Better Windows, Colder Room? The Radiator and Curtain Checks to Do First

There's a particular kind of disappointment that arrives about three weeks after a window installation. The draughts have gone, the street noise has dropped, the frames look smart — and yet, on the first properly cold evening, someone in the house says the room feels colder than it used to. It's a genuinely common complaint, and it's rarely the glass.

When you seal up a room, you change how heat moves around it. Old windows leaked air, and that leakiness was doing a job nobody asked it to do: stirring the air, dragging warmth off the radiator and pushing it across the room. Take the leak away and the whole picture shifts. What follows is a check you can work through yourself before you ring the installer or the heating engineer.

Why a tighter room can feel cooler

Three things tend to be going on at once.

First, radiators were often sized for a leakier house. If yours were specified years ago to hold a room at 20°C against draughts and single glazing, they now have spare capacity. That sounds like good news, and in a well-balanced system it is. But in a system that has never been balanced, one room can end up roasting while another stays stubbornly cool, because the pump is pushing water round the easiest circuit rather than the one that needs it.

Second, air movement has changed. Warm air off a radiator used to be mixed around the room by incidental draughts. Now it rises, spreads along the ceiling and drops — often before it reaches the sofa. The thermometer on the wall might read 20°C while the person sitting still in an armchair feels a distinct chill.

Third, comfort is radiant as well as air temperature. Glass is colder than an insulated wall, and a large pane still pulls heat off you if you sit close to it, even when it's a good unit. That effect is much smaller than it was with the old windows, but it hasn't disappeared, and it's most noticeable with big, uncurtained openings.

None of this means the installation was wrong. It means the room needs a half-hour of attention.

Start at the radiator, not the window

Get a cheap thermometer and take a reading in the middle of the room, at sitting height, an hour after the heating comes on. Then take one near the window. A difference of a degree or so is normal. A big gap tells you where the problem sits.

Now check the radiator itself:

• Is it hot all over? Cold at the top means air; cold at the bottom means sludge. Both cut output, and both are cheap to sort.
• Is anything in front of it? A sofa pushed up against a radiator is the single most common reason a room underperforms.
• Is the radiator under the window blocked? This arrangement exists for a good reason — the rising warm air forms a curtain against the cold glass. It only works if the heat can actually escape upwards.

If the radiator is hot and the room is still cold, suspect balancing rather than the window.

The TRV trap

Thermostatic radiator valves are brilliant and frequently misused.

A TRV with a setting of 3 doesn't mean "three-quarters hot". It means roughly 20°C in that room, and it will shut the radiator down once it gets there — including when the room is only warm near the ceiling. If the valve is tucked behind a curtain, inside a cabinet, or right next to a radiator that's been boxed in, it's reading the wrong temperature and cutting the heat early.

Two quick checks. Turn every TRV to maximum for an hour and see whether the cold room catches up; if it does, the valve is throttling too soon. And in early autumn, take the head off each valve and press the pin underneath — it should spring back freely. A stuck pin is a five-minute fix and a very common cause of a radiator that never comes on.

Curtains, blinds and the shelf above the radiator

Here's where new windows and old habits collide. Replacement windows often come with deeper reveals or a different board height, and curtains that used to hang clear of the radiator now drape over it.

A full-length curtain sitting on top of a radiator traps the heat between the fabric and the glass. The room gets the cold end of the deal and the window gets the warmth. If you can, finish curtains at the window board, or fit a shelf above the radiator to deflect warm air out into the room.

Blinds matter too. A blind pulled down and left a few centimetres off the sill creates a pocket of still, cold air against the glass — fine for heat loss, less good for comfort. Close blinds at dusk, open them in the morning.

Don't forget the vents

This one catches people out. New windows almost always come with trickle vents, and a lot of households shut them in October and forget until April.

A sealed room with no background ventilation gets humid. Humid air takes more energy to warm and feels colder on the skin, and it condenses on the coldest surface available — often the glass. That's how a brand-new window ends up being blamed for condensation it didn't cause.

Leave trickle vents open in kitchens and bathrooms as a minimum, and open a window a crack for ten minutes first thing in the morning. It feels counterintuitive on a cold day, but a quick air change costs far less than a permanently damp room.

A quick room-by-room walk-through

• Living room: radiator clear of furniture, curtains off the radiator, TRV unobstructed, door to the hall shut when the heating is on.
• Bedrooms: smaller radiators here are often the ones left unbalanced; check the pin and bleed.
• Kitchen: extractor running while cooking, trickle vent open, don't block the radiator with a tumble dryer or a towel rail's worth of laundry.
• Bathroom: heated towel rail plus a good extractor beats a hot radiator and a sealed window.
• Hall, stairs and landing: if the thermostat lives here, it's controlling the whole house from the coldest, draughtiest spot. Worth knowing before you blame the sitting room.

When it really is the glazing

Be honest about the symptoms. If the glass mists between the panes, if the frame feels loose, if you can see daylight where you shouldn't, or if one window is noticeably colder than an identical one next door, that's an installation or unit issue and it belongs with your installer, inside the guarantee period.

If the window is sound and the room is still uncomfortable, the answer usually sits in the heating system, the curtains or the ventilation — and all three are cheaper to fix than a second round of glazing. Give the room a winter to settle, make the checks above, and judge it in February rather than in the first cold snap of November.