How does a thermal cold-proof curtain work? The science
An anti-cold thermal curtain works by creating a layer of still air between the cold window pane and the inside of the room. This layer of air trapped by the dense fabric acts as a natural insulator, exactly like the principle of double glazing. The heavier and multi-layered the fabric (620 g/m² at Kurtens), the more effective the barrier: up to 7 °C difference measured between the window surface and the room side of the curtain*.
This guide explains the science behind textile insulation, why a thick curtain is not enough, how density and multi-layer construction determine real performance, and under what conditions the thermal gain is maximized.
The physical principle: air gap, mass, and radiation
The thermal insulation of a curtain relies on three cumulative physical mechanisms:
The still air gap (the primary mechanism)
Still air is one of the best natural insulators (thermal conductivity: 0.026 W/m.K, compared to 1.0 for glass and 0.16 for brick). When a curtain is hung in front of a window, it traps a 5 to 15 cm layer of air between the pane and the fabric. This air gap significantly reduces thermal exchange, as heat is difficult to transfer through air that is not moving.
This is exactly the principle of double glazing: two panes separated by a layer of air (or argon gas) insulate much better than a single thick pane. The curtain reproduces this effect in front of the existing window.
The mass effect (the physical barrier)
A dense and heavy fabric absorbs part of the thermal energy instead of letting it pass through. The law of mass applies in thermics as in acoustics: the heavier the material, the more it inhibits transfers. A 620 g/m² curtain absorbs 3 to 4 times more energy than a 150 g/m² curtain.
Blocking infrared radiation
In winter, the cold window pane emits infrared radiation into the room, creating the sensation of a "cold wall" even in a heated room. A dense blackout curtain blocks this radiation and prevents the window from "cooling" the ambient air by radiation. This is what eliminates the sensation of cold when you approach the window with the curtain closed.
Why a thick curtain is not enough
The difference between a thick decorative curtain and a technical thermal curtain is not a matter of marketing. It is a measurable physical difference:
| Curtain type | Density | Construction | Thermal gain | Mechanism |
|---|---|---|---|---|
| Sheer curtain | 80-150 g/m² | 1 thin layer | 0-1 °C | No barrier, air passes through the fabric |
| Thick decorative curtain | 150-300 g/m² | 1-2 standard fabric layers | 1-2 °C | Slightly slows convection |
| Mid-range thermal curtain | 300-500 g/m² | 2-3 layers with lining | 2-4 °C | Air gap + moderate mass |
| High-performance thermal curtain | 500-620+ g/m² | 3+ technical layers | 5-7 °C | Air gap + high mass + IR blocking |
The difference between 1-2 °C and 5-7 °C is considerable in practice. A 1 °C gain is imperceptible. A 5-7 °C gain means the complete disappearance of the sensation of cold near the window and a measurable reduction in heating demand.
The critical threshold is 500 g/m². Below this, the fabric does not have enough mass to create an effective thermal barrier. Above this, the three mechanisms (air gap, mass, IR blocking) operate at full capacity.
The 3 layers of a technical thermal curtain
A high-performance thermal curtain uses layers with complementary functions, not just thicker fabric:
- Layer 1 (room side): dense decorative fabric. Absorbs the first heat waves and provides aesthetic appeal. Kurtens curtains offer 7 colors (grey, off-white, cream beige, mint green, green, lilac, blue), all with the same thermal performance*.
- Layer 2 (intermediate): high-density fleece. This is the core of the insulation. The fleece traps air within its fibers and creates an insulating mass. The denser it is, the more effective it is.
- Layer 3 (window side): technical membrane. Blocks infrared radiation from the cold window pane and prevents direct thermal transmission. This layer also ensures 100% blackout*.
Kurtens anti-cold thermal curtains combine these 3 layers for a total of 620 g/m², with three performances in a single fabric: up to 7 °C thermal gain*, 100% blackout*, and 22 dB sound attenuation*.
Under what conditions is the thermal gain maximized?
The 7 °C gain* is a maximum measured under optimal conditions. In real conditions, the gain depends on several factors:
The type of glazing
- Single glazing (Ug ~5.8 W/m².K): maximum gain. The pane is very cold, the difference between the pane and the room is significant. The curtain fully exploits its potential.
- Old double glazing (Ug ~2.8-3.0 W/m².K): significant gain. The pane is less cold but remains the thermal weak point of the wall.
- High-performance double glazing (Ug ~1.1-1.4 W/m².K): smaller but perceptible gain, especially to eliminate the cold wall effect and convection currents.
