Are curtains an effective way to retain expensive heat? Our testing finds out.
When it’s time to replace your curtains, most of us only worry about cost and style. But did you know this choice can make a massive difference to how warm and comfortable your home will be over winter? We tested which types were most effective at reducing heat loss in your home.
This report is free thanks to funding from the Ministry of Health.
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If you have an insulated house, you can lose upwards of 45% of your heat through your windows. This drops to 30% in an uninsulated home, since it’s easier for heat to escape through the walls, ceiling and floors. This shows the importance of choosing wisely when it comes to your curtains or blinds; the right window coverings can save two-thirds of the heat lost through your windows.
We measured heat loss through an aluminium-framed single-glazed window fitted with different window coverings. The window was fitted to a mini-room inside our Thermal Comfort lab. The lab temperature was reduced to 4°C to simulate a chilly winter night, while an electric heater inside the mini-room beavered away to maintain a temperature of 20°C.
The different window coverings were tested for at least three hours and we measured the total power usage from the heater, along with the difference between indoor and outdoor temperature. These readings were then used to calculate how effective each window covering was at stopping heat escaping.
Cool air is denser (heavier) than warm air. When it’s cold outside, the inside air close to a window pane is cooled and tends to sink. As this cooled air sinks, it gets replaced by warmer air from other of the room. This creates a circulating air current that cools the room parts.
Curtains that aren’t sealed at the top or bottom to stop these air currents can make the situation worse by forming a channel between the window and curtain. This allows cooled air to flow continuously and chill the room faster.
We tested two types of curtains, thermal and heavy lined, cut to both sill and floor length. The thermal curtains were in a single drop and had a plastic coating bonded to the fabric. You might have expected the thermal curtains would perform better, but the extra layer of fabric in the pricier heavy lined curtains made them better insulators. If you’re getting curtains fitted, opt for floor-length as they keep in heat better than ones that sit at the sill.
We tested five types of blinds: honeycomb, roman, roller, and aluminium and wooden venetians.
Our blinds were installed within the window frame (with the exception of the romans), so there was no gap (like the one between the back of the curtain and the window frame) to allow a reverse chimney to form. Honeycomb blinds easily topped our testing for all window coverings. Air is a good insulator, as long as it’s not moving, and the honeycomb structure creates a large, still air gap between the cold window pane and the warm inside air. Also, the honeycomb blinds fitted closer to the sides of the window frame than our other tested blinds, which also helped reduce heat loss.
While their public baths may have gone out of fashion, roman blinds are still going strong. Roman blinds were the best window covering after the honeycomb blinds and secondary double-glazing options. A roman blind’s good performance comes down to the close fit it has over the window frame. This good seal, along with a close fit to the wall at the top, helps retain heat.
We also tested venetian and roller blinds. It seems obvious, but it makes a difference if you close the slats rather than leaving them open. Of the two types of venetians, wood was a better insulator than aluminium.
We also tested two types of secondary glazing: a 3M Window Insulator Kit and some acrylic, magnetic double glazing. The 3M film is available at most hardware stores and installation is a piece of cake. You start by taping it up over the frame, trimming it and then shrinking the plastic down using a hairdryer. However, it’s not without drawbacks. Once it’s on, you can’t take it down without destroying the sheet, so you’ll need to buy a new kit and restart the installation process again. Also, once it’s on, you can’t clean or open the window and, if any condensation forms on the window pane, you can’t wipe it off. The magnetic glazing does away with these drawbacks, since you can simply take them down whenever you want. However, they need to custom-made and installed, cost more and don’t perform as well.
We installed a wooden pelmet above the thermal curtain. This disrupted the airflow down behind the curtain, but only kept in an extra 2% of the heat over a floor length curtain. The improved performance probably isn’t enough to justify the cost of getting them installed, but if you like the look, or already have them in place, they’ll help out a little.
We also tried a couple of cheap “hacks”, which supposedly help curtains keep in the warmth better. One was rolling up a towel and placing it on top of the curtain rail, simulating a pelmet. This increased the performance of our thermal-backed curtain. In fact, the towel created a better seal at the top of the curtain than a regular pelmet and it interrupted the reverse-chimney effect. Our other hack was pinning a cheap polar fleece blanket up over the window frame behind the curtain. This had a positive impact on performance, but it’s not viable in the long run, unless you wanted to pin up a blanket each night.
We ran the numbers, based on our testing, to calculate what you’d save by fitting different window coverings over the winter. These figures just apply to our test room with its solitary 1.8m² window. Savings were worked out compared to an uncovered window from the beginning of June through to the end of August. We assumed an electric heater is used for six hours each night at the standard price of 26¢/kWh.
If you don’t mind the sight of a few rolled-up towels on top of your existing curtains, this free hack will buy you a tub of ice cream to celebrate the end of winter. The honeycomb blind saved the most– more than $10. You probably won’t save a fortune, especially compared to the cost of the curtains or blinds. However, the bigger savings aren’t necessarily monetary - you’d gain the benefits from living in a warmer and more comfortable home.
Our testing found any window covering is better than nothing. If you’ve got any uncovered windows, put something up. Even a cheap net curtain will help you save on your heating bill.
It might not be economical to rip down your existing curtains or blinds. However, if they’re looking a bit scruffy, your best bet is replacing them with honeycomb blinds.
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.
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.