condensation on windows

Window condensation: physics, not fate

Why glass runs with water, how to use dew point, and what changes between single glazing, double glazing and a cold reveal.

Updated 2026-09-01 · 9 min read

Water on the glass in the morning is the most read symptom in this niche and the most misread. It is not “damp coming through the walls” by default. It is indoor air meeting a pane colder than its dew point.

The physics in one sentence

Warmer air can hold more vapour. Cool it, and relative humidity rises. At 100% on a surface, water condenses. Dew point is that temperature.

Calculate dew point

Where it appears, and why that matters

A quick reading of location

Place	Typical reading	First move
Single glazing, bottom edge only	Cold pane, RH maybe acceptable	Air the room; do not blanket the radiator
Glass and sill every morning	High indoor vapour or a very cold night setback	Measure RH; laundry and showers
Reveal, shutter box, mouldy corner	Thermal bridge + RH	Pull furniture forward; consider local insulation
Summer condensation on cold pipes	Chilled water or a cellar	Insulate the pipe; this is not “window mould”

Habits that load the glass

A drying rack in the living room with the windows shut.
Pans without lids and the hood off.
A long shower with no extractor, bathroom door open onto the hall.
Heavy curtains closed over the radiator under the window: warm air never washes the pane.

Airing in winter without dumping all the heat

Short, wide-open bursts beat a hopper left ajar for hours, which chills surfaces without really replacing the air. See the winter ventilation guide.

FAQ

Does double glazing stop condensation?+

It makes glass condensation less likely because the inner pane is warmer. It can move the water onto a cold reveal or shutter box if indoor RH stays high.

Should I turn the heating off overnight to save money?+

A sharp overnight drop raises relative humidity and cools the glass: morning condensation often starts there. This is physics, not a tariff tip.

What may help (after diagnosis)

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10–25 €

Digital hygrometer (probe if you can)

Without a relative-humidity number, every tip is guesswork. A €10–25 hygrometer is the first useful purchase — before any dehumidifier.

When: Always, as the first diagnostic step.

Not when: It does not replace a moisture meter or a rising-damp survey.

Affiliate link not connected — a category, not a model review.

40–180 €

Bathroom extractor with humidistat

In a windowless bathroom this often beats a portable dehumidifier: it removes vapour at the source.

When: Bathroom mould, daily showers, poor air change.

Not when: It will not fix a thermal bridge in a bedroom.