Thermal blackout curtain: a guide to making the right choice
The thermal blackout curtain market is flooded with products labeled "thermal blackout" with no measured performance. Between a 25-euro curtain sold without a single figure and a 620 g/m² technical multi-layer curtain that blocks 100% of light and shows a temperature difference of up to 7 °C measured in the laboratory, the price gap is real, and so is the gap in construction.
This guide teaches you how to decode a product page in 5 minutes, how to test a curtain yourself, and what the price tells you (and does not tell you) about real performance. The goal: avoid the 3 most costly mistakes and invest in the right place.
The problem: "thermal blackout" means nothing in legal terms
In France, no standard defines what a curtain must block to be labeled "thermal blackout". A manufacturer can call a curtain "thermal blackout" when it lets some light through and its effect on the cold has never been measured, with no legal consequence. It is the same lack of regulation as for the term "blackout" on its own (which we explained in detail in our article blackout vs light-filtering vs opaque).
The result: on marketplaces and home decor sites, you will find "thermal blackout" curtains from 15 to 400 euros. 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 = just the thickness of the fabric. | 80-95% | Rarely measured | Rarely stated |
| 40-80 € | Lined curtain with light fleece. Moderate insulation, decent blackout. | 90-98% | Rarely measured | Rarely stated |
| 80-200 € | Technical multi-layer curtain. Genuine thermal insulation and structural blackout. | 95-100% | Varies by product | Varies by product |
| 150-400 € | High-density multi-layer curtain, made to measure. Triple performance, measured. | 100%* | Up to 7 °C* | Up to 22 dB* |
The 5 checks to make before you buy
Check 1: is the fabric weight stated?
Fabric weight (in g/m²) is useful information: it tells you how much the fabric weighs and gives an idea of its density. If the product page does not state it, ask for it. But fabric weight alone is not enough to predict performance: the construction and test results matter more.
- Light fabric, often coated: decorative curtain. Blackout varies with the coating, effect on the cold rarely measured.
- Lined fabric: mid-range curtain. Good blackout possible, thermal performance rarely quantified.
- Dense multi-layer with measured performance: high-performance curtain. Total blackout (100%), test results published. Kurtens curtains are 620 g/m², with a temperature difference of up to 7 °C measured in the laboratory.
Check 2: is the performance quantified?
A genuine thermal blackout curtain shows measured data: blackout percentage (ideally 100%), thermal gain in °C, and ideally sound attenuation in dB. If the product page simply says "thermal blackout" without a single figure, the performance has probably not been measured.
Wording that should put you on alert:
- "Excellent blackout" → exactly what %?
- "Good thermal insulation" → how many °C of gain?
- "Reduces noise" → how many dB measured?
- "Technical multi-layer fabric" → what materials in each layer?
Check 3: is the blackout structural or from a coating?
This is the most important technical distinction, and the one consumers know least about:
- Blackout from a coating: an opaque coating (acrylic, PVC) is applied to the back of the fabric. It blocks light at first, but cracks and peels after 10 to 20 washes (2 to 4 years of use). Blackout gradually drops from 100% to 80%. Typical of curtains under 60 euros.
- Structural blackout: light is blocked by a membrane built into the multi-layer construction of the fabric. It does not degrade with washing or use. Lifespan: 10 to 15 years. Typical of technical curtains over 80 euros.
How to tell: turn the curtain over. If the back is a smooth, uniform coating (often white or black), it is a coating. If the back is a technical fabric with a visible structure, it is structural blackout. To learn more, read our comparison thermal blackout vs simple blackout curtain.
Check 4: are the dimensions tailored or standard?
A thermal blackout curtain loses 30 to 40% of its effectiveness if it is the wrong size. Light and cold leaking at the sides, top and bottom cancel out part of the investment. Check that the curtain extends 15 to 20 cm beyond each side of the window and reaches the floor.
If no standard size fits your window, made to measure is the only way to guarantee full coverage. To measure correctly, see the size guide.
Check 5: is the price consistent with the stated performance?
A 620 g/m² multi-layer curtain costs more in raw materials than a light coated curtain: more fabric, technical materials, a more complex assembly. If a seller offers a "high-performance thermal blackout" curtain for 25 euros without a single measured figure, be wary of the label.
Price benchmarks:
- Basic blackout curtain, light coated fabric: 15 to 40 euros. Performance rarely measured.
- Lined blackout curtain, mid-range: 40 to 100 euros. Fine for light.
- Multi-layer thermal blackout curtain, high performance: 80 to 350 euros. Performance measured and published.
