Thermal blackout curtain: a guide to making the right choice
The thermal blackout curtain market is saturated with products labeled "thermal blackout" without any real measured performance. Between a €25 curtain that blocks 80% of light and provides a 1-2°C gain, and a multi-layer technical curtain at 620 g/m² that blocks 100% of light and provides a 5-7°C gain, the price difference is fivefold, but the performance difference is tenfold.
This guide teaches you how to decipher a product description in 5 minutes, how to test a curtain yourself, and how to understand what the price tells you (and doesn't tell you) about actual performance. The goal: avoid the 3 most costly mistakes and invest wisely.
The Problem: "Thermal Blackout" Has No Legal Meaning
In France, no standard defines what a curtain must block to carry the "thermal blackout" label. A manufacturer can sell a curtain that blocks 80% of light and provides a 1°C gain, calling it "thermal blackout" without legal consequence. This is the same lack of regulation as for the term "blackout" alone (which we detailed in our article blackout vs. dimout vs. opaque).
The result: on marketplaces and decor websites, you'll find "thermal blackout" curtains ranging from €15 to €400. This price range reflects radically different products:
| Price Range | What You Actually Get | Blackout | Thermal Gain | Acoustic |
|---|---|---|---|---|
| €15-40 | Thick polyester curtain + opaque coating. "Thermal" marketing = simple fabric thickness. | 80-95% | 1-2°C | 3-7 dB |
| €40-80 | Lined curtain with light fleece. Moderate insulation, decent blackout. | 90-98% | 2-3°C | 5-10 dB |
| €80-200 | Technical multi-layer curtain. True thermal insulation and structural blackout. | 95-100% | 3-5°C | 12-18 dB |
| €150-400 | High-density custom multi-layer curtain. Measured triple performance. | 100%* | 5-7°C* | 18-22 dB* |
5 Checks to Make Before Buying
Check 1: Is the weight indicated?
Weight (in g/m²) is the most reliable indicator of performance. If the product description doesn't indicate it, it's a red flag. A manufacturer with dense fabric will communicate it as an objective selling point.
- Less than 300 g/m²: thick decorative curtain. Partial blackout (80-95%), cosmetic thermal insulation (1-2°C).
- 300-500 g/m²: mid-range technical curtain. Good blackout (90-100%), moderate thermal insulation (2-4°C).
- 500 g/m² and more: high-performance curtain. Total blackout (100%), real insulation (5-7°C). Kurtens curtains are 620 g/m².
Check 2: Are the performances quantified?
A true thermal blackout curtain displays measured data: blackout percentage (ideally 100%), thermal gain in °C, and ideally sound attenuation in dB. If the product description simply says "thermal blackout" without any figures, the performance has probably not been measured.
Phrases that should alert you:
- "Very blackout" → how many % exactly?
- "Good thermal insulation" → how many °C gain?
- "Reduces noise" → how many dB measured?
- "Technical multi-layer fabric" → what materials in each layer?
Check 3: Is the blackout structural or by coating?
This is the most important and least known technical distinction for consumers:
- Blackout by coating: an opaque coating (acrylic, PVC) is applied to the back of the fabric. It blocks light initially but cracks and peels after 10 to 20 washes (2 to 4 years of use). Blackout gradually goes from 100% to 80%. Typical for curtains under €60.
- Structural blackout: light is blocked by a membrane integrated into the multi-layer fabric construction. Does not degrade with washing or use. Lifespan: 10 to 15 years. Typical for technical curtains over €80.
To tell: turn the curtain over. If the back is a smooth, uniform coating (often white or black), it's a coating. If the back is a technical fabric with a visible structure, it's structural blackout. To learn more, see our comparison thermal blackout vs. simple blackout curtains.
Check 4: Are the dimensions suitable or standard?
A thermal blackout curtain loses 30 to 40% of its effectiveness if it is poorly sized. Light and cold leaks on the sides, top, and bottom negate part of the investment. Ensure the curtain extends 15 to 20 cm beyond each side of the window and reaches the floor.
If no standard size matches your window, custom-made is the only way to ensure full coverage. To measure correctly, consult the size guide.
Check 5: Is the price consistent with the advertised performance?
A 620 g/m² multi-layer curtain costs more in raw materials than a 200 g/m² curtain with coating. It's physical: 3 times more fabric, technical materials, more complex assembly. If a seller offers a "high-performance thermal blackout" curtain for €25, the product cannot contain the necessary mass for real performance.
Price benchmarks:
- Basic thermal blackout curtain (150-300 g/m²): €15 to €40. Limited real performance.
- Mid-range thermal blackout curtain (300-500 g/m²): €40 to €100. Decent.
- High-performance thermal blackout curtain (500+ g/m²): €80 to €350. Measured triple performance.
