Secondary Glazing or Thermal Curtains: What to Choose to Insulate
Secondary glazing adds a second pane to your existing window for a permanent insulation gain at the glass, while a thermal curtain places a dense barrier on the inside that you draw or open at will. Secondary glazing is a semi-permanent modification, mostly useful in winter; the curtain is reversible, also works against summer heat, blocks light and dampens noise, without touching the window.
The Kurtens 3-in-1 curtain, 620 gsm, holds back up to 7 °C of difference* and blocks 100% of light. Hung on a simple rod, it is fitted and removed with no works, where secondary glazing is fixed durably onto the frame.
Secondary glazing: how it works, and its limits
Secondary glazing means fixing a second pane, of glass or plastic, onto an existing window, without changing the frame. The air gap created between the two improves the thermal insulation of old windows. Professionals claim a reduction in heat loss of around 40% under optimal conditions, for a price generally between 90 and 450 euros per window depending on the frame material.
Its limits are concrete. It is a semi-permanent modification of the frame, which often hinders opening the window, brings neither blackout nor real acoustic comfort, and is above all a winter solution against the cold. On a window you cannot or do not wish to modify, when renting in particular, secondary glazing is not ideal.
Comparison table: secondary glazing or thermal curtains
Both insulate the window, but one acts permanently at the glass, the other modulates on the inside.
| Criterion | Secondary glazing | Kurtens thermal curtain |
|---|---|---|
| Winter insulation | Good, permanent | Good, up to 7 °C difference* |
| Summer protection | Low | Yes, blocks radiation |
| Blackout and acoustic | No | 100% blackout, dampens noise |
| Window modification | Yes, fixed to the frame | None |
| Reversible | Hardly | Yes, on a rod |
| Fitting | DIY or pro | Yourself, no works |
*Data obtained under optimal test conditions. The secondary-glazing percentages are orders of magnitude communicated by the market.
Reversible versus semi-permanent: the deciding argument
This is the real difference in use. Secondary glazing is fixed onto the frame and is not designed to be removed with the seasons: it stays in place, and taking it down requires work. The thermal curtain, by contrast, hangs on a rod and comes off whenever you want, leaving no trace.
This reversibility is decisive when renting, where you cannot modify the windows, and on old or wooden windows you do not want to drill or weigh down. The curtain goes with the home, moves house, and changes with the decor.
Summer and versatility: where the curtain takes the lead
Secondary glazing is calibrated for winter, against the cold. It does little against the summer heat that comes in through solar radiation. A dense thermal curtain, on the other hand, blocks that radiation in summer and limits losses in winter, on top of blocking light and dampening noise. For the full anti-heat strategy, see our guide on how to cool a room without air conditioning. The French agency ADEME also recalls that protecting your openings from the sun is the most effective summer measure, detailed in its summer comfort guide.
How much does each solution cost
Secondary glazing represents a cost per window, generally 90 to 450 euros depending on the frame, sometimes plus a professional fitting. The made-to-measure thermal curtain costs the price of a curtain, fitted yourself, and fills several roles at once: thermal in summer and winter, blackout, acoustic and decor. At a comparable budget, the curtain covers more needs and can be moved elsewhere.
When secondary glazing makes sense, when the curtain wins
Secondary glazing makes sense on an old single-glazed window you keep transparent for the view, when the goal is only to reduce winter heat loss and modifying the frame is no problem. The curtain wins as soon as you want a reversible solution, with no works, that also acts in summer, blocks light, dampens noise and dresses the window, whether renting or not. The two can be combined on a window very exposed to the cold.
To compare with fully replacing the window, see also thermal curtains vs double glazing.
The Kurtens thermal curtain
The Kurtens 3-in-1 curtain insulates the window without modifying it. It combines three technical layers, weighs 620 gsm, holds back up to 7 °C of difference* and blocks 100% of light. Cut to measure to the exact dimensions of the window, it avoids leaks on the sides, hangs on a rod with no drilling, and stays fully reversible. It works in winter against the cold and in summer against the heat, where secondary glazing is limited to winter. Configure yours in the custom thermal curtains collection.
Frequently asked questions
Does a thermal curtain insulate as well as secondary glazing?
Both reduce heat loss through the window, by different means: secondary glazing permanently at the glass, the curtain as soon as it is drawn. A 620 gsm thermal curtain holds back up to 7 °C of difference*. The curtain also adds summer protection, blackout and noise reduction.
Is secondary glazing reversible?
Hardly. It is fixed onto the window frame and is not designed to be removed across the seasons; taking it down requires work. The thermal curtain, hung on a rod, comes off and goes back freely, which makes it better suited to renting.
Which solution to insulate a window in a rental?
The thermal curtain is the most suitable: hung on a rod, with no drilling or modification of the frame, it is fitted and removed without altering anything. Secondary glazing modifies the window, which makes it of little use when you are not the owner.
Does secondary glazing protect from summer heat?
Little. Secondary glazing is designed to reduce winter heat loss, not to block summer solar radiation. A dense thermal curtain intercepts that radiation at the window and works both ways, summer and winter.
Can secondary glazing and a thermal curtain be combined?
Yes, on a window very exposed to the cold, secondary glazing improves insulation at the glass and the curtain adds an adjustable barrier on the inside, with blackout and acoustics on top. But for most homes, the thermal curtain alone already brings clear comfort.
Is a made-to-measure thermal curtain more effective?
Yes. A curtain that is too narrow lets air leak on the sides and at the bottom. Cut to measure to the exact dimensions of the window, the curtain covers the whole opening and removes those thermal bridges, which clearly improves real insulation.
Key takeaways
Secondary glazing offers a permanent insulation gain at the glass, mostly in winter, but it is a semi-permanent modification of the window, with no blackout or acoustics. The thermal curtain is reversible, also works against summer heat, blocks light, dampens noise and fits with no works. Secondary glazing suits an old window you keep transparent, the curtain those who want a flexible, versatile solution. The Kurtens 3-in-1 curtain, 620 gsm and up to 7 °C of difference*, is cut to your exact dimensions. Configure yours in the custom thermal curtains collection.
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