The 2-minute test you can do yourself
If you already have a "thermal blackout" curtain and you doubt its performance, here are 3 simple tests:
- Blackout test: close the curtain in broad daylight and hold a flashlight behind the fabric (window side). If you see a halo of light through the fabric, it is not a true 100%. A technical multi-layer curtain lets no light through, even with a flashlight pressed against the fabric.
- Thermal test: in winter, touch the room-side face of the curtain and the window-side face. If both faces are the same temperature, the curtain is not creating a thermal barrier. A genuine thermal curtain has a room side that is noticeably warmer than the window side.
- Weight test: weigh your curtain. A 150x230 cm curtain at 620 g/m² weighs about 2.1 kg. If your curtain of the same size weighs less than 1 kg, it is under 300 g/m²: a light fabric, rarely designed to act on the cold.
When do you need a thermal blackout curtain (and when is a simple blackout enough)?
| Your situation | Simple blackout is enough | Thermal blackout recommended |
|---|---|---|
| Guest room (occasional use) | ✅ Enough darkness, insulation not critical | |
| Room with shutters closed at night | ✅ Shutters already provide insulation | |
| Main bedroom (365 nights/year) | ✅ Sleep + temperature + noise every day | |
| Baby's room | ✅ Stable temperature (WHO: 18-20 °C) + blackout + quiet | |
| Home without shutters | ✅ The curtain replaces the shutter (blackout + insulation) | |
| Window facing a noisy street | ✅ Triple performance: light + cold + noise | |
| South/west-facing picture window | ✅ Blocks summer heat + blackout + acoustic |
Key takeaways
The "thermal blackout" curtain market mixes 15-euro products and 350-euro products under the same label. The 5 essential checks: fabric weight stated, performance quantified and measured (°C, %, dB), structural blackout (no coating), suitable dimensions (made to measure if needed), price consistent with the materials.
The Kurtens made-to-measure thermal blackout curtains pass all 5 checks: 620 g/m², 100%* structural blackout, temperature difference of up to 7 °C* measured in the laboratory, sound attenuation of up to 22 dB*, made to the dimensions of your window. 14 colors, free delivery in 2 to 3 weeks. Exact-Fit Guarantee (€29.90): free remake if you make a measuring error.
*Data from tests carried out under optimal conditions.
Frequently asked questions
How can I tell a genuine thermal blackout curtain?
Check 3 things: the fabric weight is stated, the performance is quantified and measured (% blackout, °C, dB of attenuation), and the blackout is structural (built-in membrane) rather than from a coating (a surface layer that degrades). If these 3 pieces of information are missing from the product page, it is probably a thick decorative curtain renamed "thermal blackout".
How much does a good thermal blackout curtain cost?
15-40 euros: basic curtain, light coated fabric, performance rarely measured. 40-100 euros: lined mid-range, fine for light. 80-350 euros: high-performance multi-layer, made to measure, 100%*, up to 7 °C*, up to 22 dB*. The price reflects the mass of fabric and the complexity of the construction. A "high-performance" curtain at 25 euros without a single measured figure deserves suspicion.
What is the difference between a coating and structural blackout?
A coating is an opaque layer (acrylic, PVC) applied to the back of the fabric. It blocks light at first but cracks after 10-20 washes (2-4 years). Structural blackout uses a membrane built into the multi-layer construction: it does not degrade with washing and lasts 10-15 years. To identify it: turn the curtain over. Smooth, uniform back = coating. Technical fabric back = structural.
Can a 25-euro thermal blackout curtain be effective?
For partial blackout, yes. For the cold, nothing proves it: at that price, the fabric is light and its thermal performance is almost never measured. The "thermal" label alone is not proof. Look for a multi-layer curtain whose test results are published.
How can I test whether my curtain is really blackout?
Close the curtain in broad daylight and hold a flashlight (or your phone's flash) directly against the fabric on the window side. If you see a halo of light coming through the fabric on the room side, your curtain is not a true 100% blackout. A technical multi-layer curtain lets no light through, even with a strong light source pressed against the fabric.
Does a thermal blackout curtain also block noise?
It depends on the construction. A light curtain is not designed to reduce noise. A 620 g/m² multi-layer curtain such as the Kurtens 3-in-1 reduces noise by up to 22 dB* according to the brand, under test conditions. If acoustics matter to you (street-facing bedroom, working from home), check that the attenuation in dB is published. The best "thermal blackout" curtains combine all 3 functions at no extra cost.
Do I need a made-to-measure thermal blackout curtain?
Made to measure is strongly recommended, because a curtain of the wrong size lets light in at the sides and cold in at the bottom. If one of the 4 standard sizes (100x150, 150x230, 250x230, 300x270 cm) fits your window with the recommended overlaps, it will do. Otherwise, made to measure fits the curtain to the dimensions of your window to cover the whole opening.
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