Which way to hang a thermal curtain for optimal insulation
A thermal curtain with a metallic coating should be installed with the technical side facing the window: this layer reflects 60 to 80% of infrared radiation, sending heat back into the room in winter and blocking solar heat in summer. Multilayer curtains without a metallic side (like Kurtens curtains at 620 g/m²) can be installed in either direction with no difference in performance.
The direction of installation is the most frequent question about thermal curtains, but it is not the factor that most influences insulation. The curtain's dimensions, density, and attachment method have a much greater impact on actual performance. This guide covers the installation rule, the consequences of an error, and why multilayer curtains eliminate the problem of direction.
The general rule: technical side towards 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 it be absorbed by the glazing. In summer, it reflects solar radiation before it can penetrate the room.
How to identify the technical side: it is smoother, more rigid, often silver or satin. The decorative side is softer, matte, with patterns or colors. Touch test: place your hand for 10 seconds on each side. The technical side remains colder because it does not retain body heat. If in doubt, consult the manufacturer's label.
This rule applies only to curtains with an attached lining or metallic coating. Multilayer curtains with integrated insulation work differently (see next section).
What happens when a thermal curtain is installed inside out
Installing a thermal curtain inside out (technical side facing the room instead of the window) reduces insulation performance by 40 to 60% according to studies by the Centre Scientifique et Technique du Bâtiment (CSTB) on textile protections. The reflective layer then sends the room's heat towards the window, which is the exact opposite of its function.
Measured consequences:
- Heat loss: the temperature between the glazing and the curtain drops by an additional 3 to 5 °C compared to correct installation, creating a cold zone against the window.
- Condensation and mold: this cold zone traps ambient moisture between the glass and the fabric, especially in winter when the indoor/outdoor temperature difference exceeds 15 °C. Mold appears on window seals and the bottom of the curtain within a few weeks.
- Fabric degradation: permanent humidity accelerates fabric aging and damages window seals in the medium term.
- Increased energy costs: the loss of thermal efficiency results in an estimated increase of 50 to 80 euros per year on the heating bill for a 15 to 20 m² room (estimate based on a 3 °C difference in continuous heating conditions).
Important point: a curtain installed inside out retains some of its insulating properties thanks to the fabric's mass, which slows down air movement. But the reflection effect, which represents the majority of the thermal gain of a coated curtain, is canceled out.
Single-layer vs. multi-layer curtains: the direction depends on the type
Not all thermal curtains work in the same way. The table below compares the two technologies and their sensitivity to the direction of installation.
| Criterion | Single-layer curtain (metallized lining) | Multilayer curtain (integrated insulation) |
|---|---|---|
| Insulation principle | Infrared reflection by the metallized side | Pockets of still air between fabric layers |
| Installation direction | Mandatory: technical side towards the window | Indifferent: both sides are equivalent |
| Risk of error | High: 40 to 60% performance loss | None: identical performance in both directions |
| Visual aspect | Silver side visible towards the window | Aesthetic textile on both sides |
| Treatment durability | Metallized coating degrades with washing and light exposure | Performance comes from the fabric structure, not a surface treatment |
| Typical thermal gain | 2 to 4 °C (varies depending on coating quality) | 5 to 7 °C (varies depending on density and number of layers) |
Multilayer curtains: when direction no longer matters
Multilayer 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: the air trapped between the layers is the best natural insulator. Performance does not depend on a reflective side, but on the integrated structure of the fabric.
With a multilayer curtain, the direction of installation is irrelevant. Both sides are textile, aesthetic, and decorative. No silver side to hide towards the window, no risk of installation error. The curtain is installed like a classic curtain, with the same thermal performance in both directions.
Durability is also superior: a metallic coating gradually degrades with washing and light exposure (10 to 20% loss of reflection after 2 years of daily use). A multilayer curtain retains its properties because performance comes from the fabric itself, not from a surface treatment.
