Condensation feels arbitrary until you see what it is actually comparing: the temperature of your glass against the dew point of your air. Two houses can differ enormously in both while looking identical from the street.
Four things account for nearly all of it, and they are worth knowing in order, because only some of them are worth spending money on.
1. How cold the glass gets
This is the largest variable. Single glazing on a freezing night can sit close to outdoor temperature. Modern double glazing with a low-emissivity coating and a warm-edge spacer might be twenty degrees warmer at the same moment, and triple glazing warmer still.
The frame matters as much as the pane. An aluminium frame without a thermal break conducts cold straight through and is often the first thing in the room to run with water, even when the glass itself is fine. This is why condensation so often appears at the very bottom edge of a unit — the spacer and frame are the coldest part of the assembly.
2. How much moisture the household produces
Two adults and no laundry is a different house from four people, a vented-indoors dryer, long showers and pans without lids. A family of four generates two to three gallons of water vapor a day; the avoidable share of that is large.
This is the variable most people can change fastest, and it is free. It is also the one that explains the classic case of two identical flats where only one has streaming windows.
3. How airtight the house is
Older, draftier houses ventilate themselves continuously whether anyone opens a window or not, and that constant slow air exchange takes moisture out with it. Newer or recently retrofitted homes are far tighter — which saves energy and removes that accidental drying.
This is why damp problems so often appear immediately after new windows are fitted or a house is air-sealed. Nothing got wetter; the leaks that were quietly removing moisture were closed. Where that happens, trickle vents or mechanical ventilation are not an upgrade, they are the missing half of the retrofit.
4. Whether warm air can reach the glass
Heavy curtains closed against the window, a blind pulled down, or furniture across a radiator under the sill all cut the glass off from room air. The pocket behind them runs several degrees colder and considerably more humid than the room, and condenses first.
It costs nothing to change and it is regularly the whole explanation. Curtains a few inches clear of the glass during the day, radiators unobstructed, and blinds up while the room is in use.
The takeaway. Check them in cost order: airflow to the glass, then moisture production, then ventilation, and only then the glazing. The first two are free and account for a surprising share of the difference between two houses on the same street.
Common questions
Why do only some windows in my house have condensation?
The coldest ones condense first. North-facing rooms, single-glazed or metal-framed units, rooms with the heating off, and windows behind closed curtains all run colder than the rest of the house.
Did new windows cause my condensation problem?
Indirectly, often. New windows are much more airtight than the ones they replaced, so the constant background ventilation that used to remove moisture disappears. Trickle vents or mechanical ventilation replace it.
Should I keep curtains open to stop condensation?
During the day, yes, or at least clear of the glass. Closed curtains cut the window off from room air, so the glass and the air trapped against it both get colder.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions