter, and plug it back in.

  1. Unplug the unit (most HRVs plug into a nearby outlet) and unlatch the front panel.
  2. Slide out the foam or mesh filters and wash them in the sink with warm water and a little dish soap. Let them dry before they go back.
  3. If the honeycomb core slides out easily, follow the door sticker or manual: most cores rinse with a gentle shower spray. No dishwasher, no pressure washer.
  4. Check the two outdoor hoods for the unit (intake and exhaust) — lint, leaves, or snow screening them off will choke it just like a dirty filter.
  5. Reassemble, plug in, and set the control: continuous low ventilation, dehumidistat near 30–40% for winter. Put a reminder in your phone to wash the filters every two or three months during the heating season.
  6. If the unit won't run, rattles, or one airstream is dead, an HRV service visit is a routine call for a ventilation/HVAC contractor.
schedule45 min savings$0–20

visibilityHow to check

  1. Run the bath fan and hold a single square of toilet paper to the grille. It should stick firmly. If it flutters or falls, the fan is weak, clogged with lint, or the duct is blocked.
  2. Step outside and find the vent hood for each fan (wall or roof). With the fan running, the flapper should be open and you should feel air. A flapper that never moves means the duct is disconnected or blocked.
  3. If you can safely peek into the attic, look for a fan duct lying loose or ending in the insulation — moisture dumped into a Canadian attic in January becomes frost, then a "roof leak" in the spring thaw. That termination fix is a job for a contractor if you're not comfortable up there.
buildNo tools schedule10 min

handymanHow to fix it

Use the fans every time and long enough; clean lint off the grilles; and if a duct is disconnected or dumps into the attic, have it properly extended to an outside hood.

  1. Make the habit automatic: fan on with the shower, off 30 minutes after. A wind-up timer or the switch's built-in timer (if it has one) does the remembering for you.
  2. Vacuum the lint off the fan grille — a felted grille can cut airflow dramatically, and it's a two-minute job.
  3. If the toilet-paper test failed or the outside flapper never opens, the duct is blocked, crushed, or disconnected. Re-seating an accessible duct is doable; anything buried in the attic or ceiling is a fair job to hand off.
  4. If a fan vents into the attic (or nowhere), have it ducted to a proper exterior hood — a routine visit for a ventilation contractor, and one of the best moisture fixes a Canadian house can get.
schedule30 min savings$0–25

visibilityHow to check

  1. Wipe the inside face of the glass with a dry cloth. If the fog wipes clean, it's surface condensation — work the humidity causes instead.
  2. If it doesn't wipe clean from inside, check the outside face too (frost and dew sit out there on some mornings).
  3. Haze, droplets, or a milky stain that no cloth can reach — often worst in the middle of the pane, sometimes with mineral streaks — sits between the panes. That points to a failed seal in that window's glazing unit.
buildNo tools schedule5 min

verifiedWhy this one goes to a pro

Replacing a sealed unit means handling a large, heavy pane of glass — often on a ladder — and getting the measurement, spacers, and seal right so the new unit doesn't fog next winter. Glass shops do this every day, usually for a few hundred dollars per window, and the frame stays put. There's no shame and no urgency: a fogged unit is a cosmetic and efficiency issue, not a safety one.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

handymanHow to fix it

Let a little room air reach the glass: crack blinds a few centimetres, keep curtains off the sill, and open coverings during the day.

  1. Overnight, leave blinds cracked a few centimetres at the bottom and keep heavy curtains from resting on the sill, so room air can wash the glass.
  2. Open the coverings fully during the day and let the windows dry out.
  3. If the covered windows stay wet anyway, the humidity itself is high — go back to the hygrometer check.
schedule5 min savingsusually free
sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.