How to Choose Your Thermal Curtain: The Expert Guide 2026
To choose a truly effective thermal curtain, five criteria make the difference: fabric density (minimum 500 g/m² for real performance), the presence of a multi-layer insulating lining, measured thermal performance (gain in °C or R-value), adaptation to the exact dimensions of your window, and dual seasonality (cold AND heat protection).
A quality thermal curtain, correctly sized, can increase the temperature by up to 7°C between the glazed surface and the interior of the room*. However, the majority of curtains sold online as "thermal" are merely thick curtains with no real insulating performance. This guide provides you with the keys to differentiate and invest in the right product.
What is a thermal curtain and how does it work?
A thermal curtain acts as an insulating barrier between the cold surface of your window and the warm air inside your home. Windows account for 10 to 15% of heat loss in a home according to ADEME, even with double glazing. The thermal coefficient Ug of a double-glazed window (1.1 to 3.0 W/m²K) remains 3 to 7 times higher than that of an insulated wall (0.2 to 0.5 W/m²K). The curtain blocks the circulation of cold air by trapping a layer of still air between the fabric and the glass.
Performance relies on multi-layer construction. A simply thick curtain (150-300 g/m²) slightly slows down thermal exchanges, with a gain of 1 to 2 °C. A true multi-layer thermal curtain (500 g/m² and more) incorporates specific technical layers: outer fabric, high-density fleece, insulating membrane. The gain reaches 5 to 7 °C*.
What many consumers ignore: a thermal curtain also works in summer. The same physical principle applies in reverse; the curtain blocks external heat transmitted by the sun-exposed glass. The RE2020 environmental regulation, which came into force in 2022, reinforces summer comfort requirements in new buildings, but older homes do not benefit from this. The thermal curtain is one of the few solutions that addresses thermal comfort in both summer and winter without requiring renovations.
The 5 criteria for choosing a truly effective thermal curtain
Criterion 1: Fabric density (minimum 500 g/m²)
Fabric weight is the first indicator to check. The law of mass in thermal insulation is the same as in acoustics: the denser the fabric, the more it traps still air in its structure, and it is this still air that insulates. Here are the performance thresholds by density:
- 150 to 300 g/m² (standard thick curtain): thermal gain of 1 to 2 °C. Cosmetic effect, no real insulation.
- 300 to 500 g/m² (mid-range thermal curtain): gain of 2 to 4 °C. Noticeable but limited improvement.
- 500 g/m² and more (high-performance thermal curtain): gain of 5 to 7 °C. Real insulation, elimination of the cold wall effect.
Kurtens curtains use a high-density fabric of 620 g/m², specifically selected for its insulating properties. Thermal performance is integrated from the design stage, not added as an afterthought to a decorative fabric.
Criterion 2: Insulating lining, the true technical differentiator
The lining is what separates a real thermal curtain from a simply thick curtain. A technical lining acts as a thermal reflector: it reflects interior heat back into the room in winter and blocks solar radiation in summer. Without an insulating lining, a curtain only slows down exchanges; it does not block them.
Always check the composition indicated by the manufacturer. If the product sheet mentions "blackout fabric" without detail on the thermal lining, the actual performance will be limited. Look for terms such as: "multi-layer," "insulating membrane," "technical thermal lining."
Criterion 3: Measured performance (gain in °C or R-value)
This is the most objective criterion and one that most sellers avoid communicating. A thermal curtain worthy of the name should display measured performance: either a temperature gain between the glass and the room, or an R-value (thermal resistance, expressed in m²K/W).
For reference, the thermal resistance of a curtain is between R 0.1 (standard thick curtain) and R 0.4-0.6 (high-performance multi-layer curtain). By comparison, a double-glazed window has a thermal resistance of approximately R 0.3 to 0.9. A good thermal curtain therefore adds 30 to 60% additional thermal resistance to your window.
Kurtens curtains offer up to 7 °C of measured thermal gain*. Concretely: if the temperature at the contact of your window is 8 °C on a winter evening, it will be 15 °C on the room side behind the curtain. This is the difference between a permanent cold sensation near the window and real comfort.
If a manufacturer does not communicate any measured data, ask yourself: why? Technical performance, when it exists, is a powerful selling point. Its absence is rarely an oversight.
Criterion 4: Dimensions, why custom-made is superior for insulation
A thermal curtain only works if it fully covers the window, with no gaps on the sides, top, or bottom. Every uncovered centimeter is a thermal bridge through which cold air infiltrates. A standard-sized curtain (widths 140, 240, or 300 cm) systematically leaves gaps: a 137 cm window with a 140 cm curtain has 1.5 cm of gap on each side. The heat loss due to these lateral leaks is estimated at 1 to 2 °C.
