Why it forms
Everyday life puts moisture into the air. Cooking, showering, drying washing and simply breathing all raise the humidity in a room, and warm air holds more water than cold air. When that air touches a colder surface it can no longer hold the moisture, and it releases it as the film you see on the glass.
So condensation is not, in itself, a fault. It is a signal about the balance between moisture, warmth and ventilation in the room.
Which side of the glass is telling you what
Condensation on the inside of the glass points to too much moisture and too little ventilation in the room. Condensation on the outside, on a cold morning, is the opposite: it means the glass is staying cold because the window is insulating well and keeping the room's heat inside, which is a good sign rather than a bad one.
Condensation between the two panes is different again. That means the sealed unit has failed and moisture has found its way inside it, and it is the one type a working double-glazed sash should never show.
Where the window helps
A warm double-glazed sash keeps the inner pane closer to room temperature, so there is less of a cold surface for moisture to gather on than there is on old single glazing. That reduces how readily condensation forms in the first place.
Just as important is ventilation. Concealed trickle vents are fitted to meet Part F of the Building Regulations, letting a room change its air slowly without opening a window to the cold. Moving that damp air out gently is what keeps the glass clear, and both sashes tilt inward so wiping down a sill or clearing a misted morning is done from inside the room.
A window cannot dry a room on its own. It gives you a warmer surface and a controlled way to let moisture out, and the rest is how the room is used and heated.
What you can change in the room
Cover pans while cooking, run an extractor in the kitchen and bathroom, avoid drying washing on radiators, and heat the house steadily rather than in short cold-to-hot bursts. Leaving the trickle vents open in the worst months does more than people expect.
If condensation is sitting between the panes rather than on the surface, that is a failed unit and the window needs attention. Send us a photograph and we will tell you what you are looking at.
Questions we get asked
Why do my sash windows get condensation?
Warm, moist air in the room meets the colder surface of the glass and releases its moisture. It is usually a sign of high humidity and low ventilation rather than a faulty window.
Is condensation a sign my window is faulty?
Not on the surface. Condensation on the inside points to room humidity, and on the outside it means the window is insulating well. Only condensation trapped between the panes is a fault, and that is a failed sealed unit.
How does ventilation stop condensation?
It lets the damp air leave the room slowly and steadily. Concealed trickle vents fitted to meet Part F do this without opening a window, which is what keeps the glass clear.
What is condensation between the panes?
It means the sealed double-glazed unit has failed and moisture has entered it. Wiping the glass will not clear it because the moisture is sealed inside, so the unit needs replacing.
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Talk to us about your windows
Send a photograph of the front of the property and roughly how many windows are involved, or your schedule of sizes for a supply-only order, and we will tell you what is realistic before anyone comes out.
Call 0800 861 1450, message us on WhatsApp, or email info@sashwindowfactory.co.uk.