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What to check, and in what order

There is condensation on my windows

Which surface it is on decides everything. On the INSIDE it is room air meeting a cold pane — a moisture and ventilation question. On the OUTSIDE it means the glazing is performing well. BETWEEN the panes it is a failed seal, and that is the only one of the three that is a fault in the window itself.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

Condensation forms when air meets a surface colder than its dew point. On a window that can happen in three places, and they are three unrelated situations that people routinely treat as one.

INSIDE the glass is by far the most common and is almost never a window fault. Warm, moist room air meets the coldest surface in the room, which is the glazing, and the moisture comes out of it. The window is simply where it shows first — which makes it a useful indicator of the room's humidity rather than a defect.

OUTSIDE the glass, on the outer face, looks alarming and means the opposite: the outer pane is cold because very little heat is escaping through the unit to warm it. It happens on clear, still nights with modern glazing, and it is evidence that the unit is working.

BETWEEN the panes is the one real fault. A sealed unit contains a dried cavity, and if the perimeter seal fails, moist air enters and condenses on the inner faces where nothing can wipe it. That cannot be cleaned or dried and it does not recover — the unit is replaced.

So the first action is not to buy anything. It is to work out which surface the water is on, which takes a fingertip.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. Too much moisture in the room air

    Most common

    On the INSIDE face, wipeable, worst in the morning, in kitchens and bathrooms, and in rooms that have been closed up. Often with mould at the frame or in the corners of the reveal.

    How to check: Wipe it. If it comes away on the cloth, it is on the inside and this is the first family to consider.

  2. Too little ventilation to carry the moisture away

    Most common

    Inside face, and strongly linked to habits — trickle vents closed, an extractor that is not used or not ducted outside, drying washing indoors.

    How to check: Look at the trickle vents and run the extractor with a sheet of paper against it. A fan that will not hold paper is moving very little air.

  3. A cold spot — thermal bridging at the frame or reveal

    Common

    Inside face but concentrated at the edges, the frame, or the reveal rather than across the glass, often with mould following that line.

    How to check: Whether the water and any mould trace the frame rather than covering the pane. A pattern that follows structure is following the cold.

  4. A failed sealed unit

    Common

    BETWEEN the panes. Cannot be wiped from either side, often with a misty bloom or visible tide marks that come and go with the sun.

    How to check: Wipe both faces. Moisture that survives both is inside the unit, and that is conclusive.

  5. Normal external condensation

    Common

    On the OUTSIDE face, on clear still mornings, clearing as the sun rises — and typically on the newest, best-performing windows in the house.

    How to check: Which side wipes clear. External condensation is not a fault and needs no action.

  6. A new source of moisture in the building

    Less common

    A sudden change rather than a seasonal one — appearing after building work, a new plaster finish, a leak, or a change in how the house is used.

    How to check: Whether anything changed around the time it started. New plaster and screed release a great deal of water for weeks.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the entire diagnosis for two of the three situations and it takes seconds. Moisture surviving both wipes is between the panes, which is the only genuine window fault here.

  2. 2

    Mornings in bedrooms and bathrooms points at overnight moisture and ventilation; a single cold room points at a heating or thermal-bridge issue; everywhere at once points at the whole building's humidity.

  3. 3

    These are the two provisions specifically intended to carry moisture out, and both are commonly closed, blocked, or ducted into a loft rather than outside.

  4. 4

    It separates a general humidity problem from a local cold spot, which have different remedies — one is about the air, the other about the construction.

  5. 5

    New plaster, a new screed, a new occupant, a leak or a change of heating pattern all shift the moisture balance, and condensation that started abruptly usually has an abrupt cause.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Vapor Barrier Dew-Point Condensation Risk Calculator →CFM Ventilation Calculator →Window Wall Area-Weighted U-Factor Calculator →

When to stop and get somebody in

  • Where moisture is between the panes — the unit has failed and cannot be dried or cleaned, so the question is replacement rather than remedy.
  • Where mould keeps returning over a substantial area after cleaning and improved ventilation, particularly where anyone in the building has a respiratory condition.
  • Where condensation and damp appear together at the same place low on a wall, which suggests water arriving from somewhere rather than from the air.
  • Where an extractor cannot be shown to discharge outside — ducting into a loft moves the moisture rather than removing it, and causes a second problem above the ceiling.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.