Curtain dimensions
A curtain that is too narrow or too short allows cold to pass through the sides and bottom. A curtain that does not extend 15 to 20 cm beyond each side of the window loses 30 to 40% of its thermal efficiency. The cold bypasses the fabric exactly like a draft bypasses an obstacle.
For maximum performance, the curtain should:
- Extend 15 to 20 cm beyond each side of the window
- Reach the floor (maximum 2-3 cm gap)
- Be placed 5-10 cm from the window pane (optimal space for the air gap)
This is why custom-made is superior to standard sizes: a curtain made to the exact dimensions of the window eliminates side leaks and exploits 100% of the insulating potential. Consult the size guide to measure correctly.
Outdoor temperature
The colder it is outside, the greater the difference between the window pane and the room, and the greater the impact of the curtain. On a -5 °C night, the gain is maximal. On a 10 °C day, the gain is smaller because the basic difference is lower.
Does the thermal curtain also work in summer?
Yes. The same physical principle applies in reverse. In summer, the sun-exposed window pane reaches 40 to 50 °C. The closed thermal curtain blocks solar radiation (60 to 80% of the heat) and prevents the hot window from heating the room by radiation and convection.
This is a decisive advantage compared to winter-only insulation solutions (film, secondary glazing): the curtain protects against cold in winter AND heat in summer. The environmental regulation RE2020 also imposes summer comfort requirements in new buildings, proving that summer overheating is a recognized problem. To delve deeper into this topic, consult our article room too hot in summer: 5 solutions.
Key takeaways
A thermal curtain works thanks to three physical mechanisms: the still air gap (natural insulator), the mass effect (dense fabric absorbs thermal energy), and the blocking of infrared radiation. Performance depends on density (minimum 500 g/m² for real gain) and multi-layer construction (3 layers with complementary functions).
Discover our custom-made anti-cold thermal curtains: 620 g/m², up to 7 °C thermal gain*, 100% blackout*, and 22 dB sound attenuation*. 7 colors, manufactured to exact dimensions, free delivery in 2 to 3 weeks. To compare with double glazing, consult our article thermal curtain vs double glazing.
*Data from tests conducted under optimal conditions.
Frequently Asked Questions
How does an anti-cold thermal curtain work?
A thermal curtain creates a layer of still air between the cold window pane and the room. Still air is an excellent natural insulator (conductivity 0.026 W/m.K). The dense fabric (620 g/m² at Kurtens) absorbs thermal energy and blocks infrared radiation from the window. Result: up to 7 °C difference between the window surface and the room side of the curtain*.
What is the difference between a thick curtain and a thermal curtain?
A standard thick curtain (150-300 g/m²) provides only 1-2 °C gain because its mass is insufficient to create an effective thermal barrier. A technical multi-layer thermal curtain (500+ g/m²) achieves 5-7 °C thanks to three layers with complementary functions (dense fabric + fleece + membrane). The critical threshold is 500 g/m²: below this, insulation is cosmetic.
How many degrees can be gained with a thermal curtain?
The gain depends on the density of the curtain and the type of glazing. Decorative curtain (150-300 g/m²): 1-2 °C. Mid-range thermal curtain (300-500 g/m²): 2-4 °C. High-performance thermal curtain (500+ g/m²): 5-7 °C. The gain is maximal on single glazing (very cold window) and smaller on recent double glazing, but perceptible in both cases.
Does a thermal curtain work in summer against heat?
Yes. The same physical principle applies in reverse: in winter the curtain prevents cold from the window from entering the room, in summer it blocks 60 to 80% of solar radiation and prevents the hot window (40-50 °C in full sun) from overheating the room. One curtain for both seasons.
Is the thermal curtain effective on double glazing?
Yes. Even recent double glazing (Ug 1.1 to 3.0 W/m².K) has a thermal resistance 3 to 7 times lower than an insulated wall (U 0.2 to 0.5 W/m².K). The window remains the thermal weak point of the dwelling. The curtain adds an additional layer of insulation, eliminates the cold wall effect, and reduces convection currents along the glazing.
Why should the curtain extend beyond the window?
A curtain that stops at the edge of the window allows cold to bypass the fabric through the sides and bottom. Cold air flows down the pane, passes under the curtain, and rises into the room. An overhang of 15-20 cm on each side and a floor-length drop close these passages. Without an overhang, the curtain loses 30 to 40% of its thermal efficiency.
What is the thermal conductivity of air?
Still air has a thermal conductivity of 0.026 W/m.K, making it one of the best natural insulators. For comparison: glass has a conductivity of 1.0 W/m.K (40 times more conductive), brick 0.16 W/m.K, and glass wool 0.035 W/m.K. This is why trapping a layer of still air between the window and the curtain is so effective: the air does the insulation work.
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