Heavy vs. lightweight curtains: what's the real impact on insulation?

Comparative figures: performance by weight category

The critical jump is between 300 and 500 gsm. Below 300 gsm, insulation is cosmetic (1-2 °C, 5-7 dB). Above 500 gsm, it becomes real and measurable (5-7 °C, 18-22 dB). It is in this range that the curtain goes from "a little better than nothing" to "I no longer feel the cold / I no longer hear the street".

Why weight determines performance

For thermal insulation: mass traps air

Still air is one of the best natural insulators (thermal conductivity: 0.026 W/m.K). A dense fabric traps air within its fibrous structure. The heavier the fabric, the more fibers it contains, the more air it traps, and the better the insulation. A 620 gsm curtain contains 3 to 4 times more fibrous material than a 150 gsm curtain, hence 3 to 4 times more trapped air.

In parallel, the mass of the fabric itself creates a barrier against thermal transfer by conduction. The cold (or heat in summer) must pass through a greater thickness of material, which slows down transmission.

For sound insulation: the law of mass

The law of mass in acoustics, formalized by Berger (1911), states that every doubling of a wall's surface mass improves insulation by approximately 6 dB. Applied to curtains:

• 150 gsm → 300 gsm (doubling) = +6 dB
• 300 gsm → 600 gsm (doubling) = +6 dB additional

This explains why a 150 gsm curtain attenuates 5-7 dB and a 600 gsm curtain attenuates 18-22 dB: the weight has been multiplied by 4 (2 doublings), the theoretical gain is +12 dB, which corresponds to actual measurements.

For blackout: thickness blocks light

A thin, light fabric allows some light to pass through the fibers (you can see through it when held up to the sun). A dense, heavy fabric is too thick for light to penetrate. From 400-500 gsm, blackout reaches 95-100% without the need for chemical coating. The blackout is "structural" (due to the mass of the fabric) and not "chemical" (due to a coating that degrades).

The "pleats vs. weight" debate: what you need to know

Some guides claim that a light but heavily pleated curtain (fullness factor 2 to 2.5) can insulate as much as a heavy, flat curtain. This is partially true for acoustics, but overall misleading:

• Acoustics: pleats create a "labyrinth" that absorbs sound waves through multiple reflections. A light 200 gsm curtain that is heavily pleated can achieve 8-12 dB (instead of 5-7 dB flat). But a heavy 620 gsm curtain, even slightly gathered, reaches 18-22 dB*. Weight remains the dominant factor. Pleats improve a light curtain but do not transform it into a high-performance curtain.
• Thermal: pleats create pockets of still air between the undulations. Slight gain (0.5-1 °C better than a flat curtain). But the mass of the fabric remains the main factor. A pleated 200 gsm curtain will never achieve the 5-7 °C of a 620 gsm curtain.
• Blackout: pleats do not improve blackout. They even create micro-openings between the undulations through which light infiltrates. For blackout, a flat or slightly gathered curtain is superior to a heavily pleated curtain.

Verdict: pleats are a bonus, not a substitute for weight. Pleating a light curtain improves it a little. But the same energy (and the same budget) invested in a heavier curtain gives a significantly superior result.

When a lighter curtain is sufficient

A 620 gsm curtain is not necessary in all situations. Here are cases where a 300-500 gsm curtain does the job correctly:

• South-facing living room, no noise problem: if the goal is only to filter the sun in summer and keep some heat in winter, a 300-400 gsm curtain (2-4 °C gain) is sufficient.
• Guest bedroom (occasional use): for a few nights a month, a mid-range curtain at 50-100 euros is a good compromise.
• Window with already efficient shutters: if you close shutters every evening (blackout + insulation by the shutters), the curtain only needs to complement, not do all the work.
• Purely decorative sheer: if the goal is purely aesthetic (filtering light without blocking, ensuring privacy during the day), a 100-150 gsm sheer is perfect. No need for weight.

When weight is non-negotiable

• Bedroom facing a busy street: nocturnal noise (55-70 dB) requires a minimum attenuation of 18 dB for proper sleep. Only a curtain of 500 gsm and above reaches this threshold.
• Baby's room: infants wake up with 10-20 lux of light and are sensitive to sound peaks. 100% blackout and 18+ dB sound insulation require 500+ gsm.
• Accommodation without shutters: the curtain entirely replaces the function of the shutter (blackout + insulation). It must provide all the performance alone. 500+ gsm mandatory.
• Single glazing or old double glazing: basic heat loss is high. The curtain must compensate as much as possible. 500+ gsm for a gain of 5-7 °C*.
• Home cinema: complete darkness (less than 1 lux) requires a structurally 100% blackout curtain. Only a 500+ gsm multi-layer curtain guarantees this without coating.

Diminishing returns: beyond 620 gsm, is it worth it?

The law of mass predicts a gain of 6 dB for every doubling of mass. But the gains are diminishing in terms of usefulness:

• From 150 to 300 gsm (+6 dB): transition from 5-7 dB to 11-13 dB. This goes from "no perceived difference" to "noticeable improvement." Very significant useful gain.
• From 300 to 600 gsm (+6 dB): transition from 11-13 dB to 18-22 dB. This goes from "better" to "real comfort, comparable to a closed door." Significant useful gain.
• From 600 to 1,200 gsm (+6 dB): transition from 18-22 dB to 24-28 dB. A gain of 6 dB, but the curtain would weigh 10 kg per panel, be impossible to handle, and cost twice as much. Marginal useful gain relative to the constraints.

The equilibrium point is at 500-620 gsm: maximum performance achievable with a curtain that can be handled daily, at an affordable price, and compatible with standard curtain rods. This is precisely why Kurtens curtains are 620 gsm: the optimal point between performance and practicality.

Discover our custom curtains: 620 gsm, 22 dB sound attenuation*, 100% blackout* and up to 7 °C thermal gain*. 7 colors, made to exact dimensions, free delivery in 2 to 3 weeks. To choose the right criteria, consult our soundproof curtain buying guide.

*Data from tests conducted under optimal conditions.

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