How to insulate windows and cut heat loss

You don't need to replace your windows to make a real difference. Most homes can cut window-related heat loss considerably with draught-proofing, better curtains and a few low-cost film kits — all before even thinking about new glazing.

Why windows lose so much heat

A window loses heat in two main ways: air leaking through gaps around the frame and edge, and heat conducting through the glass itself (and the frame). Single-glazed windows — a single pane of glass with no trapped air — conduct heat very freely. Double-glazed units trap a layer of gas between two panes, which slows that conduction considerably. Triple glazing adds a third pane for even greater resistance.

Older homes often have single glazing and frames that have warped or shrunk over time, leaving gaps that let cold air whistle in. Even modern double-glazed windows develop problems: failed seals that let moisture inside the unit, worn rubber gaskets, and draughty frame edges. Understanding which problem you have tells you which fix to prioritise.

It's worth noting that windows are not the only — or usually the largest — source of heat loss in a home. Loft insulation and wall insulation tend to deliver bigger savings. But windows are visible, they create cold spots and discomfort near them, and many fixes are cheap enough that they're worth doing regardless. See our home insulation guide for the bigger picture.

Assess your windows first

Before spending any money, spend ten minutes checking what you actually have. This shapes where to focus.

• Single, double or triple glazing? Hold a lit candle or lighter near the glass (carefully). A single reflection means single glazing; two reflections mean double; three mean triple. Single glazing is the highest priority for improvement.
• Check for draughts. On a windy day, hold your hand around the frame perimeter, the opening sashes and the letterbox or cat-flap if nearby. Even a small gap can cause a noticeable cold draught and ongoing heat loss.
• Look at the glass for a white haze or moisture between panes. This means the double-glazed sealed unit has failed — the insulating gas has escaped. The unit needs replacing, not just improving from the outside.
• Check the frames. Timber frames can shrink, warp and crack. UPVC frames degrade over many years. Gaps between frame and wall can also let in cold air — check around the outside too.
• Note which windows face which direction. North-facing windows lose heat without gaining much sun. South-facing windows gain useful solar heat in winter, so blocking them entirely with heavy treatments is a trade-off.

Low-cost DIY fixes

These options cost very little and can be applied without professional help. Start here before considering larger investments.

Draught-proofing strips and sealants

Gaps around window frames and between opening sashes and frames are one of the most common and fixable problems. Self-adhesive foam strips, brush strips and rubber-seal strips can be pressed into gaps in the frame to block the draught. For gaps between the fixed frame and the wall, a flexible, paintable filler or silicone sealant works well.

This is one of the most cost-effective things you can do in any home. Our full draught-proofing guide covers every type of gap and the right materials for each situation.

Thermal curtains and pelmets

A heavy or thermal-lined curtain creates a layer of still air between the room and the cold glass, reducing heat loss and improving comfort near the window. To work well, curtains need to:

• Reach the floor or the window sill (not stop mid-air where cold air can circulate underneath).
• Overlap the wall on both sides, not just cover the glass.
• Have a pelmet or deep heading above — without one, warm room air rises and spills over the curtain rail, creating a convection loop that makes the curtains much less effective.
• Be closed consistently at dusk rather than after dark when the room has already cooled.

Thermal-lined curtains are widely available at a range of price points. Roman or roller blinds with a thermal backing can work too, particularly for smaller windows.

Secondary glazing film kits

These kits consist of a thin, clear plastic film stretched over the window opening and attached with double-sided tape. When tightened with a hairdryer, the film becomes almost invisible and creates a thin air gap between itself and the glass that meaningfully reduces heat loss. They are particularly effective on cold, draughty single-glazed windows. Most kits are designed for seasonal use — applied in autumn and removed in spring — though some are clear enough to leave year-round.

Insulating window panels and reusable inserts

Rigid insulating panels — cut to fit inside the window reveal — can be placed in the window at night or during cold spells and removed during the day. They are more work to manage than curtains but offer greater insulation, particularly over single glazing. Cellular or honeycomb blinds also trap air in their cells and provide meaningful insulation, especially in the "top-down, bottom-up" style that lets in light while keeping the lower portion covered.

Internal shutters, solid blinds and insulating roller blinds

Solid wooden or fabric-filled shutters that close snugly against the frame are an older solution that works well. They stop draughts and trap air. Where original period shutters survive in older homes, using them is free and effective. New internal shutters are an investment but a durable one. Insulating roller blinds — often described as "blackout" or "thermal" — are a more affordable option and can be fitted inside the window recess without drilling into the wall.

How to apply window insulation film

1. Clean the frame thoroughly. The double-sided tape must bond to a clean, dry surface. Wipe the frame with a damp cloth, let it dry completely, then wipe with rubbing alcohol or white vinegar. Any grease or dust will cause the tape to peel early.
2. Measure the window opening. Measure the width and height of the area you want to cover — usually the reveal just inside the frame. Add a few centimetres on each side so the film overlaps the tape with room to spare. Cut the film to size.
3. Apply the double-sided tape around the perimeter. Stick the tape to the frame all the way around, pressing firmly into corners. Leave the backing paper on for now.
4. Peel the backing and attach the film at the top. Peel the backing paper from the top strip of tape first. Press the top edge of the film onto it, keeping it straight and centred.
5. Work down both sides, keeping the film taut. Peel the backing from one side, stretch the film gently outward and press it down. Repeat on the other side, then do the bottom, pulling lightly as you go. The film will look wrinkled — that's expected.
6. Use a hairdryer on medium heat to shrink the film tight. Hold the hairdryer about 5–10 cm from the surface and move it across the film in steady passes. The film will tighten and become clear. Work from the centre outward. Do not hold the dryer in one spot or use the highest heat setting, which can melt or distort the film.
7. Trim any excess film. Once tight and clear, use scissors or a craft knife to trim neatly along the outer edge of the tape. Check all edges are sealed with no gaps.

