How to Clean Mould Off Windows (and Stop It Coming Back in Rain-City Vancouver)

Mould needs three things: a spore, something to eat, and water. Spores are already in every home on earth, and the dust and skin cells that settle in a window track are food. The only variable you control is the water, and on a window that water is almost never a leak. It is condensation.

Here is the mechanism. Warm indoor air holds a lot of moisture. Your window glass is the coldest surface in the room, because it is a thin sheet separating you from the outdoors. When that warm, moist air touches cold glass, the air right at the surface cools past its dew point and gives up its water as droplets. Those droplets run down the pane and pool exactly where the glass meets the frame, which is why the black growth starts in the bottom corners and works outward rather than appearing in the middle of the pane.

Metro Vancouver makes this worse than most of Canada, and not for the reason people assume. Vancouver International Airport records precipitation on well over 160 days a year under Environment and Climate Change Canada's 1991 to 2020 climate normals. Our winters are wet rather than deeply cold, so the outdoor air stays close to saturated for months and homes get sealed up and heated without the hard freeze that would dry the air out. Then there is the housing stock. Plenty of the older inventory in North Vancouver and on the east side still runs single glazing or 1970s aluminum frames that conduct cold straight through, and the result is glass that sits below the room's dew point every night from October to March.

This is why replacing the cleaner never fixes it. You are not fighting dirt. You are fighting a surface that is wet for eight hours a night, every night, for half the year.

The first thing most people do when they see something black on a windowsill is search whether it is toxic black mould. It is a reasonable fear and a poor question. Health Canada's household guidance is explicit that its recommendations apply regardless of the species growing in the building, and that in most cases there is no need to measure concentrations in the air or identify which mould you have. The level of risk depends on how much growth there is and how long it has been damp.

Health Canada also does not recommend air testing for mould in most situations, on the grounds that an air test does not tell you anything useful about health and does not address the cause. Both statements point the same direction: skip the identification step, find the moisture, remove the growth.

That does not mean shrug it off. Damp indoor environments are associated with respiratory symptoms, and anyone in the home with asthma or an existing allergy will notice a mouldy room before the rest of the household does. The practical read is somewhere between panic and denial. A palm-sized patch on a vinyl sill is an ordinary Saturday-morning job. Growth that returns within two weeks of a proper clean, or that has crept past the frame into the drywall, is telling you the moisture source is still running.

Most of the widely circulated advice on cleaning mould off windows opens with a bleach dilution. It is confident, it is specific, and for a Canadian reader it contradicts the most authoritative guidance available. Health Canada's guide to addressing moisture and mould indoors says it twice, once in the section on washable surfaces and once in the general prevention checklist: do not use bleach to clean up mould. What it recommends instead is scrubbing with an unscented soap or dishwashing detergent solution, then drying the surface completely and quickly.

The reasoning becomes obvious once you look at what household bleach actually is. It is a small amount of sodium hypochlorite dissolved in a great deal of water, which makes it mostly water. On glass, that is fine, because the growth is sitting on top of a surface it cannot penetrate. On a bare wood sill or a painted reveal, the chlorine reacts and spends itself at the surface while the water carries on soaking into the material, straight down to the part of the growth that is actually anchored there. You bleach the colour out, the patch looks clean, and three weeks later it is back looking exactly the same, because you fed it.

There is a second problem that matters more in a small condo than a house. Bleach is a respiratory irritant, and window cleaning tends to happen in the smallest, most closed-up room. Mixing it with anything ammonia-based, which includes a lot of glass sprays, produces chloramine gas. Detergent has none of these failure modes and does the job that actually matters, which is lifting the growth off the surface mechanically rather than trying to sterilize it in place.

For completeness, the other popular internet answer is a baking soda paste. It works reasonably well as a mild abrasive on non-porous surfaces, for much the same reason detergent does, and we have written about where baking soda genuinely helps and where it gets oversold. The short version is that any of these approaches beats bleach on a window, because none of them depends on soaking the surface.

Look at a window from the inside and count the materials. There is glass. There is a frame, in vinyl, aluminum or wood. There is a sill, which may be vinyl-wrapped, painted MDF or bare timber. There is a track full of grit. There is a bead of silicone or a rubber gasket sealing the glass to the frame. And there is the reveal, the painted plaster or drywall returning into the room. Six materials, but only three ways of behaving once mould lands on them, and Health Canada draws the first line: materials that can be properly cleaned are the non-porous ones, and it lists glass, metals, plastics and vinyl by name.

Everything on the non-porous side of that line is genuinely fixable with a cloth and detergent. Everything on the porous side is a different conversation, because the growth has anchored into the material rather than sitting on it. You can remove what is on the surface, which is worth doing, but if the staining stays after the mould is gone, that is pigment in the grain and no amount of scrubbing will pull it out.

Silicone is its own category. It is flexible, slightly tacky, and mould gets into the micro-texture and into the seam where the bead meets the glass. Surface cleaning improves it. If the bead has gone properly black through its thickness, the honest answer is that it needs cutting out and re-running rather than cleaning, which is a tube of sealant and an afternoon.

