Window Condensation in Winter: What Saint-Laurent Homeowners Should Know About Wet Glass and Dirty Sills

Condensation on the inside of your windows in winter happens when warm, humid indoor air touches glass that's been chilled by the cold outside. A little fog along the bottom edge on a very cold morning is normal. Water running down the pane every day, black spots on the sills or fog trapped between two layers of glass are signs something needs attention, and cleaning only fixes part of it.

Saint-Laurent has a real mix of housing, from 1950s and 1960s bungalows and duplexes to newer condos and townhouses around Bois-Franc. Condensation shows up in all of them, just in different ways.

Why windows sweat once the heat comes on

Air holds moisture. The warmer it is, the more it can carry. When that air hits a surface colder than its dew point, the moisture turns back into water. In a Montréal winter, the coldest surface in most rooms is the window glass, and the coldest part of the glass is the bottom few centimetres, near the frame. That's where the fog and the drips start.

Everyday life puts a lot of water into the air: cooking, showers, drying laundry inside, houseplants, a humidifier set too high, even breathing in a closed bedroom overnight. Newer, tighter homes hold onto that moisture more than drafty older ones did. So a renovated bungalow with new windows and better insulation can actually see more condensation than it did before the work, simply because the air no longer leaks out.

Canadian housing guidance generally suggests keeping indoor humidity lower when it's very cold outside, often somewhere around 30 to 35 percent. A cheap hygrometer from a hardware store tells you where you stand.

What condensation does to your windows

The water itself isn't the only issue. Repeated wetting and drying leaves things behind.

• Film and drip marks on the glass. As condensation dries, it leaves a hazy film of whatever was in the air: cooking grease, dust, smoke from candles or a fireplace. Over a winter that builds into streaky lines running down from where the drops formed.
• Mould on frames, sills and caulking. Water pooling on the sill every morning is an easy place for mould to grow, especially in corners and along rubber gaskets. You'll often see small black spots there first.
• Dirty tracks. On sliding windows and patio doors, water runs into the track and mixes with dust and grit. The result is a dark paste that's hard to get out with a cloth.
• Damage to wood. Wooden sills and frames that stay wet can swell, lose their finish and eventually rot. Painted sills start to peel.

Condensation between the panes is a different problem

If the fog is trapped between the two layers of a double-glazed window and you can't wipe it from either side, the seal on that unit has likely failed. Moist air has got into the space between the panes. No cleaning will fix that, inside or out. The sealed unit usually needs to be replaced, and that's a job for a window installer.

It's worth checking for this in fall, while you can still see clearly. A crew cleaning your windows will often notice a cloudy unit and mention it, but they can't repair it.

What you can do yourself

Most of the fix is about moisture, not cleaning:

• Run the kitchen and bathroom fans while you cook and shower, and for a while after.
• Avoid drying laundry on racks indoors if you can, or do it in a room with the door closed and the fan on.
• Open curtains and blinds during the day. Heavy curtains trap cold, humid air against the glass all night.
• Keep furniture a little away from windows and heating vents so warm air can reach the glass.
• Turn down or switch off a humidifier if the windows are wet most mornings.
• Wipe the sills dry on bad mornings so water doesn't sit.

If the windows still run with water after all that, a heating and ventilation contractor can look at whether the home has enough air exchange.

Where a professional clean fits

Cleaning won't stop condensation, but it does deal with what condensation leaves behind, and clean glass is easier to keep dry and easier to see through on short winter days.

Altara's flat rate covers the exterior: glass, frames and sills, cleaned with pure deionized water on a water-fed pole, with screens rinsed in place. You choose a tier by window count on the pricing page. Exterior work is seasonal and runs roughly from March to November, so late October and early November are the last chances this year.

Interior glass is an optional paid add-on, and interior cleaning runs year-round. That makes it a good fit for the condensation season. Track cleaning is also an optional add-on, and it's the one that matters most for sliding windows and patio doors where water collects. Screen removal and washing is a separate add-on. Heavy mineral staining, skylights and solariums are quoted separately.

A few honest limits. Altara cleans glass, frames, sills and tracks. Mould that's grown into caulking or wood may need the caulking redone or the wood treated, which is repair work. Same for fogged sealed units. If the crew sees either, they'll point it out.

The pole reaches up to three storeys from the ground, which covers most Saint-Laurent houses and townhouses. Taller buildings or homes with awkward access get a site visit first. You can read more on the Saint-Laurent service page.

If you see a streak after the visit, report it within 24 hours and the crew comes back to fix it at no charge.

Frequently asked questions

Is some condensation on windows in winter normal?

Yes. A thin band of fog at the bottom of the glass on the coldest mornings is common in Montréal homes and usually clears as the day warms up. It becomes a concern when water runs down the glass daily, pools on the sills or leads to mould on the frames.

Why are my new windows getting more condensation than the old ones?

New windows and insulation make a home more airtight, so moisture from cooking, showers and breathing stays inside instead of leaking out. The glass may also be the only cold surface left in the room. Better ventilation, using exhaust fans and keeping humidity in check usually help.

Can window cleaning remove fog between the panes?

No. Fog between the two layers of a double-glazed window means the seal has failed and moisture has entered the sealed unit. It can't be reached from either side, and the unit typically has to be replaced by a window installer.

Can I book interior window cleaning in the middle of winter?

Yes. Exterior cleaning pauses for the cold months, but interior work runs year-round. Interior glass and track cleaning are optional paid add-ons, so mention what you need when you get in touch.

Get your windows ready for the cold

Count your windows and pick a tier on the pricing page, then add interior glass or tracks if condensation has been leaving marks. If you have a sliding door full of black grit or a few cloudy units you want looked at, send us a message and mention it.