This is why custom-made thermal curtains offer significantly superior insulation. At Kurtens, curtains are made to the exact dimensions of your window, up to 270 cm in height. The fabric covers the entire opening, eliminates lateral thermal bridges, and ensures ground contact that blocks cold air circulation.
Criterion 5: Dual seasonality, cold AND heat protection
An often-overlooked criterion: a good thermal curtain is not only useful in winter. In summer, south or west-facing windows turn your interior into a greenhouse. Solar radiation through a double-glazed window can raise a room's temperature by 5 to 10 °C in a few hours. A quality thermal curtain blocks this radiation and significantly reduces temperature rise.
Check that the manufacturer mentions this dual function. If the curtain is only presented as an "anti-cold" solution, it probably lacks the technical layer that blocks radiation in summer.
Summary table: the 5 criteria to check
| Criterion | Minimum threshold | Optimal performance | Warning sign |
|---|---|---|---|
| Fabric density | 500 g/m² | 600+ g/m² | "Thick curtain" with no grammage specified |
| Lining | Technical multi-layer | 3+ layers (fabric + fleece + membrane) | "Blackout fabric" without technical detail |
| Measured performance | Communicated °C gain | 5-7 °C or R 0.4+ | No numerical data |
| Dimensions | Full window coverage | Custom-made to the nearest cm | Standard sizes only |
| Dual seasonality | Cold + heat protection | Confirmed solar radiation blocking | Presented as "anti-cold" only |
Thermal curtain vs. blackout curtain: the confusion to avoid
This is one of the most widespread confusions in the market. A blackout curtain blocks light. A thermal curtain insulates temperature. These are two distinct functions that rely on different technologies.
A blackout curtain uses an opaque coating or a very dense fabric to prevent light from passing through. It can offer slight thermal insulation due to mass effect (1 to 2 °C), but this is not its primary function. Conversely, a thermal curtain is designed to block heat transfer, which does not necessarily imply blocking light.
The problem: many e-commerce sites sell blackout curtains under the name "thermal blackout curtain" without any real insulation data. The term "thermal" is used as a marketing argument. How to recognize a true thermal curtain? Look for numerical performance data (gain in °C, R-value) and a precise description of the technical layers.
The most judicious choice: a curtain that truly combines both performances. Kurtens curtains offer 100% blackout* AND up to 7 °C thermal gain* AND 22 dB sound attenuation* in a single custom-made thermal and soundproof blackout curtain. Each performance is measured, not just claimed.
Thermal curtain vs. double glazing: complement or alternative?
A thermal curtain does not replace double glazing. These are two complementary solutions. Double glazing reduces transfers through the glass (Ug from 1.1 to 3.0 W/m²K). The curtain creates an additional air barrier between the glass and the room (thermal resistance R from 0.3 to 0.6 m²K/W).
Even with recent double glazing, your windows remain the thermal weak point of your home: thermal resistance 5 to 10 times lower than an insulated wall. Adding a custom-made thermal curtain significantly improves the overall insulation of the room.
Cost comparison: a custom-made thermal curtain costs between 100 and 350 euros depending on the dimensions. Replacing a window with double glazing starts at 500 euros and easily exceeds 1,500 euros including installation, not to mention the lead times and work involved. A thermal curtain can be installed in 5 minutes and produces an immediate effect.
If you have double glazing, the curtain optimizes your existing insulation. If you have single glazing (Ug 5.8 W/m²K) and replacement is not planned, the thermal curtain is the most cost-effective solution.
For which room should you choose a thermal curtain?
Bedroom: insulation and blackout for quality sleep
The bedroom is the room where the thermal curtain has the most daily impact. The WHO recommends a temperature of 16 to 18 °C for sleeping. A thermal curtain maintains this stable temperature overnight without overheating. Combined with blackout (less than 5 lux recommended for sleep), it guarantees optimal rest conditions. Particularly critical for baby rooms, where maintaining a constant temperature is essential.
Living room and large openings: the thermal weak point
Bay windows and large openings are the surfaces in a home with the highest heat loss. A 3 m² bay window with standard double glazing (Ug 2.8) loses as much heat as 10 to 15 m² of insulated wall. A wide thermal curtain, sized exactly to your bay window, transforms this heat-losing surface into an insulated area. It is often in the living room that the gain in comfort is most noticeable.
