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Curtains

There’s a huge variety of curtains – and our testing shows some do little to help retain inside warmth. The way they’re installed is more important than the curtain type.

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The problem

Glass is a good conductor of heat. So in the winter when temperatures inside are higher than out, heat escapes through glass.

Double glazing is required for windows installed in most new builds and renovations. But for existing homes retrofitting double glazing is expensive and sometimes difficult – and it’s not an option for tenants. So are curtains an effective way to retain that expensive heat?

What we did

We measured heat loss through wooden- and aluminium-framed windows fitted with netting curtains, heavy lined curtains, and sill and floor-length ready-made thermal curtains. We also tested other window systems such as pelmets and secondary glazing. To do this we built an insulated wall inside a cooled chamber and installed first a wooden- and then an aluminium-framed window.

One side of the wall was cooled to -4°C and the other heated to +26°C. That’s a 30°C temperature difference across the window/wall assembly.

To get maximum and minimum heat loss reference values, we measured the energy required to keep the warm side at 26°C. We did this first with the window bare, and then with it covered with a thick layer of polystyrene foam on both sides.

For each window-curtain configuration we measured how much energy it took to keep the “warm” side at the controlled temperature. We logged the cold- and warm-side temperatures, power consumption, and time.

Reverse chimneys

Cool air is denser (heavier) than warm air. During cold weather, the air close to a room’s window pane is cooled and tends to sink. When the cooled air sinks it’s replaced by warmer air from other parts of the room. This creates a circulating air current which cools the room – and it isn’t desirable.

Curtains that aren’t sealed at top or bottom to stop these air currents can make the situation worse by forming a channel between the window and curtain for the falling cooled air.

Stopping that “reverse chimney” air current is crucial to improving window heat loss. It seems simple but our testing shows it’s not that easy.

What we found

For heat retention, secondary glazing was the most effective window system we tested. You don’t need a professional to apply secondary glazing – and kits such as the one we tested are inexpensive from DIY stores. Interestingly, previous testing has shown secondary glazing to be almost as effective as full double glazing

Surprisingly, old-fashioned net curtains were also effective at reducing heat loss. We think this is because the netting, which was installed close to the panes, interferes with downward air movement over the window. Net curtains are worth having – especially because they’re relatively cheap.

We tested ready-made sill and floor-length thermal curtains as well as floor-length custom-made heavy lined curtains. The floor-length curtains of both types were more effective than sill-length curtains, which weren’t a lot better than having no curtains at all. The heavy custom curtain worked better than the cheaper thermal curtains of both types. With both floor-length curtains, making sure the curtains touched the floor was important. Otherwise cooled air escaped out of the gap between the curtain and floor – increasing the overall heat loss.

For both types of frame we covered the window frame with a polar-fleece blanket and thermal curtains over the blanket.

Heat loss was substantially reduced. It proves it’s not the insulation qualities of the curtain material that matters; it’s stopping airflow around the window as much as possible.

Wood vs aluminium

Houses built before 1970 mostly have wooden frame windows. Later, aluminium frames took over.

Curtain-for-curtain, the aluminium-frame window lost more heat than the wooden frame equivalent. Wood is a better heat insulator than aluminium, so less heat is lost through the frame.

The secondary glazing kit worked well on aluminium frames – it achieved a heat retention value similar to what was achieved by the same type of kit fitted to the wooden frame.

But all the curtains performed poorly on aluminium frames compared with the equivalent wooden-frame results. With the thicker wooden frames, the curtains hung slightly closer to the window. The netting curtains in particular were recessed inside the wooden frame but this was not possible with the aluminium frames. The greater air-gap seems to have allowed a freer flow of air behind the curtain – and so there was increased heat loss.

Pelmets

We installed a pelmet above the curtains on both the wooden- and aluminium-framed windows to see if there was decreased heat loss. There was – but not by much. Air still leaked from under the curtains. We also think it was leaking over the top of the curtain (under the pelmet) and around the sides of the curtain at the top.

Our conclusion is that pelmets help – but they’re not a complete answer.

Using a fan

During some of our test runs we ran a small desk fan in the test chamber – and in those runs we noted a significant reduction in heat loss compared with the runs where there was no fan. It appears the turbulently tumbling air from the fan interferes with establishment of “reverse chimney” airflows.

If you have a heating appliance that doesn’t have an inbuilt fan (such as a woodburner or some electric heaters), using a separate fan should help with heat retention.

Report by Bill Whitley.

Our tips

3 things matter when it comes to heat retention.

Sealing the curtains effectively
The material a curtain is made from is much less important than “sealing” the curtain so that air movement is stopped. Heat is lost, not so much through curtain fabric, but by air moving between curtain and window.

  • Mount curtains as close as possible to the window frame.
  • Position nets as close to the window as possible.
  • If a pelmet is fitted, minimise any air-gap between the top of the curtain and the pelmet.
  • Make floor-length curtains touch the floor.
  • Make curtains a generous width, so they overlap the window frames at the sides.

Using net curtains
The textured surface of net curtains appears to interfere with the “reverse chimney”. In our testing, nets also worked better on wooden frames than on aluminium. The nets on the wooden-frame windows were mounted so the netting was in contact with the top of the window frame – and we think this contributed to the results we observed.

Using a fan
Our testing shows that using a fan significantly reduces heat loss. If your heating appliance doesn’t have an inbuilt fan, look at buying a separate fan for the room.

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