Glass curtains or insulated curtains:which keeps the heat in

Insulated glass curtains keep the heat in far better. Single glass has a U-value of about 5.7 W/m²K, while double glazing with an air gap is about 2.9, almost half. A low-e coating brings it down to 1.7. Double glazing pays off if you heat the terrace in winter; for spring to autumn use, single glass is usually enough.

The difference is all in the glass. Standard glass curtains use one pane of toughened glass per panel: they stop wind and rain, but a single pane does very little to hold back heat. Insulated glass curtains have two panes in every panel with a layer of dry air between them, and that layer is what cuts the heat loss. Both types open the same way and stack neatly to one side.

One point people often miss: the glazing is only part of the enclosure. Heat also escapes through the joints between panels, through the aluminium tracks and, above all, through an uninsulated roof. A terrace with double glazing under a thin metal roof can end up colder than you would expect, so it is worth looking at the whole space before choosing what goes in the frames.

Glazing is compared by its thermal transmittance, the U-value: how many watts pass through one square metre for every degree of difference between inside and out. The lower the figure, the warmer the pane. The reference table published by the Castilla-La Mancha regional government for window replacement gives 5.7 W/m²K for single glass, 2.9 for a 4/12/6 double glazed unit and 1.7 for the same unit with a low-e coating.

Now picture a January night in Alicante. With 20 °C inside and 6.3 °C outside, the average January minimum in the AEMET climate normals for Alicante, the difference is 13.7 degrees. Each square metre of single glass then loses about 78 watts, double glazing about 40 and low-e double glazing about 23. On a terrace with ten square metres of glass, switching from single to double glazing saves around 380 watts of heating for as long as the cold lasts.

It depends on when you use it. Alicante winters are mild: in the AEMET series for 1981 to 2010, January averages 11.7 °C, with highs of 17 °C and lows of 6.3 °C. On a sunny day an enclosed terrace warms up on its own even behind a single pane, and the city gets around 2,851 hours of sunshine a year. The trouble starts at dusk, when a single pane gives that warmth back to the night air within an hour or so.

Spain's building code, the Código Técnico de la Edificación, places the city of Alicante in climate zone B4, as the Normatia technical sheet for Alicante shows. The letter B stands for a mild winter and the 4 for the hottest summer band on the scale. In practice the night chill matters if the terrace is your winter dining room, while in summer what counts is ventilation and shade rather than insulation.

Double glazing makes sense when the terrace becomes another room that you will heat. It also helps on a busy street, because two panes damp noise better than one, and on penthouse enclosures high up, where the winter wind chills the panes quickly. These are the cases where it usually earns its keep:

If you mostly use the terrace from March to November, and in winter only step out to hang the washing or sit in the midday sun, a standard curtain is usually enough. Before paying for double glazing you will not make use of, check the roof over the terrace first: a poorly insulated roof loses more heat than any glazed wall, so that is often the better place to spend.

• A winter dining or living room with a heater or heat pump
• A terrace on a noisy street or near bars
• A top floor penthouse exposed to the north wind
• A restaurant terrace that stays open in January and February
• A porch you want to bring into the house

An enclosed terrace loses heat in five places at once: the glass, the joints between panels, the aluminium tracks and profiles, the roof and the floor. The same Castilla-La Mancha table gives a metal frame without a thermal break 5.7 W/m²K, exactly the same as a single pane, and one with a break wider than 12 mm 3.2. Without that break, aluminium lets the cold through just as readily as plain glazing does.

Glass curtains have no vertical profiles, so there is little aluminium in the system, but the joints between panels matter a great deal: once a seal wears, cold air gets in even through double glazing. On a winter terrace it pays to check the seals and spare parts every season and to think about the roof, which on a penthouse is usually the biggest surface. Which roofs and seals LUNEKS pairs with insulated glass curtains

As a rule, yes. Water vapour condenses on any surface colder than its dew point. On a January night a lone pane gets very close to the outdoor temperature, so with a heater on, cooking or a crowd inside, it soon runs with droplets. In a double glazed unit the inner pane stays warmer, because the air gap separates it from the cold outside, and much less moisture settles on it.

Condensation on the inner face never disappears completely, but a few minutes of airing each day keeps it in check. Droplets inside the unit, between the two panes, are a different matter: they mean the edge seal of the double glazing has failed, as the Wikipedia article on insulated glazing explains, and that unit needs inspecting or replacing because it will not dry out by itself.