Double-glazing condensation · The GGF, EPA and Energy Saving Trust guides read on 7 November 2026

Where does the condensation sit on the glass? Find what it means

Three very different phenomena share one name. On the room-side face, water shows that moist air met a surface cooled below its dew point. Between the panes, water points at the sealed unit itself. On the outer face, water is often proof the glass is doing its job well. Choose the place, enter the room’s temperature and humidity, and the diagnosis follows the guides’ physics and practical steps.

A double-glazed unit in cross-section, droplets in the three possible places. The chosen place lights up; the explanation changes completely from place to place.

What the chosen place means

Example: droplets indoors, room at 21 °C and 60% humidity

Water from the room air

On the room-side face, the water is moist air meeting glass cooled below its dew point.

Approximate dew point of the described air: 12.9 °C.

Any surface in the room cooled below that temperature gathers water, and the glass is usually the coldest. At 60% you sit above the band the EPA recommends for homes, below 60% and ideally between 30 and 50%: short, frequent airing lowers humidity without chilling the walls.

The droplets’ place states the cause, before any formula#

The finger test separates the worlds. If you can wipe the water from the room side, the phenomenon plays out between your air and the glass temperature. The British Glass and Glazing Federation’s guide states it plainly: condensation on the room-side surface means the surface temperature is too low for the water-vapour content of the room’s air. It happens on single glazing, and also on double or triple units in unheated rooms.

If no finger reaches the water on either face, the water sits in the cavity between the panes. There, a hermetically sealed double unit has no business holding moisture: the guide calls it a failure of the seal. Room vapour cannot cross an intact sheet of glass, which is why a dehumidifier cannot fix this case. The unit is inspected and, usually, the glass unit is replaced while the frame stays. Do not confuse it with the light, passing misting of secondary glazing added over an old window: that cannot be hermetically sealed, and some migration of moist air between the leaves is to be expected.

If the water sits on the outer face, usually in the morning and only on some mornings, the window earns a compliment. With modern coated glass, the house’s heat reaches the outer pane so weakly that it stays cold and can fall below the outdoor dew point in clear, cold conditions, typically in spring and autumn. The guide calls the phenomenon strong evidence that heat is not escaping from the house through the window.

The dew point: your air’s temperature threshold#

Warm air carries plenty of invisible water; cooled, its capacity shrinks. The temperature at which the air becomes full is called the dew point: below it, the surplus vapour leaves the air as liquid water on the first cold surface it meets. The calculator uses the Magnus approximation over water, anchored on hard-to-move checkpoints: at 20 °C and 50% relative humidity, the dew point is 9.3 °C; at 20 °C and 60% it is 12.0 °C; at 21 °C and 60% it is 12.9 °C; at 22 °C and 55% it is 12.5 °C. The formula tracks published tables across the household ranges, without laboratory pretensions.

Two readings fall out of the numbers. First, humidity moves the threshold hard: at the same room temperature, going from 50 to 60% lifts the dew point by nearly three degrees, so far warmer surfaces start gathering water. Second, the glass temperature is neither the outdoor temperature nor the room’s: the inner pane sits somewhere between, colder towards the lower edge and the corners, where the spacer and frame pull the cold in. That is why indoor condensation almost always begins as a band at the foot of the glass, and why the honest comparison asks for a thermometer stuck to the pane a palm’s width from the edge, on the morning the water appears.

The steps, in the order the guides give them#

For indoor condensation, the US Environmental Protection Agency’s palette is short: keep humidity below 60%, ideally between 30 and 50%; dry wet surfaces quickly and cut the moisture source; increase ventilation and air movement; insulate cold surfaces; raise the air temperature. The Energy Saving Trust, the UK’s energy-efficiency body, adds the domestic arithmetic: a household produces up to around a hundred litres of moisture a week through showers, cooking, laundry and the occupants’ breath; the cooker hood and bathroom fan deserve running at least 15 minutes after the fact, laundry dried outdoors while the weather allows, trickle vents kept open and clear.

