Condensation and mould on window sills once the heat goes on

The furnace comes on, the nights drop toward freezing, and one morning there is water on the bottom of the glass and a grey line along the sill. If that line is mould, cleaning alone will not keep it away while the water keeps arriving. This post covers what the water is telling you, the humidity your windows can actually handle in a GTA winter, and how to deal with the sill itself.

Why does water collect on window sills once the heat is on?

Warm indoor air carries water vapour, and when it touches a surface colder than its dew point, the vapour turns into liquid. Natural Resources Canada uses window condensation as its standard example of this, and adds that on a very cold window the water appears as frost instead.

The glass is usually the coldest surface in a heated room, and the bottom edge of it is colder still, so that is where the drops form first. They run down by gravity and sit on the sill. NRCan's window chapter also points at the old aluminum-box spacers between the panes of many sealed units, which lose heat and add to condensation along the edges. If the water lines up along the lower edge of the glass, that is the usual reason.

Before going further, check which surface the water is on. Fog between the panes of a sealed unit is a failed seal, and fog on a storm window is warm air leaking past the inner sash. Both are covered in our post on taking window screens off before winter. Everything below is about water on the room side of the glass.

How much humidity can your windows take on a cold night?

Less than you might expect, and the limit falls as the outdoor temperature falls. Health Canada recommends keeping indoor relative humidity between 30 and 50 per cent. NRCan publishes a table of the highest indoor humidity, at 21 °C inside, at which a window stays dry. Here are its rows for the temperatures a GTA winter actually reaches, with the window types as NRCan names them.

Now put that next to the local weather. Environment and Climate Change Canada's 1991 to 2020 normals for Toronto Pearson give an average overnight low of 0.2 °C in November, -5.0 °C in December and -8.9 °C in January, and an average of 13.4 January nights that drop below -10 °C. On one of those nights a plain double-glazed window starts to condense at 44 per cent. So a house kept at 50 per cent, the top of the Health Canada range, will have wet sills in January even if it was dry in October.

The practical rule follows from the table: keep the humidity near 30 to 40 per cent once the nights go below freezing, and lower it further if the windows are older. NRCan's own summary is lower humidity in winter and higher in summer.

Where the extra moisture comes from

NRCan estimates that a family of four adds about 50 litres of water a week to the air of a house through ordinary living. The biggest share, 38 litres, is breathing and skin. Cooking three meals a day adds 6.3 litres and showers add 2.4. In a tight, well-sealed house much of that stays inside, which is why NRCan warns that new windows can make condensation worse: the old leaky ones were letting moist air out.

If you sealed draughts this autumn, expect the humidity to rise, as covered in caulking and weatherstripping before the heat goes on. These are the sources worth checking first, taken from Health Canada's and NRCan's lists:

• If a humidifier is running, unplug it. Health Canada's checklist for condensation on windows and sills says to unplug and remove humidifiers, and one built into the furnace can be turned down or off while you watch the sills for a week.
• Dry laundry in a dryer that vents outdoors rather than on racks inside, because Health Canada notes that hanging wet laundry indoors raises the humidity. Keep the dryer's outside vent clear of snow and leaves.
• Run the bathroom exhaust fan during every shower and for at least 30 minutes afterward, and check that it vents outside and not into the attic.
• Use the range hood whenever you cook, preferably on its highest setting, and cook on the back burners when you can.
• Move firewood out to the garage or shed. NRCan also lists houseplants and aquariums among household moisture sources, which matters in a room full of plants.

Getting warm air to the glass

A window covered by a heavy curtain or a closed blind sits colder than one open to the room. Health Canada's window checklist says to keep coverings open so warm air reaches the glass, to keep baseboard heaters and heating vents clear of furniture, and to leave interior doors open so air can move. NRCan makes the same point: curtains and blinds are not airtight, so moist room air still gets behind them to a window that the fabric is keeping cold.

In a bedroom the door is often shut at night, the people sleeping there are breathing into a small room, and the blind is down. Opening the blind in the morning and wiping the glass for a few weeks will tell you whether air movement alone fixes that room.

How to clean mould off a window sill

You can do this yourself, and for a sill you should. Health Canada names window sills as a washable surface and says small areas of mould can be cleaned with proper precautions. Mould begins to grow within 48 hours where moisture sits, so deal with it the week you see it.

1. 1Put on glasses, gloves and an N95Health Canada's minimum gear is safety glasses or goggles, household disposable gloves and a well-fitting disposable N95 respirator. A plain dust mask does not count. Keep children, anyone with asthma and pets out of the room while you work.
2. 2Wipe up the standing water firstSoak up what is on the sill and in the corners of the frame with a cloth or paper towel. Cleaning a wet sill just moves the mould around.
3. 3Scrub with unscented soap and waterUse a cloth and an unscented soap solution, such as a few drops of unscented dish detergent in warm water. Get into the corners where the sill meets the frame and the track, where the line of growth usually sits.
4. 4Dry it completely and quicklyHealth Canada's wording is to dry completely and quickly. Use a dry cloth, then leave the blind up and the air moving. A fan pointed at the window helps.
5. 5Bag the cloths and wash your handsPut the used cloths and towels in a plastic bag and take them straight outside.

When the mould is more than a cleaning job

Health Canada sizes a mould problem by area, and the size decides who should deal with it.

Size is one trigger. The other is mould that comes back after you cleaned it, which Health Canada treats as a sign the moisture source has not been removed and a reason to bring in a professional. Many small patches throughout the house can also point to a bigger moisture problem. Health Canada does not recommend testing the air for mould in most situations, so a quote that starts with air sampling is worth questioning.

Water that stains the wall below the window when it rains, with no fog on the glass, points to a leak in the window or the wall. Health Canada's first step is to repair the building envelope before cleaning anything. If you rent, Health Canada's advice is to speak to the property owner about any moisture or mould problem.

A two-week routine once the nights go below freezing

1. Put a hygrometer in the worst room and read it each morning.
2. Turn the furnace humidifier down or off, and stop drying laundry on racks indoors.
3. Run the bathroom fan for 30 minutes after every shower and the range hood every time you cook.
4. Open blinds and curtains in the morning, and move furniture off the heating vents.
5. Wipe the glass and the sills dry each morning, as Health Canada's checklist says, until they stay dry on their own.
6. Clean any mould that was already there, using the steps above.
7. If the sills are still wet with the humidity near 30 per cent, look at ventilation or the windows themselves, and see NRCan's advice on increasing ventilation in its Keeping the Heat In guide.

Some water on the bathroom and kitchen windows after a shower or a pot of soup is normal, and NRCan says occasional condensation is not a problem. The sign to act on is water that is there every morning on windows in rooms where nobody cooks or bathes. The vents you keep clear for the glass are the same ones covered in cleaning before the heating season.