Water On The Windows Every Morning: What It Does To The Walls

Where the damage actually happens

Water on glass looks bad and does nothing. Water that runs down and pools on the sill is the problem, because it soaks into the frame joint, the sealant, and the wall directly underneath, and it does it every night for months.

That is why the first sign is rarely the window. It is black spotting in the corner of the frame, a soft patch of paint on the wall under the sill, or curtains that have greyed along the bottom edge.

Where the moisture comes from

• Showers, which is why a bathroom with a weak fan pushes humidity into the whole apartment.
• Cooking, particularly anything boiling without a lid.
• Drying laundry indoors, which is the single largest source in most apartments and the most common.
• Breathing, genuinely: two people asleep in a closed bedroom produce a noticeable amount overnight.
• Unvented dryers, and any humidifier running unattended.

Drying laundry on a rack in a closed room is the one to change first. It puts litres of water into the air over a night, and it lands on the coldest surface in the apartment, which is the window.

What actually works

• Wipe the glass and the sill every morning, which takes a minute and prevents almost all of the damage.
• Ventilate briefly and fully rather than leaving a window ajar all day: wide open for five to ten minutes clears the moist air without cooling the walls.
• Run the bathroom fan long after showers, and the extractor while cooking.
• Keep the bedroom door open at night if the room runs wet, or leave a window on its vent position.
• Move furniture a few centimetres off exterior walls, so air can pass behind it.
• A dehumidifier in the worst room, which is the direct solution when the others are not enough.

Do not seal the room tighter

The instinct when a room is damp is to close it up, which does the opposite of helping. Moisture has to leave somewhere, and a sealed room simply moves the condensation to whatever surface is coldest, usually behind furniture where nobody sees it until there is mould.

The same applies to heating a room hard with no ventilation. Warm air holds more moisture, so it feels better and the water still arrives on the glass at night.

Cleaning what has already grown

Black spotting on frames and sealant is surface mould at first and it comes off. Wash the area, dry it fully, and keep it dry. Silicone that has gone black through its thickness is a replacement rather than a cleaning job.

Do not paint over a damp patch on the wall, and do not simply wipe it and consider it handled if it returns within days. A mark that comes back is telling you the water is still arriving.

Where it stops being yours

Single glazing, failed double-glazed units with haze between the panes, windows that no longer close properly, missing trickle vents, a bathroom fan that does not move air, and cold patches on a wall that are colder than the rest of it are all building items.

Report them with the pattern rather than the complaint: water on the glass every morning, the wall under the sill going soft, the fan not holding a tissue. Named symptoms produce an inspection, and the word damp on its own usually produces advice about opening windows.

The humidity side of a bathroom is covered in the guide to a bathroom with no window, and closets on exterior walls, which fail the same way for the same reason, are in the musty closet guide.