How do i reduce heat loss through windows in winter? (2026)

Overview

Windows lose heat in two main ways: warm indoor air leaks out through gaps, and heat passes through the glass and frame even when the window is shut. The quickest, cheapest improvements are usually draught-focused: check for worn seals, gaps around frames, loose latches and cracks in old sealant. If the window closes properly but still feels cold, the next step is to add insulation on the room side with close-fitting curtains, thermal blinds or a removable secondary glazing panel. These measures work best when they fit tightly, because uncontrolled air movement around the edges can undo much of the benefit. If your windows are single glazed, damaged, rotten, warped or misting between panes, simple DIY measures may not be enough. In that case, a repair or replacement assessment may save more heat and improve comfort long term. Also remember that windows are part of the wider building envelope: if nearby walls, vents or doors are very draughty, fixing only the glass may have a limited effect. A practical approach is to identify the worst-performing windows first, treat those, and then decide whether low-cost seasonal measures are enough or whether you need a more permanent upgrade.

Who this is for

Homeowners and tenants who want practical ways to make existing windows less draughty and less cold in winter, especially in older homes.

What you’ll need

• clean cloths
• mild cleaner
• torch
• replacement weather seals or draught strip suitable for your window type
• caulk or sealant suitable for interior or exterior window joints, if needed
• caulking gun, if using cartridge sealant
• curtains, thermal lining or close-fitting blind
• measuring tape
• temporary secondary glazing kit or removable window insulation film, if using
• manufacturer instructions for any product you install

Before you start

Work out whether the heat loss is mainly from air leakage, cold glass, or both. On a cold or windy day, feel carefully around the frame, opening edge and sill for draughts, and look for visible gaps, damaged seals, condensation, rot, cracked putty or failed glazing units. Check whether you rent the property, as some fixes may need the landlord's permission. Also check whether the window is an emergency escape route before fitting anything that could hinder opening.

Step-by-step

1. 1

   Inspect each window properly

   Open and close the window fully. Look for worn or missing weather seals, gaps between frame and wall, cracked sealant, loose catches, damaged putty, warped sashes and any sign that the window does not pull tightly shut. Note which windows feel draughty and which simply feel cold through the glass.

   Why: You need to match the fix to the problem. A draught problem needs sealing; a cold-glass problem needs added insulation or better glazing.

2. 2

   Clean the surfaces before any repair

   Wipe the frame, sash edges and any area where you plan to stick, seal or fit a product. Remove dust, grease, flaky old material and moisture. Let the area dry as required by the product instructions.

   Why: Seals, tapes and films usually fail early if applied to dirty or damp surfaces.

3. 3

   Stop obvious draughts at the opening parts

   Fit or replace weather stripping or compressible seals where the sash or opening section meets the frame. Make sure the product suits the window style and does not stop the window from shutting or locking properly. If the latch is loose and adjustable, tighten or adjust it so the closed window pulls in firmly without forcing it.

   Why: Air leakage around opening joints is often the biggest cause of winter discomfort and is usually the easiest heat loss to reduce.

4. 4

   Seal fixed gaps around the frame

   For non-moving joints between the frame and the surrounding wall or trim, remove failed sealant if necessary and apply a suitable replacement sealant. Do not seal designed drainage paths or ventilation features built into the window system. If you are unsure what is intentional, check the manufacturer's guidance.

   Why: Fixed perimeter gaps let cold air in and warm air out, but blocking drainage or required ventilation can create moisture and durability problems.

5. 5

   Add an insulating layer on the room side

   Fit close-fitting curtains, thermal-lined curtains or a blind that sits near the window and covers it well. For higher performance on problem windows, consider a removable secondary glazing panel or a seasonal insulation film system fitted strictly to the product instructions.

   Why: A still layer of air and an extra barrier reduce heat transfer through cold glass and can improve comfort even when the window itself is not being replaced.

