Quick answer
Condensation on windows means the air inside your home is too moist for how cold the glass is. Health Canada recommends keeping indoor humidity between 30% and 50%. Run bathroom and kitchen fans, don't dry laundry indoors, open blinds, and use a dehumidifier if water keeps forming. Wipe sills dry every day so mould can't grow. Water between the two panes means the seal has failed and the glass unit needs replacing.
Many Surrey homes get wet windows in fall and winter. It is often worst first thing in the morning. You may see water running down the glass, puddles on the sill, or black spots on the frame and caulking.
Condensation is not a leak. The water comes from inside your home. The good news is that most of it can be fixed with habits that cost little or nothing.
Why BC windows get wet in fall and winter
Everyday life puts water into the air: showers, cooking, breathing, and drying clothes. Natural Resources Canada explains that condensation happens in colder weather when moist indoor air touches cooler surfaces. Windows are usually the coldest surface in a room, so water forms there first.
In the Lower Mainland, fall and winter bring many cool, rainy weeks. Windows stay closed and the heat is on, so moisture made indoors has fewer ways to leave. HealthLink BC notes that large differences between inside and outside temperatures can cause water to condense on windows and lead to mould on frames or sills.
What indoor humidity should be
Health Canada recommends keeping relative humidity in your home between 30% and 50%. It says to run a dehumidifier if moisture is condensing on cold surfaces such as window panes, or if humidity is above 50%.
A small digital humidity meter (hygrometer) from a hardware store will show your level. Check it in the morning, in the bedrooms, when condensation is usually worst.
If your windows are wet every morning but the meter reads well under 50%, look at airflow instead. Closed blinds and blocked heat vents can keep glass cold even when the air is not very humid.
How to stop condensation on windows
- Run the bathroom fan longer. Health Canada says to keep the exhaust fan running for at least 30 minutes after a shower or bath.
- Use the kitchen range hood every time you cook. Health Canada recommends using it when cooking, preferably on the highest setting.
- Make sure fans vent outside. Health Canada says your clothes dryer, stove, kitchen and bathroom fans should all vent to the outdoors, not into the attic or a cupboard.
- Don't dry laundry on racks indoors. Health Canada warns that hanging wet laundry indoors can raise indoor humidity. Use a dryer that vents outside, and clean the lint tray every time.
- Open window coverings during the day. Heavy curtains or blinds can trap cold and moisture against the glass, according to Health Canada.
- Keep heat moving past the glass. Natural Resources Canada says airflow across the glass helps keep it warmer. Don't block heating vents or baseboard heaters under windows with furniture or drapes.
- Turn on your HRV. If your home has a heat recovery ventilator, Natural Resources Canada says to make sure it is on.
- Use a dehumidifier in damp rooms and basements, or if humidity stays above 50%.
- Wipe sills and frames dry every day. Health Canada says this keeps water from dripping and causing mould to grow.
Condensation between the panes
If the fog or water is between the two layers of glass, you cannot wipe it off and the tips above will not help. Natural Resources Canada says this happens when the seal is broken, and the fix is to replace the insulating glass unit. Often only the sealed glass needs replacing, not the whole frame. Ask a window company to check.
If you are replacing whole windows, Natural Resources Canada suggests ENERGY STAR certified models, which stay warmer on the inside and get less condensation. BC Hydro currently lists a window and door rebate of up to $2,000 through the Home Renovation Rebate Program, run with the Province and FortisBC. There are rules: a licensed contractor must install the windows, the windows must meet an energy rating, and you must apply within six months of the invoice date. Eligibility also depends on your home, so check before you buy.
Is the black spotting mould, and is it harmful?
Black, grey or green spots on window frames, caulking or nearby walls are usually mould. HealthLink BC says mould can trigger allergic reactions. Studies found more coughing, wheezing and headaches in people who live in homes with dampness and visible mould.
Mould keeps coming back until you fix the moisture. Cleaning without lowering humidity only buys a few weeks.
How to clean window mould safely
Health Canada sorts mould by size:
| Size | What it looks like | Who should clean it |
|---|---|---|
| Small | Three or fewer patches, total area less than 1 m² (about 10 sq. ft.) | You can clean it yourself with proper steps and protection |
| Medium | More than 3 patches, or patches between 1 m² and 3 m² | Often safe to clean yourself with proper steps; consider a professional |
| Large | A single patch larger than 3 m² (about 32 sq. ft.) | Health Canada recommends a qualified professional |
Most window-sill mould is small. To clean it:
- Wear safety glasses or goggles, a well-fitting N95 mask, and household gloves. These are Health Canada's minimum.
- Health Canada says susceptible people should not be in the same room or near the area while mould is being cleaned.
- Scrub smooth surfaces with water and dish detergent. Health Canada says there is no need to use bleach, and its moisture guide says not to use bleach on mould.
- Don't soak the area. HealthLink BC says the surface needs to dry completely.
- Throw away mouldy porous items you can't clean, such as fabric blinds or cardboard.
- Dry the area well, then fix the humidity so it does not return.
Safety: Health Canada suggests hiring a professional if there is a large amount of mould or if it keeps coming back. Mould on drywall, inside walls, or with a musty smell and no visible cause can mean a hidden leak. Get it checked.
Renting? Who deals with mould in BC
The Residential Tenancy Branch lists "dealing with minor mold issues on windowsills, tubs and showers" as a tenant responsibility. Landlords are responsible for most repairs, and the Branch says landlords should fix water damage quickly to avoid more serious problems like mould growth.
- Wipe sills, run fans, and keep humidity down. That part is on you.
- If fans don't work, windows leak, or mould keeps spreading, ask the landlord for repairs in writing. Email, a letter or a text is fine. Keep a copy.
- If repairs are not done in a reasonable time, you can apply for dispute resolution with the Residential Tenancy Branch.
Frequently Asked Questions
Is some condensation on windows normal in winter?
A little on the bottom edge of the glass on very cold mornings is common. Water that runs onto the sill, collects every day, or causes black spots means the air is too humid or the window is too cold. Check your humidity.
Should I open a window to stop condensation?
Health Canada suggests opening windows when weather permits to keep your home ventilated. In cold, wet weather, running exhaust fans and an HRV is usually a better everyday fix.
Can I use bleach on window mould?
Health Canada says there is no need for bleach. Use water and dish detergent, wear gloves, goggles and an N95 mask, and dry the area afterwards.
Why is there water between my double-pane windows?
The seal around the glass has failed. Natural Resources Canada says the insulating glass unit should be replaced. Wiping and dehumidifying will not fix it.
My landlord says the mould is my fault. What can I do?
Minor mould on sills, tubs and showers is a tenant job under Residential Tenancy Branch guidance. Leaks, water damage and broken fans are the landlord's to fix. Ask in writing, keep records, and apply for dispute resolution if repairs are not made.
Official sources
- Guide to addressing moisture and mould indoors — Health Canada
- Mould — Health Canada
- Condensation — Natural Resources Canada
- Indoor air quality: Mould and other biological contaminants — HealthLink BC
- Repairs and maintenance — Residential Tenancy Branch, Province of B.C.
- Window and door rebates — BC Hydro
General information only, not professional advice. Rules and prices change, so check the official sources above before you act. Spotted something out of date? Tell us.