Kurtens custom-made curtains are high-density multilayer curtains (620 g/m²) that combine three performances in a single curtain:
- Up to 7 °C thermal gain between the glazing surface and the room side*
- 100% certified blackout*
- 22 dB sound attenuation*
No metallic side, no risk of wrong direction, an aesthetic finish on both sides. Available as custom-made thermal curtains in the exact dimensions of your window.
What matters more than direction: curtain dimensions
The direction of installation influences the performance by 40 to 60% on a single-layer curtain, but the curtain dimensions influence the performance by 50 to 70% on all types of curtains. A thermal curtain that is too small allows cold air to pass through three leakage points, regardless of the direction of installation.
The 3 thermal leakage points of an undersized curtain:
- The sides: a curtain that is too narrow leaves side gaps where cold air flows down along the glazing, passes under the curtain, and rises into the room by convection. Solution: allow 15 to 20 cm overhang on each side of the window.
- The bottom: excessive clearance between the curtain and the floor creates a draft of cold air due to a chimney effect. Solution: the hem should lightly touch the floor, with a maximum of 2 to 3 cm clearance.
- The top: a curtain rod positioned too low leaves a space above the curtain through which warm air escapes. Solution: position the curtain rod at least 10 cm above the top of the window.
For windows with atypical dimensions, custom-made curtains guarantee optimal coverage without compromise. Consult the Kurtens size guide to determine the exact dimensions suited to your configuration.
To delve deeper into the subject, the complete guide to choosing your thermal curtain covers materials, number of layers, weight, and measured performance. And to compare opacity levels, consult the blackout vs. light-filtering vs. opaque comparison.
*Data from tests conducted under optimal conditions.
Frequently asked questions
Should the direction of the thermal curtain be changed between summer and winter?
No. For a metallized-lined curtain, the technical side remains facing the window all year round: it reflects indoor heat in winter and blocks solar heat in summer. For a multilayer curtain like Kurtens curtains, the direction is irrelevant regardless of the season: the thermal performance is identical in both orientations.
How do I recognize the thermal side of my curtain?
The thermal side is generally smoother, more rigid, and often silver or satin. The decorative side is softer and matte. Touch test: place your hand for 10 seconds on each side. The side that remains cool to the touch is the technical side (it does not retain body heat). If both sides have the same textile appearance, your curtain is probably a multilayer model, and the direction does not matter.
Is an improperly installed thermal curtain still effective?
Partially. The mass of the fabric blocks air movements and provides basic insulation, but the infrared reflection effect is canceled. The performance loss is estimated at 40 to 60% for a single-layer curtain with a metallic coating. Additional consequences: increased condensation between the glazing and the curtain, risk of mold on the seals, and an estimated heating cost increase of 50 to 80 euros per year for a 15 to 20 m² room. Multilayer curtains eliminate this risk because their performance does not depend on the direction of installation.
Can a thermal curtain be installed on an entrance door?
Yes. Entrance doors are the second point of thermal heat loss after windows, especially in older homes. The installation rule remains the same: technical side towards the door. To completely cover the frame without leaving side gaps, a custom-made curtain is particularly recommended. Allow 15 to 20 cm overhang on each side and a hem that lightly touches the floor (maximum 2 to 3 cm clearance) to maximize insulation.
Does a thermal curtain replace double glazing?
No, it complements it. A thermal curtain creates an additional barrier that reduces thermal exchanges between the glazing and the room. The gain is greater on single glazing (where the basic heat loss is high) than on double glazing. A Kurtens multilayer curtain provides up to 7 °C of thermal gain*, making it a significant complement even for double glazing. It is also the only removable and transportable insulation solution, ideal for renters who cannot modify windows.
How many degrees does a thermal curtain save?
The gain depends on the type of curtain and the quality of the glazing. A single-layer thermal curtain with a metallized lining generally provides 2 to 4 °C of gain between the glazing and the room. A high-density multilayer curtain like Kurtens curtains (620 g/m²) provides up to 7 °C of thermal gain*. On single glazing, the gain is more pronounced because the basic heat loss is higher. These measurements are taken by measuring the temperature at the glazing surface and on the room side, under optimal conditions.
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