Why Windows and Doors Are Where the Heat Goes

In most UK homes, windows and doors are the weakest part of the thermal envelope. Lofts can be topped up and cavity walls filled, but glazing is harder to improve without replacing it entirely. Heat escapes in two quite different ways, and it pays to know which one you are dealing with.

The first is conduction — warmth passing straight through glass, frame and panel. The second is air leakage — the steady trickle of warm air squeezing out through gaps around opening lights, letterboxes and door bottoms. A draughty single-glazed sash on a blustery January evening is losing heat by both routes at once, which is why some rooms never seem to warm up properly.

Older properties deserve a gentler approach. Victorian and Edwardian homes were built to breathe, so sealing every gap can trap moisture in the fabric. Prioritise the openings you actually use and keep existing ventilation paths — air bricks, trickle vents and chimney flues — doing their job.

Draught-Proofing: Start With the Cheapest Wins

Before spending anything on glazing, deal with the gaps. Seals cost a few pounds per window and can be fitted in an afternoon with a screwdriver, scissors and a damp cloth.

• Sash windows: brush or wiper seals along the parting beads, plus a foam or rubber strip at the meeting rail where the two sashes overlap.
• Casement windows: self-adhesive foam or rubber gasket around the opening light, and a check that the handle pulls the window firmly against it.
• Door bottoms: a brush strip, a hinged flap or a rubber blade draught excluder, chosen to suit your threshold height.
• Letterboxes: a brush seal on the inside face, and a keyhole cover over the lock.
• Chimneys: a chimney balloon or a ventilated cap — leave some airflow so the flue can dry out.

A simple test on a windy day: run a damp hand around the frame and feel for cold movement, or hold a sheet of paper near the edges to see if it flutters. Fit seals on a dry, mild day so the adhesive cures properly, and leave trickle vents alone — they are there for a reason.

Secondary Glazing: Trapped Air Does the Work

Secondary glazing means adding an independent pane inside the existing window, creating a sealed pocket of air that slows heat transfer. It is often far cheaper than replacement glazing, and because it sits behind the original frame it is frequently acceptable in conservation areas and listed buildings where new windows are not.

Options range from magnetic acrylic sheets for awkward shapes to rigid sliding panels in aluminium or timber. A few practical points:

• Fix the panel to the reveal rather than the window frame, so the original window still opens and closes freely.
• Choose at least one opening section in habitable rooms, so you can still ventilate without dismantling the whole thing.
• Clean and dry the existing window before fitting, and check whether your design needs a small drainage gap to let moisture escape.
• Keep the outer window's own seals in good order — secondary glazing works best as part of a system, not a substitute for basic maintenance.

As a bonus, the extra pane takes the edge off traffic noise, which is often the first thing people notice.

Curtains, Blinds and Shutters as an Extra Layer

Textiles will not replace sealed glazing, but they add a useful buffer. Interlined or thermally lined curtains, drawn at dusk and opened during the day, keep warmth in and let low winter sun do some free heating on south-facing windows.

Hang curtains as close to the wall as you can and let them reach the sill or floor, so cold air does not tumble down behind them. A pelmet or a simple valance stops warmth sneaking up the gap at the top. Blinds with side channels, honeycomb blinds and solid shutters all create a still layer of air next to the glass.

One caution: never let curtains or long blinds cover a radiator. Trapping heat against the wall wastes it. Tuck them behind the radiator, or shorten them so warm air can rise into the room.

Ventilation: The Balance That Prevents Condensation

Seal everything and you will soon meet the other problem: condensation. Cooking, showering and drying laundry push litres of moisture into the air each day, and it settles on the coldest surface available — usually glass, frames and external wall corners. Left alone, that leads to damp patches, mould and a stuffy, unhealthy atmosphere.

• Keep trickle vents open and fit them where they are missing.
• Run kitchen and bathroom extract fans for ten to fifteen minutes after use, and check the filters and ducting are clear.
• Purge ventilate: open a couple of windows wide for five to ten minutes, two or three times a day in winter, rather than leaving one ajar for hours.
• Aim for indoor relative humidity of roughly 40–60 per cent. An inexpensive hygrometer takes the guesswork out of it.
• Dry laundry outdoors or in a vented dryer where possible, and keep furniture a small gap away from external walls.

Putting It Together: A Sensible Order of Work

Start with draught seals and door excluders, because they are inexpensive and immediately noticeable. Next, address the worst-performing windows — usually north-facing singles and bay windows — with secondary glazing. Then add thermal curtains to bedrooms and living rooms. Finally, review ventilation so the improvements you have made do not create a moisture problem.

Revisit the seals each autumn: they harden, shrink and lift over time, and a fifteen-minute check before the cold arrives will keep your home warmer, quieter and cheaper to run all winter.