Which way to hang a thermal curtain for optimal insulation
A thermal curtain with a metallic coating is hung with the technical side facing the window: this layer reflects part of the infrared radiation, which helps keep heat on the room side in winter. Multi-layer curtains without a metallic side (such as Kurtens curtains at 620 g/m²) can be hung either way round with no difference in performance.
Which way to hang it is the most frequent question about thermal curtains, but it is not the factor that most affects insulation. The curtain's dimensions, its density and how it is mounted have a much greater impact on real performance. This guide covers the hanging rule, the consequences of getting it wrong, and why multi-layer curtains eliminate the question of direction.
The general rule: technical side facing the window
The thermal side of an insulating curtain must face the window. This side, usually metallic or satin-finished, reflects 60 to 80% of infrared radiation depending on the type of coating (vaporized aluminum or metallized polyester film). In winter, it sends indoor heat back into the room instead of letting the glazing absorb it. In summer, it reflects solar radiation before it enters the room.
How to identify the technical side: it is smoother, stiffer, often silver or satin. The decorative side is softer and matte, with patterns or colors. Touch test: place your hand on each side for 10 seconds. The technical side stays cooler because it does not retain body heat. If in doubt, check the manufacturer's label.
This rule only applies to curtains with an attached lining or a metallic coating. Multi-layer curtains with built-in insulation work differently (see next section).
What happens when you hang a thermal curtain the wrong way round
Hanging a thermal curtain the wrong way round (technical side facing the room instead of the window) reduces its insulating performance by 40 to 60% according to studies by the Centre Scientifique et Technique du Bâtiment (CSTB) on textile window coverings. The reflective layer then sends the room's heat towards the window, the exact opposite of its function.
Measured consequences:
- Heat loss: the technical side no longer does its job: the space between the glazing and the curtain gets colder, against the window.
- Condensation and mold: this cold zone encourages condensation between the glass and the fabric, especially in winter, which can lead to mold on the window seals and the bottom of the curtain. Air the room and keep ventilation clear.
- Fabric damage: constant moisture speeds up the aging of the fabric and damages the window seals in the medium term.
- Extra energy cost: the loss of efficiency is paid for in comfort near the window, and the heating has to compensate more.
Important: a curtain hung the wrong way round keeps some of its insulating properties thanks to the mass of the fabric, which slows air movement. But the reflection effect, which accounts for most of the thermal gain of a coated curtain, is cancelled out.
Single-layer vs multi-layer curtain: the direction depends on the type
Not all thermal curtains work the same way. The table below compares the two technologies and how sensitive each is to the hanging direction.
| Criterion | Single-layer curtain (metallic lining) | Multi-layer curtain (built-in insulation) |
|---|---|---|
| Insulation principle | Infrared reflection by the metallic side | Pockets of still air between the fabric layers |
| Hanging direction | Mandatory: technical side facing the window | Irrelevant: both sides are equivalent |
| Risk of error | High: 40 to 60% loss of performance | None: identical performance either way |
| Appearance | Silver side visible from the window | Attractive fabric on both sides |
| Durability of the treatment | The metallic coating degrades with washing and exposure to light | Performance comes from the structure of the fabric, not a surface treatment |
| Typical thermal gain | Varies with the quality of the coating | Up to 7 °C* (Kurtens 3-in-1, laboratory measurement) |
Multi-layer curtains: when direction no longer matters
The multi-layer thermal curtains use several layers of high-density fabric that create pockets of still air between the window and the room. The principle is similar to double glazing: air trapped between the layers is the best natural insulator. Performance does not depend on a reflective side, but on the built-in structure of the fabric.
With a multi-layer curtain, the hanging direction makes no difference. Both sides are fabric, attractive and decorative. No silver side to hide towards the window, no risk of hanging it wrong. The curtain is hung like a classic curtain, with the same thermal performance either way.
Durability is also better: a metallic coating gradually degrades with washing and exposure to light (10 to 20% loss of reflection after 2 years of daily use). A multi-layer curtain keeps its properties because the performance comes from the fabric itself, not from a surface treatment.