6. 6

   Use curtains and blinds correctly

   Close them before the room cools down in the evening, and open them when useful winter sun can warm the space. Try to reduce large side gaps where warm air can circulate behind the covering. Keep fabric clear of heaters and avoid covering vents you are meant to leave open.

   Why: Even good window coverings work poorly if warm room air can freely flow behind them or if they are used in a way that blocks safe heating or ventilation.

7. 7

   Check for moisture and ventilation issues after sealing

   Over the next few days, watch for increased condensation on the glass or frame, mould growth, or stale air. If condensation worsens, improve moisture control by using extractor fans where provided, covering pans when cooking, drying clothes appropriately and ventilating as needed.

   Why: A tighter home is usually warmer, but reduced uncontrolled airflow can make indoor moisture more noticeable if ventilation is poor.

8. 8

   Decide whether repair or replacement is the better long-term fix

   If the window is rotten, badly warped, hard to shut, cracked, has failed double glazing, or remains very cold and draughty after basic measures, get a professional assessment for repair, reglazing or replacement.

   Why: Temporary or minor fixes cannot solve underlying structural or glazing failures.

Why this works

Heat is lost through windows by air leakage, conduction through glass and frames, and air movement caused by cold surfaces. Draught-proofing cuts uncontrolled airflow, while curtains, blinds and secondary glazing add resistance to heat flow and help keep the inner surface warmer. Warmer inner surfaces also reduce the feeling of radiant cold and can lessen condensation risk when used alongside adequate ventilation.

Common mistakes to avoid

• Using sealant on parts of the window that need to move, drain or ventilate
• Applying adhesive strips or films to dirty, damp or flaky surfaces
• Fitting draught seals so thickly that the window will not shut or lock properly
• Assuming thicker curtains alone will fix a major frame draught
• Ignoring condensation and mould after making the window area more airtight
• Blocking required trickle vents or other intentional ventilation openings

Troubleshooting

Compare your options

Alternatives

• Use temporary rope caulk or removable seasonal draught products if you need a non-permanent fix
• Improve whole-room heat retention with door seals, loft or ceiling insulation, and controlled ventilation
• Rearrange furniture so seating is away from the coldest window if upgrades must wait

Pro tips

• Prioritise the worst room-facing windows first, especially bedrooms and main living spaces
• On sunny winter days, open curtains to gain free solar warmth, then close them before the room cools
• If one window is much colder than others, compare how tightly it shuts and whether its seals are visibly flatter or damaged
• Keep a note of which products you used and where, so you can replace like-for-like later if they perform well

Safety notes

• Do not obstruct emergency escape windows if the room relies on them for egress
• Take care when working near glass, especially on older or damaged panes
• Use ladders carefully and only within your competence when accessing higher windows
• Keep fabrics, films and plastic materials clear of heat sources and follow the product safety instructions

Legal & regulatory notes

If you rent, get permission before making alterations beyond simple removable measures. In some homes, especially heritage or protected buildings, changing windows or external appearance may need approval from the relevant local authority. Replacement windows and glazing work may also need to meet current local building requirements, so check with your council or building authority before ordering major work.

Cost considerations

Low-cost measures such as draught-proofing and better curtain use usually give the fastest payback. Secondary glazing and professional repairs sit in the middle, while full replacement is a bigger investment that may make sense where windows are already failing or due for renewal.

Frequently asked questions

Sources & references

Guidance on this page is traced to documented sources. Last checked 25 September 2026.

• Energy Saving Trust · industry

  Supports general guidance on reducing heat loss through windows, including draught-proofing, curtains and glazing-related efficiency improvements.

• U.S. Department of Energy - Energy Saver · government

  Supports the principles of air sealing, weatherstripping, caulking and the use of window coverings or storm/secondary approaches to reduce heat loss.

• BRANZ · industry

  Supports New Zealand-relevant good practice on building performance, moisture, ventilation and maintenance considerations around windows and building envelopes.

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