The GGF insists on ventilation without offending draughts: briefly opening a window changes the air, and the cool dry air that enters costs little to reheat beside the warm moist air it replaces. The same guide places radiators under windows precisely to keep the inner pane warm, and recommends, in cold weather, some form of permanent background heating over rare strong blasts. The practical order in a Romanian flat stays: measure and see your band; remove moisture at source (hood, fan, lids, laundry outside); air briefly and often; let heat reach the glass; only then consider the dehumidifier.

What a window cannot diagnose, and when the problem is another#

Repeated indoor condensation alone does not prove leaks, rising damp or a wall defect. It can mean only a winter of laundry dried on radiators. Conversely, a home’s serious damp has other signs: stains climbing from the floor, mould in corners far from windows, walls damp to the touch on dry days too. The diagnosis here is a first triage, and window and building firms hold their own interest in the outcome; the glazing unit’s warranty and its written inspection outweigh an opinion by phone.

Measurement limits remain: cheap hygrometers can err by a few percent, a sensor two metres from the glass reads different air from the layer hugging the pane, and behind a heavy curtain a pocket of near-outdoor-temperature air forms. That is exactly why the page shows an “approximate” dew point and compares with margins, not tenths of a degree. If the water appears only in winter, indoors, at the foot of the pane, and clears after airing, you are in the ordinary story of modern well-sealed flats, where the new windows cut the old ones’ accidental ventilation.

Frequently asked questions#

The new window mists indoors worse than the old one. Is it defective?

Usually it shows airtightness rather than a defect: old windows let moist air escape through their leaks, new ones keep it in the house. If the water sits on the room-side face, the story belongs to humidity and glass temperature; the dew point computed above frames it. A glazing defect reads elsewhere, between the panes.

How can I tell whether the water is between the panes or just on the glass?

Wipe both accessible faces with a dry cloth. If the mist stays identical, viewed from both sides, it sits in the cavity between the panes. In a hermetically sealed unit that means a lost seal and the road to warranty or replacing the glass unit; in added secondary glazing, air migration between the leaves is to be expected.

In the morning my glass is wet outside, the neighbour’s is dry. What does that mean?

It may point to better insulation, but orientation, shelter and wind matter too. Modern coated glass lets little heat reach the outer pane, which stays cold and can gather the morning dew, while weaker glazing warms its outer pane with heat escaping from the house. The GGF guide calls external condensation evidence that heat is not escaping through the window. Two neighbours can also differ in the facade’s orientation, the window’s shelter, wind or surrounding vegetation, so one dry pane does not make yours the winner or theirs the loser.

At 21 °C and 60% indoors, how much should water on the glass worry me?

The dew point of that air is about 12.9 °C, so an inner pane below that threshold will gather water, normally towards the lower edge in winter. The EPA recommends humidity below 60%, ideally between 30 and 50%: at 50% and the same temperature the threshold drops by nearly three degrees and many wet mornings disappear. Measure the glass before buying appliances.

Does a dehumidifier help?

It helps in the indoor case, because it lowers humidity and with it the dew point. It does nothing for water between the panes, where the problem belongs to the glazing unit, and it has no role for outdoor water, which is a good sign. Before it, short frequent airing, the hood and the fan after showers, and laundry dried out of the flat are worth trying.

Why does the water appear first at the foot of the glass?

Because the glass is coldest there: the spacer between the panes and the frame pull the cold towards the lower edge, and cooled air slides down and settles at the base. The pane’s centre can sit above the dew point while the first centimetres beside the frame are below it; that is why the good measurement is taken near the lower edge.

Sources and how they were checked#

The three diagnoses follow the GGF guide read on 7 November 2026: condensation on the room-side surface means a surface too cold for the room’s vapour; within the cavity of a hermetically sealed unit it denotes a failure of the seal; on the outside, with modern coated glass, it is evidence that heat is not escaping through the window. The humidity band (below 60%, ideally 30–50%) and the preventive actions are the EPA’s, read the same day. The dew point is computed with the Magnus approximation over water (coefficients 17.625 and 243.04 from the literature, checked against published tables: 20 °C at 50% gives 9.3 °C; 21 °C at 60% gives 12.9 °C) and remains an approximation. The 1.5 °C margin above the dew point, at which the calculator says the glass sits “at the edge”, is an editorial heuristic, not an EPA or other standard threshold; the glass temperature must be measured, not inferred from the weather outside.