Persistent condensation on your windows in winter means indoor humidity is too high for your home’s cold surfaces, and glass is just where you can see it. The same moisture is condensing on window frames, cool wall corners, closet walls and attic sheathing, which is where mold starts. Health Canada recommends keeping indoor relative humidity around 30 per cent in cold weather, and the fix is ventilation plus moisture control, not wiping the glass.
Windows are your home’s humidity gauge. Here is how to read it, why the Fraser Valley makes this harder than most of Canada, and how to dry the house out before mold makes the decision for you.
Why do windows sweat in winter?
Air holds water vapour, and warm air holds more than cold air. When indoor air touches a surface cold enough, its vapour condenses into liquid, exactly like a cold drink can in summer. In winter the coldest surfaces you can see are your windows, so they fog and stream first. The two variables are surface temperature and indoor humidity: older aluminum framed and early double glazed windows run cold, and every shower, pot of pasta, load of indoor drying laundry and sleeping human adds vapour.
The part that matters for mold: your glass is the coldest visible surface, not the coldest surface. Wall corners behind furniture, closet walls on exterior faces, window frames and caulking, and the attic’s roof sheathing all run cold too. When windows are wet every morning, some of those hidden surfaces are damp too, and unlike glass, drywall and wood feed mold. Black spotting on window frames, sills and nearby caulking is typically the first visible growth, and the pattern behind it is the same one in our bathroom mold guide.
How much indoor humidity is too much?
Health Canada’s residential guidance recommends keeping relative humidity below about 50 per cent in warm months and around 30 per cent in cold weather, precisely because winter condensation on cold surfaces drives mold growth. A digital hygrometer costs about $20 and removes the guesswork; put one in the main living area and one in the problem bedroom.
Then use your windows as the tiebreaker. The colder the outdoor temperature, the colder your glass and the lower the indoor humidity you can carry without condensation. If the hygrometer says 45 per cent but the windows stream every cold morning, 45 per cent is too high for your particular windows this particular week. Dry glass, not a single magic number, is the target.
Why is this worse in the Fraser Valley?
Because our winter air is mild but nearly saturated. From roughly October through April, Abbotsford, Chilliwack, Mission and Langley sit under weeks of rain and fog with outdoor relative humidity frequently in the 80s and 90s. That has two consequences. First, ventilation helps less per minute than it does in a cold, dry prairie winter, so it must run longer; the incoming air still dries your house (cold air warmed up becomes dry air), but slowly. Second, homes here spend six months without the deep freeze that would force furnaces to run constantly and incidentally dry the house. Mild, damp and closed up is the exact recipe for condensation, which is why streaming windows are practically a regional pastime and why mold pressure here is higher than the postcard climate suggests.
Where else is that moisture condensing?
The checklist worth walking with a flashlight, in rough order of likelihood:
- Window frames, sills and caulking: black speckling here is established growth, not dirt
- Behind furniture on exterior walls: pull the bed and dressers away from the wall and look
- Closet walls and corners on exterior faces, where air barely circulates
- Bathroom ceilings and walls, the highest humidity room in the house
- The attic: the same indoor humidity leaks upward and condenses on roof sheathing, which is how the Fraser Valley’s most common hidden mold problem starts. Our attic mold guide explains why bathroom fans are usually involved.
- Basement walls and floors, cold by nature; ongoing dampness there is its own topic and a core part of our basement and bathroom mold service
If you find spreading growth, a persistent musty smell, or staining you cannot explain, a mold inspection with a moisture meter answers in an hour what guessing cannot. And if the growth is established, symptoms in the household are worth taking seriously; see mold and your health.
How do you actually stop window condensation?
Reduce moisture in, increase moisture out:
- Run bathroom fans during and 30 minutes after every shower. A timer switch makes it automatic.
- Use the range hood every time you cook, and put lids on pots.
- Vent the dryer outside and never dry laundry on racks indoors in winter; a single load releases litres of water.
- Run exhaust ventilation longer than feels natural. In our climate, hours per day of fan runtime is normal, not wasteful. Homes with an HRV should run it continuously in winter.
- Open blinds and curtains during the day and keep furniture off exterior walls; trapped air pockets run cold and damp.
- Crack bedroom windows or leave doors ajar overnight if bedroom glass streams.
- Turn off humidifiers and check the aquarium and houseplant population in problem rooms.
- Dehumidify stubborn zones. A basement dehumidifier from fall to spring is standard equipment in this valley.
Give the changes two weeks. Most homes see dramatically drier glass; a home that does not, despite honest ventilation, may have a moisture source (crawlspace ground moisture, a slow leak, a disconnected fan duct) worth finding professionally.
The bottom line
Wet windows are cheap information: the house is telling you its humidity exceeds what its cold surfaces can carry, and mold takes the same message as an invitation. Ventilate more, produce less vapour, verify with dry glass, and investigate anything already black or musty. If the spotting has arrived or you want the moisture source found across Abbotsford, Chilliwack, Mission or Langley, call Fraser Valley Mold at the number at the top of this page or request a free quote.
Frequently Asked Questions
Is some window condensation in winter normal?
Brief fogging after showers or cooking, or a thin edge of mist on the coldest mornings that clears by mid morning, is common and not alarming. Condensation that pools on sills, persists all day, appears on many windows daily, or leaves black spotting on frames and caulking means indoor humidity is consistently too high for your surfaces.
What indoor humidity should I keep in winter to stop condensation?
Health Canada recommends keeping indoor relative humidity below about 50 per cent in warmer months and around 30 per cent in colder weather. In practice, if windows are wet every morning, your humidity is too high for your window surfaces regardless of the number, so ventilate more and cut moisture sources until the glass stays mostly clear.
Will new windows fix my condensation problem?
Sometimes, partly. Better windows have warmer interior glass, so less moisture condenses on them. But if the underlying humidity stays high, the moisture simply condenses on the next coldest surfaces instead, like wall corners and closets, where you cannot see it. Fix ventilation and moisture sources first; upgrade glass for comfort and efficiency.
Why is condensation between the panes of glass?
Fog or droplets trapped between the panes of a double glazed unit mean the unit's edge seal has failed and the insulating gas layer is compromised. That is a window repair issue, not a humidity issue: the sealed unit needs replacing. It says little about your indoor air, unlike condensation on the room side surface of the glass.
Why do only my bedroom windows stream with water?
Bedrooms combine closed doors, hours of two people breathing, cooler overnight temperatures and often trickle heat, which pushes relative humidity up overnight in a small volume of air. Leaving bedroom doors ajar, cracking a window slightly, and running the home's exhaust fans longer usually improves bedroom glass noticeably within days.
Published July 10, 2026 · Last updated August 20, 2026 · Fraser Valley Mold