Windows: comfort, condensation and what is worth replacing

Windows are where most of a room's comfort problems originate. They are the coldest surface in winter, the largest heat gain in summer, the source of the draught you can feel with the window shut, and the place condensation appears first.

They are also expensive to replace, which makes it worth understanding what actually improves them — because several cheaper interventions outperform new glazing, and one common upgrade delivers less than people expect.

The numbers on the sticker

Two figures describe a window's thermal behaviour and they pull in opposite directions.

U-value measures heat loss, in watts per square metre per kelvin. Lower is better. Single glazing is around 5, older double glazing around 2.8, modern double glazing with a low-emissivity coating and argon fill around 1.2 to 1.4, and good triple glazing around 0.8. A solid wall for comparison might be 0.3, which is why windows dominate.

G-value, or solar heat gain coefficient, measures how much solar energy passes through. Higher means more free heat in winter and more overheating in summer.

The tension is real. A south-facing window with a high g-value is a useful winter heat source and a summer problem. A low g-value glass reduces overheating and also reduces useful winter gain and daylight. There is no single correct answer, which is why orientation matters: higher g-value on south-facing glass with external shading, lower on west-facing glass which catches the hot late sun with no shading possible from an overhang.

You will also see a window energy rating in some countries, which combines both figures into a letter grade. Convenient, and it conceals the orientation question above.

What actually makes a window uncomfortable

Three distinct effects, often confused with each other.

Radiant loss to cold glass. You radiate heat towards any cold surface, and a large cold window makes a room feel colder than its air temperature suggests. This is why sitting by a window in winter is unpleasant even in a warm room, and why improving the glass improves comfort more than the U-value alone implies.

The cold downdraught. Air in contact with cold glass cools, becomes denser and falls, spreading across the floor. It feels exactly like a draught from a gap and it happens with a perfectly sealed window. This is why radiators were traditionally placed under windows — the rising warm air counteracts it — and why moving a radiator away from a window can make a room feel worse.

Actual air leakage, around the frame where it meets the wall rather than around the opening part. This is the one that draughtproofing fixes, and it is frequently the largest of the three.

Knowing which you have determines the fix. A downdraught is not solved by sealing; a leak is not solved by better glass.

Cheaper interventions that outperform replacement

In rough order of value per pound.

Draughtproofing around the frame. Sealing the junction between frame and wall, and fitting brush or compression seals to the opening parts. Cheap, immediate, and it addresses the biggest of the three effects.

Heavy, well-fitted curtains, closed at dusk. Genuinely effective, and the fitting matters more than the fabric: a curtain that seals at the top with a pelmet or track close to the ceiling, extends well beyond the reveal at the sides, and reaches the sill or floor, traps a layer of still air. A curtain hanging loose in front of a window lets the downdraught out at the bottom and the warm air in at the top, and it does considerably less.

Insulating blinds. Honeycomb or cellular blinds fitted tightly into the reveal, with side channels if available, perform surprisingly well — comparable in some tests to adding a layer of glazing, at a fraction of the cost.

Secondary glazing. An additional internal pane with a wide air gap. Where it is often better than replacement: the gap can be much wider than in a sealed unit, which improves both thermal and acoustic performance, and it retains the original window — which matters in a listed or period property where replacement may be prohibited.

Low-emissivity window film on single glazing, which reduces radiant loss modestly for very little.

Shutters, internal or external. External shutters closed at night are excellent, and closed by day in summer they are the best overheating control available.

The honest position: if your existing double glazing is sound, replacing it with new double glazing rarely pays back on energy alone. Replace when the units have failed, the frames are rotten or the windows do not open safely. Replace single glazing more readily, and consider secondary glazing where the original must stay.

Failed units, and how to spot them

A sealed double glazed unit fails when its perimeter seal degrades and moisture enters the cavity.

The signs: misting or condensation between the panes, which cannot be wiped off; a permanent haze or dirt film on the inner surfaces; and sometimes visible water pooling at the bottom of the cavity in cold weather.

A failed unit has lost most of its insulating benefit, because the gas fill has gone and the cavity is damp. The good news is that the unit can often be replaced without replacing the frame, which is far cheaper than a new window. Get a quote for both.

Condensation on windows, and what it tells you

Condensation on the inside of glass is normal in cold weather and tells you two things: the glass is below the dew point of the room air, and there is enough moisture in the room to reach it.

It is a useful diagnostic rather than a fault. Heavy condensation on bedroom windows every morning means the room's humidity is high and ventilation is inadequate — and that same moisture is also condensing on the colder corners of the walls where mould grows. The window is simply the visible one.

Condensation on the outside of a window is a good sign: it means the glass is cold because the heat is staying inside.

Where automation helps: a humidity sensor and a temperature sensor in the room let you calculate the dew point and see it coming. Better still, ventilate on absolute humidity and the problem largely disappears.

Automating windows and shading

Blinds and curtains on sunrise and sunset, which handles both heat and privacy without anyone remembering.

Shading that tracks the sun, closing on the elevation that actually reaches each window rather than all at once. This is the single most effective cooling automation in a house.

Closing everything at dusk in winter, which turns a good curtain into a nightly insulation layer.

Opening on a temperature comparison, ventilating only when outdoor air is cooler than indoor.

Rain and wind conditions on any automated opening, and a wind threshold on external blinds and awnings, which are easily damaged.

Window contacts pausing heating in that room.

Condensation warning, comparing room dew point with expected glass temperature.

One practical note on motorisation: retrofit blind and curtain motors are now genuinely good, battery or mains, and the awkward part is almost always the mounting rather than the electronics. Measure carefully and check the weight rating against a lined curtain, which is heavier than people expect.

What we would do

1. Work out which of the three problems you have — radiant, downdraught or leakage — with a thermometer, an infrared thermometer and an incense stick.
2. Draughtproof the frames first.
3. Fit curtains or blinds properly — sealed at the top, wide at the sides, down to the sill — and close them at dusk.
4. Keep radiators under windows where they already are.
5. Replace failed units rather than whole windows.
6. Consider secondary glazing for single glazing you must keep.
7. Add external shading to the window that overheats, which is the best summer investment available.
8. Automate shading on sun position and closing at dusk.

Items two and three cost very little and change how a room feels immediately, which is more than a new window will do for the same money.