The silicone bead is where most people give up. It is the last centimetre of the job, it is soft, and the growth sits in the seam rather than on the face. Detergent and a soft brush worked along the bead, left to dwell for a few minutes and then rinsed and dried, will lift most of it. What will not work is a scraper or a stiff wire brush, because scoring the silicone gives the next generation of growth more texture to hold onto, and a nicked bead lets water behind the seal.

The reveal is the other one. That painted return between the frame and the room catches the water that runs off the pane, and because it is usually finished in a flat matte paint rather than a wipeable satin, it drinks it. This is the surface people scrub hardest and get the least out of. If your reveals go patchy every winter, the durable answer is not a better cleaner, it is a coat of a washable satin or semi-gloss finish, which turns a porous surface into a wipeable one.

Both of these are worth a moment of honesty about what a clean can achieve. We do a lot of windows across Metro Vancouver, and we tell every client the same thing. We can take the growth off. We can leave the glass, frames, tracks and sills genuinely clean. But if the silicone has gone black right through its thickness, no cleaner on the market is going to make it white again.

Everything above removes the growth. None of it changes the reason the growth appeared. Health Canada's answer to that is a number: keep the home's relative humidity between 30 and 50 percent, and run a dehumidifier whenever moisture is condensing on cold surfaces such as window panes or the reading goes above 50 percent. A basic hygrometer is a cheap hardware-store item and turns this from a guess into a measurement, which is worth doing before you buy a dehumidifier you may not need.

The prevention list Health Canada gives specifically for condensation on windows, frames and sills is short and unusually concrete, and one item on it surprises nearly everyone. Keep window coverings open so warm air can reach the glass, because heavy curtains and blinds trap the cold and moisture against the window. If you have ever pulled back a drape in a spare room in February and found a black frame behind it, that is the mechanism, and it also explains why the mould is always worst in the room nobody uses.

The rest of the list is about where the moisture comes from before it ever reaches the glass. A long shower puts a remarkable amount of water into the air, as does a pot boiling without the range hood on, as does a rack of laundry drying in the living room, which is a very Vancouver habit in a rainy month. Getting that moisture out at the source is far easier than pulling it back out of the air later.

The province publishes a policy guideline that divides up maintenance and cleaning between landlord and tenant, and it has a section devoted to windows. Residential Tenancy Policy Guideline 1 states that the tenant is responsible for cleaning the inside windows and tracks during and at the end of the tenancy, and it names removing mould specifically. The same section makes the landlord responsible for cleaning the outside of the windows at reasonable intervals, and for providing clean windows in a reasonable state of repair at the start of the tenancy.

There is a line worth knowing on the other side of the ledger too. In the section on window coverings, the guideline says the tenant is not responsible for water stains due to inadequate windows. That is the distinction the whole argument usually turns on. Cleaning the growth off the sill is a tenant job. A window unit that has failed, a seal that has gone, or a building envelope that is letting water in is a repair issue, and it belongs to the landlord. If you are renting and the mould reappears within weeks of a proper clean in a home you are ventilating sensibly, that is worth reporting to your landlord in writing, and the Residential Tenancy Branch is the place to take any question about who is responsible for what.

The practical consequence at the end of a tenancy is simple enough. Window tracks and sills are one of the most commonly failed items on a move-out inspection, partly because they are easy to overlook and partly because a whole winter of condensation is sitting in them. If you would rather not gamble a damage deposit on it, they are included as standard in our move-out cleaning scope. For anything beyond cleaning, the Residential Tenancy Branch is the right place to take the question, not a cleaning company.

Health Canada sorts mould by area. Three or fewer patches totalling one square metre or less counts as small, and that is a job you can do yourself with gloves, a mask and reasonable care. More than three patches, each under a square metre but adding up to less than three square metres, is medium: an expert assessment is recommended, though in most cases it can still be cleaned up with proper procedure. A single patch larger than three square metres is large, and Health Canada says a qualified professional should assess and clean it. The US EPA draws a comparable line at about ten square feet, roughly a three-foot by three-foot patch.

For a window, almost everything you can see falls on the small side of that line. The question is what you cannot see. Four signs say the water is inside the assembly rather than sitting on it. Growth pushing through the reveal into the wall. Drywall that feels soft under a thumb. A musty smell with no visible source. And the same patch back within two weeks of a thorough clean in a home whose humidity you already have under control. Any one of those and you are treating a symptom, not the cause. That is a building problem, and it needs an assessment rather than a cloth. Our wider guide to mould and mildew around the home covers how to tell the difference room by room.

The middle ground is where most people actually sit: the growth is surface-level, it is not structural, and there is simply more window than anyone wants to spend a Saturday on. That is ordinary work for us. A window cleaning visit takes the glass, frames, tracks, sills and seals as one job rather than the glass alone, which matters here because the corners and channels are exactly where the growth lives. If the damp season has left its mark beyond the windows, a deep clean is the visit that gets into the bathroom grout, the shower doors and their tracks, the vents and the accessible interior glass, which are the other places a wet Vancouver winter shows up first.