Bigger upgrades: secondary glazing and double glazing

Once you have addressed draughts and added good curtains, the remaining heat loss is mostly through the glass itself. If your windows are single-glazed, or if you have failed double-glazed units, there are two main upgrade paths.

Secondary glazing systems

Secondary glazing means fitting a separate, independent panel — usually glass or thick acrylic — inside the existing window frame, leaving a gap between the two layers. This is a well-established approach in older or listed buildings where replacing the original windows is not permitted. It can achieve thermal performance close to standard double glazing and also significantly reduces noise.

Secondary glazing panels are available as hinged, sliding or removable systems. They can be fitted by a competent DIYer in some cases, though professional fitting gives a better seal. They are generally less disruptive and less expensive than replacing the windows entirely.

Double and triple glazing

Replacing single-glazed windows with double or triple-glazed units is the most effective thermal improvement for the window itself. Modern double-glazed units use an argon or krypton gas fill and often a low-emissivity (low-e) coating on the glass to further reduce heat loss. Triple glazing reduces heat loss further still and is standard in new buildings in colder northern European climates.

New glazing is a significant investment, and payback periods depend on your current energy costs, how cold your climate is, and whether you can access grants or incentives. Government schemes that support window upgrades vary considerably by country and change over time — check with your national energy agency or local authority for current offers before committing. Always get multiple quotes from certified or registered installers. Look for industry registration schemes in your country (such as FENSA in England and Wales, or equivalent bodies elsewhere) that ensure work meets building regulations.

If your budget is limited, prioritise the coldest, most draughty rooms and the largest windows first. A single large north-facing window can lose far more heat than several small ones.

Condensation, damp and ventilation

Adding insulation to windows can change how moisture behaves in a room, and it's worth understanding this to avoid creating problems. Condensation forms when warm, moist air meets a cold surface. Single-glazed windows are prone to this because the glass itself is very cold. When you add film, shutters or secondary glazing, the original glass surface gets even colder on its inner face — which can actually increase condensation on the glass (though the secondary surface stays warmer and drier).

More importantly: when you seal up draughts, you reduce background ventilation. Bathrooms and kitchens produce a lot of moisture; without some air movement, that moisture stays in the home and can lead to condensation on walls and mould growth. Always maintain adequate ventilation — use extractor fans in bathrooms and kitchens, leave trickle vents in window frames open, and ensure background ventilation is working. See our guide to damp and mould for more on keeping moisture under control.

Keeping cool in summer

Windows that let heat in during summer can drive up cooling loads just as much as they lose heat in winter. Reflective or solar-control window film reflects a portion of incoming solar radiation before it enters the room, reducing heat gain without permanently blocking the view. It is distinct from insulating film used in winter — most solar-control films are left in place year-round.

External shading — awnings, external shutters, shutters, or fast-growing climbing plants on a trellis — is generally more effective than internal blinds because it stops solar heat before it enters the glass. Internal blinds and curtains still help once some heat is through the glass, but they cannot return heat that has already entered the room as efficiently as external shading can prevent it from entering in the first place.

For south- and west-facing windows in particular, think about shading before summer arrives. Adjustable external awnings offer the flexibility to shade in summer and retract in winter to allow solar gain when it's useful.

Renters: non-permanent options

If you rent, you may be limited in what you can fix permanently. But you have more options than you might think, and many of the most effective measures require no drilling or adhesives at all.

• Self-adhesive foam draught strips around window frames — these peel off without damage in most cases; remove carefully and patch any marks when leaving.
• Secondary glazing film kits — double-sided tape on painted frames generally peels cleanly when removed at low heat with a craft knife.
• Heavy thermal curtains or blinds — these are your furniture and go with you when you move.
• Cellular or honeycomb blinds in tension-fit frames — these sit inside the window recess with no fixings and can be taken down.
• Reusable rigid insulating panels cut to fit the reveal — stored away in summer and placed in winter.
• Draught-excluding snakes or strips placed on window sills for the season.

If you find significant draughts, failed double-glazed units, or visible gaps around the frame, your landlord has a responsibility in many countries to maintain the property in a habitable state. Raise it with them in writing, noting the effect on comfort and heating costs. Your national tenant rights body can advise on obligations in your specific situation.

Your window insulation checklist

• Check all windows for draughts and seal gaps with appropriate strips or sealant.
• Fit or replace curtains so they reach the floor and have a pelmet overhead.
• Apply secondary glazing film to the coldest single-glazed windows this autumn.
• Keep trickle vents open and extractor fans working to maintain ventilation.
• Consider secondary glazing panels for listed buildings or where full replacement isn't viable.
• If upgrading to double or triple glazing, get multiple quotes and check local incentive schemes first.
• For summer, add reflective film or external shading to south- and west-facing windows.