How to Install Thermal Curtains to Better Insulate Your Windows

Windows are often the weak point of a home's insulation: even in good condition, they let a significant share of the heat escape to the outside, and the resulting draughts create a persistent feeling of cold. Before thinking about replacing the joinery, there is a simple, inexpensive solution: thermal curtains. Equipped with an insulating lining, they form an extra barrier that reduces losses through the windows and noticeably improves the perceived comfort, especially at night. Here is how to choose and hang them well for a visible result from the first winter.

How do thermal curtains work?

A thermal curtain combines a fabric layer and an insulating lining, often polyester or polyester wadding, which traps a layer of still air between the curtain and the window. This still air acts as a natural insulator: it slows down the transfer of heat and blocks the feeling of cold radiation. According to the usual estimates, closing thick curtains in front of a window can reduce heat loss through it by around 10 to 25 %. The real gain depends on the quality of the joinery, the temperature difference between inside and outside, and above all on the curtain being properly closed and fitted to the window.

Choosing your thermal curtains

• The thermal lining: look for curtains with an insulating lining or with eyelets, a "technical fabric" type specifically designed for insulation
• The density of the fabric: a thick, tightly woven fabric is more effective than a light sheer
• The length: prefer curtains that come down to the floor, or even that "puddle" slightly on the floor, to limit draughts
• The width: allow in total 1.5 to 2 times the width of the window for a gathered look that covers the glazing well
• Dark colours or dark linings bring a marginal extra thermal gain in winter and reduce heat in summer, but the look remains above all aesthetic
• Models with a gathered eyelet or pleated heading are the simplest to hang on a rail

Installing a suitable curtain rail

1. Measure the width of the window and add 20 to 30 cm on each side so the curtain can clear the glazing completely
2. Choose a rail strong enough to support the weight of a thick thermal curtain: check the maximum load stated
3. Position the rail 10 to 15 cm above the frame, or as close to the ceiling as possible, to hide the lintel
4. Locate the fixing points with a spirit level, avoiding cracked areas and choosing wall plugs suited to the wall (plasterboard, brick or concrete)
5. Drill, insert the wall plugs and screw in the brackets, then fit the rail and its finials
6. Hang the curtains by sliding on the rings or eyelets, then check the height and the correct coverage of the window

Hanging well to maximise effectiveness

• Close the curtains at dusk and before cutting the heating during the eco period
• During the day, open them to benefit from the free solar gain, especially in sunny weather
• Make sure the curtain covers the whole glazing and the frame, without leaving a strip uncovered
• If possible, place a "cold" rail or a small draught excluder at the floor to block the downdraught
• Complement with weatherstripping around the window if it is very old: the curtain does not stop air infiltrations

Maintaining your curtains and combining solutions

Thermal curtains are generally machine washable at 30 °C on a gentle cycle, following the instructions on the label. Wash them about once a year, checking that they do not shrink before re-hanging them. To go further, combine them with other insulation measures: shutters and blinds closed at night, sealed door bottoms, replaced window seals and, in the longer term, double glazing. Thermal curtains do not replace insulation of the joinery, but they form an effective first layer that is reversible and aesthetic, ideal when renting or while waiting for heavier work.

The mistakes to avoid

• Choosing a simple fine sheer thinking it insulates: the fabric must be thick and lined
• Installing the rail too close to the glazing: the insulating layer of air disappears
• Closing the curtains systematically in full daylight: you lose the free solar gain
• Hanging curtains that are too short, which let the draught through at the bottom
• Overloading a too-fragile rail with a heavy thermal curtain: risk of falling and deformation

Hanging thermal curtains is one of the simplest and cheapest insulation solutions there is: a few dozen euros per window, an afternoon's work and an immediate gain in comfort from the first winter. The effect is all the more visible when the joinery is old and the nights are cold. By combining them with good heating management and a few draught-proofing measures, you reduce the feeling of cold, the draughts and the heating consumption, without any major insulation work.