Office and telecommuting: thermal comfort without additional heating
Working from home has created a new need: to stay comfortably seated near a window for hours. Without a thermal curtain, the proximity of a cold window (8-12 °C in winter) generates a feeling of discomfort even in a room heated to 20 °C. The thermal curtain eliminates this cold wall effect and brings the room-side surface temperature to 15-17 °C, allowing comfortable work without increasing heating.
Entrance door: the No. 1 draft in the house
The entrance door is often the primary source of drafts. Seals age, frames deform, and cold air infiltrates. A thermal curtain installed in front of the door creates an immediate barrier. This is a particularly effective solution in old buildings where the stairwell is not heated. For this use case, consult our article on soundproof curtains, as entrance doors often combine thermal and sound problems.
*Data from tests performed under optimal conditions.
Frequently Asked Questions
Does a thermal curtain really make a difference in temperature?
Yes, provided you choose a curtain with real insulating performance. A high-density thermal curtain (500 g/m² and more), custom-sized, can create a gain of 5 to 7 °C* between the glass and the room. A standard thick curtain (150-300 g/m²) offers only 1 to 2 °C. The difference is immediately felt: disappearance of the cold sensation near windows, reduction of drafts, more homogeneous temperature in the room.
What is the difference between a thermal curtain and an insulating curtain?
In common usage, both terms refer to the same type of product: a curtain designed to limit temperature transfer through windows. "Thermal insulating curtain" and "thermal curtain" are synonymous. In contrast, a "sound insulating curtain" refers to a product focused on sound insulation. For this distinction, consult our soundproof vs. acoustic guide.
Does a thermal curtain also work in summer against heat?
Yes. A quality thermal curtain insulates in both directions: it blocks cold in winter and solar radiation in summer. A south or west-facing window can raise a room's temperature by 5 to 10 °C in a few hours due to the greenhouse effect. The thermal curtain blocks this radiation and significantly reduces temperature rise, limiting the need for air conditioning.
How to recognize a quality thermal curtain?
Three elements to check: measured and communicated performance (temperature gain of 5 to 7 °C, or R-value of 0.4 to 0.6), a precise description of the multi-layer composition (minimum 3 technical layers, density 500 g/m² and more), and dimensions adapted to your window (custom-made or wide choice of sizes). If the manufacturer provides no numerical data, the curtain is probably a standard product with a marketing claim.
Standard or custom-made thermal curtain: what is the impact on insulation?
The impact is significant: an additional 1 to 2 °C gain for custom-made. A standard curtain that does not perfectly cover the window creates thermal bridges through which cold air infiltrates (sides, top, bottom). Custom-made ensures complete coverage of the opening and optimizes insulation. At Kurtens, each curtain is made to exact dimensions, up to 270 cm in height.
Can blackout, thermal, and soundproof features be combined in a single curtain?
Yes. Kurtens technical curtains combine 100% blackout*, up to 7 °C thermal gain*, and 22 dB sound attenuation* in a single custom-made curtain, with a density of 620 g/m². This combination avoids stacking multiple curtains at the window and simplifies installation while offering three measured performances.
Do I need a thermal curtain if I already have double glazing?
Yes. Even recent double glazing (Ug 1.1 to 3.0 W/m²K) has a thermal resistance 5 to 10 times lower than an insulated wall (U 0.2 to 0.5 W/m²K). Your windows remain the thermal weak point of your home. The curtain adds an additional layer of insulation (R 0.3 to 0.6) that complements double glazing. The gain is particularly noticeable for large glazed surfaces and windows exposed to the north or prevailing winds.
From what weight is a curtain truly thermal?
Below 300 g/m², the thermal effect is cosmetic (1 to 2 °C). Between 300 and 500 g/m², the improvement is noticeable but limited (2 to 4 °C). From 500 g/m² with a multi-layer construction, the thermal gain becomes significant (5 to 7 °C). The most performing models exceed 600 g/m². Grammage alone is not enough: the multi-layer structure (minimum 3 technical layers) is essential to achieve superior performance.
Choosing the right thermal curtain: our recommendation
A thermal curtain is a concrete investment in the daily comfort of your home, provided you choose the right product. The five criteria in this guide (density 500+ g/m², multi-layer lining, measured performance, custom-made, dual seasonality) allow you to distinguish a truly performing curtain from an ordinary curtain sold under a marketing label.
Discover our custom-made thermal curtains: 620 g/m², up to 7 °C measured thermal gain*, made to your exact dimensions in 1 to 2 weeks, available in 7 colors. And to combine all three performances, our custom-made blackout thermal and soundproof curtain combines 100% blackout*, 7 °C thermal gain*, and 22 dB sound attenuation*.
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