Kurtens made-to-measure curtains are high-density 620 g/m² multi-layer curtains that combine three kinds of performance in a single curtain:
- A temperature difference of up to 7 °C between the glass surface and the room side, measured in the laboratory*
- 100% blackout*
- Up to 22 dB of sound attenuation*
No metallic side, no risk of hanging it the wrong way, an attractive finish on both sides. Available as a made-to-measure thermal curtain in the exact dimensions of your window.
What matters more than direction: the size of the curtain
The hanging direction affects performance by 40 to 60% on a single-layer curtain, but the size of the curtain affects performance by 50 to 70% on all types of curtain. A thermal curtain that is too small lets cold air through at three leak points, whichever way it is hung.
The 3 heat leak points of an undersized curtain:
- The sides: a curtain that is too narrow leaves gaps at the sides where cold air flows down the glass, passes under the curtain and rises back into the room by convection. Solution: allow 15 to 20 cm of overlap on each side of the window.
- The bottom: too large a gap between the curtain and the floor creates a cold draft through a chimney effect. Solution: the hem should skim the floor, with a gap of 2 to 3 cm at most.
- The top: a rod mounted too low leaves a gap above the curtain through which warm air escapes. Solution: mount the rod at least 10 cm above the top of the window.
For windows with unusual dimensions, made to measure guarantees optimal coverage with no compromise. See the Kurtens size guide to determine the exact dimensions for your setup.
To learn more, the complete guide to choosing a thermal curtain covers materials, number of layers, weight and measured performance. And to compare opacity levels, see the blackout vs light-filtering vs opaque comparison.
*Data from tests carried out under optimal conditions.
Frequently asked questions
Should I turn my thermal curtain around between summer and winter?
No. For a curtain with a metallic lining, the technical side stays facing the window all year round: it reflects indoor heat in winter and blocks solar heat in summer. For a multi-layer curtain such as Kurtens curtains, the direction makes no difference whatever the season: thermal performance is identical in both orientations.
How do I recognize the thermal side of my curtain?
The thermal side is usually smoother, stiffer and often silver or satin. The decorative side is softer and matte. Touch test: place your hand on each side for 10 seconds. The side that stays cool to the touch is the technical side (it does not retain body heat). If both sides look like the same fabric, your curtain is probably a multi-layer model, and the direction does not matter.
Is a thermal curtain hung the wrong way round still effective?
Partly. The mass of the fabric blocks air movement and provides basic insulation, but the infrared reflection effect is cancelled out. The loss of performance is estimated at 40 to 60% on a single-layer curtain with a metallic coating. Further consequences: more condensation between the glazing and the curtain, a risk of mold on the seals, and extra heating costs estimated at 50 to 80 euros per year for a 15 to 20 m² room. Multi-layer curtains eliminate this risk because their performance does not depend on the hanging direction.
Can I hang a thermal curtain over a front door?
Yes. Front doors are the second biggest source of heat loss after windows, especially in older homes. The hanging rule stays the same: technical side facing the door. To fully cover the frame without leaving gaps at the sides, a made-to-measure curtain is particularly recommended. Allow 15 to 20 cm of overlap on each side and a hem that skims the floor (gap of 2 to 3 cm at most) to maximize insulation.
Does a thermal curtain replace double glazing?
No, it complements it. A thermal curtain creates an extra barrier in front of the glazing. The effect is generally more noticeable on single glazing than on double glazing. The Kurtens multi-layer curtain shows a temperature difference of up to 7 °C measured in the laboratory*, which makes it a useful complement, without replacing the performance of the window. It is also a removable, portable solution, ideal for renters who cannot change their windows.
How many degrees can you gain with a thermal curtain?
The result depends on the curtain, the glazing and the installation. A high-density multi-layer curtain such as Kurtens curtains (620 g/m²) shows a temperature difference of up to 7 °C, measured in the laboratory between the glass surface and the room side*. This is not a guaranteed rise in room temperature. On single glazing, the effect is generally more noticeable.
More curtain tips
Explore our latest articles