The 2-Minute Self-Test
If you already have a "thermal blackout" curtain and doubt its performance, here are 3 simple tests:
- Blackout test: close the curtain in broad daylight and place a flashlight behind the fabric (window side). If you see a halo of light through the fabric, it's not a true 100%. A technical multi-layer curtain lets no light pass through, even with a flashlight pressed against the fabric.
- Thermal test: in winter, touch the side of the curtain facing the room and the side facing the window. If both sides are at the same temperature, the curtain does not create a thermal barrier. A true thermal curtain has a room side that is significantly warmer than the window side.
- Weight test: weigh your curtain. A 150x230 cm curtain at 620 g/m² weighs approximately 2.1 kg. If your curtain of the same dimensions weighs less than 1 kg, it is below 300 g/m² and its performance is limited.
When You Need a Thermal Blackout (and When a Simple Blackout is Enough)
| Your Situation | Simple Blackout is Enough | Thermal Blackout Recommended |
|---|---|---|
| Guest room (occasional use) | ✅ Sufficient darkness, non-critical insulation | |
| Room with shutters closed at night | ✅ Shutters already provide insulation | |
| Master bedroom (365 nights/year) | ✅ Sleep + temperature + noise daily | |
| Baby's room | ✅ Stable temperature (WHO: 18-20°C) + blackout + quiet | |
| Home without shutters | ✅ Curtain replaces shutter (blackout + insulation) | |
| Window facing noisy street | ✅ Triple performance: light + cold + noise | |
| South/west-facing bay window | ✅ Blocks summer heat + blackout + acoustic |
Key Takeaways
The "thermal blackout" curtain market mixes products from €15 and products from €350 under the same label. The 5 essential checks: indicated weight (minimum 500 g/m² for real performance), quantified performance (°C, %, dB), structural blackout (no coating), adapted dimensions (custom-made if necessary), price consistent with materials.
Kurtens custom thermal blackout curtains pass all 5 checks: 620 g/m², 100% structural blackout*, measured thermal gain up to 7°C*, sound attenuation 22 dB*, custom-made to the nearest centimeter. 7 colors, free delivery in 2 to 3 weeks. Exact-Fit Guarantee (€29.90): free remanufacture in case of measurement error.
*Data from tests performed under optimal conditions.
Frequently Asked Questions
How to recognize a true thermal blackout curtain?
Check 3 elements: the weight is indicated and exceeds 500 g/m², the performances are quantified (% blackout, °C gain, dB attenuation), and the blackout is structural (integrated membrane) not by coating (surface coating that degrades). If these 3 pieces of information are missing from the product description, it is probably a thick decorative curtain rebranded as "thermal blackout."
What is the price for a good thermal blackout curtain?
€15-40: basic thick curtain, 80-95% blackout, 1-2°C thermal gain. €40-100: mid-range lined, 90-98%, 2-3°C. €80-350: high-performance custom multi-layer, 100%*, 5-7°C*, 22 dB*. The price reflects the fabric mass and construction complexity. A "high-performance" curtain at €25 cannot physically contain enough material to perform.
What is the difference between coating and structural blackout?
Coating is an opaque layer (acrylic, PVC) applied to the back of the fabric. It blocks light initially but cracks after 10-20 washes (2-4 years). Structural blackout uses an integrated membrane in the multi-layer construction: it does not degrade with washing and lasts 10-15 years. To identify: turn the curtain over. Smooth, uniform back = coating. Technical fabric back = structural.
Can a thermal blackout curtain at €25 be effective?
For partial blackout (80-95%), yes. For thermal insulation, no. At €25, the curtain weighs 150-250 g/m², which is insufficient to create a thermal barrier (maximum 1-2°C gain, imperceptible). The "thermal" label is marketing. For a real thermal gain (5-7°C), a minimum of 500 g/m² is needed, which costs €80 and more in raw materials.
How to test if my curtain is truly blackout?
Close the curtain in broad daylight and place a flashlight (or your phone's flash) directly against the fabric on the window side. If you see a halo of light passing through the fabric on the room side, your curtain is not a true 100% blackout. A technical multi-layer curtain lets no light pass through even with a strong source pressed against the fabric.
Does a thermal blackout curtain also block noise?
It depends on the density. A 200 g/m² curtain only attenuates 3-7 dB (almost imperceptible). A 620 g/m² multi-layer curtain attenuates 22 dB* (street noise is reduced by four). If acoustics are important to you (bedroom facing the street, remote work), check that the dB attenuation is communicated. The best "thermal blackout" curtains combine all 3 functions without extra cost.
Do I need a custom thermal blackout curtain?
Custom-made is highly recommended because a poorly sized curtain loses 30 to 40% of its performance (light passing on the sides, cold infiltrating from below). If one of the 4 standard sizes (100x150, 150x230, 250x230, 300x270 cm) corresponds to your window with the recommended overlaps, it is suitable. Otherwise, custom-made to the nearest centimeter ensures complete coverage and maximum performance.
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