Curtains and blinds

Are curtains an effective way to retain expensive heat? Our testing finds out.

living room interior

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.

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This report is free thanks to funding from the Ministry of Health.

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The problem

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.

Our test

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.

Reverse chimneys

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.

Curtains

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.

Which blinds are best?

Honeycomb blinds

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.

Secondary double glazing

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.

Do pelmets or low-cost tricks help?

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.

What savings can be made?

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.

Summary

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.

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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Member comments

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Lynnette J K.
12 Sep 2019
Net curtain fitting

Should my nets be touching the glass, or leaving a small gap between them and the glass? (I will be fitting from very top of window to rest on the sill)

Ernest & Jenny C.
07 Jun 2019
Honeycomb blind

What size did you test? I believe it is available in 10 mm & 20 mm sizes

Consumer staff
12 Jun 2019
Honeycomb blind

Hi Ernest & Jenny C
We tested the 20mm sized honeycomb blinds.
Kind regards
James le Page - Technical Writer.

Madeleine F.
06 Jun 2019
polar fleece curtains

I lined the curtains with polar fleece blankets from the Warehouse only $10 for queen size. They come in a variety of colours. It has made a huge difference to heat loss. On some windows and doors I just use the blankets which are just the right length. Highly recommend as a cheap way to keep heat in. You can just cut holes in blanket and thread curtain wire through and hook up if you don't have curtain tracks.

Dave K.
05 Jun 2019
3M plastic sellotape fixed heat saver.

I put this plastic sheet on our windows on the south side of the house last winter.The glazing is lead light with each sash having approx 31 diamond shaped pieces of glass and I was amazed, to say the least, at the difference it made to the warmth and dryness.
Every winter,previously, we had pools of water on the sills that had to be sponged off.Now the sills are dry and we get no condensation on the glass.
I was highly sceptical about this product before I tried it,now I would recommend it to anyone wanting a cheap fix.

Oneironaut
03 Jun 2019
(1) What about shutters? (2) Not all honeycomb blinds offer equal thermal insulation

It would be helpful if you could expand this report to include information on the thermal performance of wooden shutters. They are expensive, but they are very popular nowadays, and they are said to provide excellent thermal insulation. Do they?

Also, were the honeycomb blinds you tested translucent ones or blackout ones, and did they have single-compartment cells or multi-compartment cells containing internal membranes? Blackout blinds (most of which have foil-lined cells) offer better insulation than translucent blinds, and both types are widely available from different suppliers. Honeycomb blinds with multi-compartment cells (also known as ‘architella’-type cellular blinds) offer better insulation than blinds with single-compartment cells, but as far as I know, only one brand offers architella-type blinds in New Zealand, and they’re much more expensive than other brands. Thus, for readers buying honeycomb blinds to be sure they’re getting ones with the type of thermal performance you describe, we need to know whether the specific blinds you tested were translucent or blackout, and whether they had single-compartment cells or multi-compartment cells. Can you please provide these details?

Consumer staff
04 Jun 2019
(1) What about shutters? (2) Not all honeycomb blinds offer equal thermal insulation

Hi Oneironaut,
We can't say how effective the wooden shutters would be but, assuming they have a good seal around the window frame, and shut tightly, they should do a reasonable job.
The honeycomb blinds were blackout, single-compartment.
Kind regards
James le Page - Technical Writer

Gary Beath
28 May 2019
1. Using a fan and 2. Roman Blinds

What exactly do you mean by using a fan? Where would you put it in the room and in which direction would you set it to blow - towards the windows, away from, etc?
I wonder what you mean by Roman Blinds because you've got $$$ beside them compared with $$ beside the honeycombs or curtains? I just went and priced honeycombs and they are $$$$. Roman blinds should be cheaper to make than curtains, unless what you mean by Roman blinds is different from what I know them as.

Consumer staff
30 May 2019
Fans and Roman Blinds

Hi Gary,
A fan running in the room in any direction should make the air more turbulent and hopefully interrupt the reverse chimney effect. I'd be positioning it near the heater to also help get the warmth circulating around the room a bit faster.
The $-$$$ are indicative of the prices we paid. There will probably be regional variance in the prices charged for the different window coverings.
Kind regards
James le Page - Technical Writer

jenny g.
27 Jun 2019
1 Fan & 2. Cheap pelmet -

Hi Gary
Based on the Consumer articles I trialed a fan.
FAN -I am using a 30cm dia GOLDAIR fan sitting on the floor on the low setting.
It works well and distributes the heat to the far corner of the open plan room which is 9m away from the fan.
I think that the fan also mixes the layers of air (hot air under ceiling with cold by floor) and disrupts the reverse chimney effect created by the windows and doors.
The fan noise is not too intrusive. (It is just above noise level of the fan in the oven)
I angle the fan at 45 degrees to vertical (horizontal same) into the corner of the room adjacent to the wood burner, diagonally opposite to the furthermost part of the room. Room is 3.1m x 4.7m + another room 6.5m x 3.2m – opening 2.4m wide 2.5 m high between the rooms. Both rooms are 2.7m high.
CURTAINS - The curtains touch and ‘puddle’ on the floor.
CHEAP PELMET - I made a cheap pelmet out of 75mm wide white elastic @ $2 linear/m, run under and over the curtain rod brackets, elastic held to sides of brackets with bulldog clips to seal air gap between wall and curtain rod.
Room colour is off -white so the pelmet is unnoticable.
To block the air at both ends of the curtain I unhook the end curtain hook, fold the curtain 90 degrees back to touch the wall and fix the curtain end to the rod bracket with a bulldog clip.

Jan
28 May 2019
Honeycomb Duet Blinds

We were sleeping in our studio unit for a few weeks during a very cold Otago winter. One morning we woke up to find that condensation had frozen between our duet blinds and double glazed window. Clearly the Duet blinds were not letting heat through.

Linda T.
28 May 2019
Condensation

We used to have terrible condensation and used your suggestions but the problem is by having curtains close to the windows you then get wet curtains with mould. Our problem of condensation is now solved by having a DVS system with an air warmer and not having the curtains sealing round the windows. Also a warmer house that warms up more quickly. And we also installed double glaze windows in wooden frames. Will also add the new windows didn’t stop condensation. Only dvs system did.

Alison J.
28 May 2019
Condensation in windows

I live high up in Wellington hills. When I first lived here without a ground vapour barrier under the house condensation poured down the windows like a river. Once the plastic polythene was installed under the house it improved out of sight. There is no insulation in the walls that doesn't help and the fact that the deck is built straight on to the front of the house. Till I painted it with a wood preservative and painted that again, it has improved but still gets some condensation.

Shan L.
27 May 2019
Make of blinds

Do you have the make and model of the blinds you used in your test? In particular the honeycomb.

Consumer staff
28 May 2019
Make of blinds

Hi Shan,
We don't have the exact make and models of the curtains and blinds. Instead, we just used a local curtain and blind supplier to come and fit them and focus on the type rather than the brand. If you are in the market for new blinds, any company that can supply you with honeycomb blinds will deliver a product that performs on par with the blinds we tested.
Kind regards
James le Page - Technical Writer

Paul W
26 May 2019
Vertical blinds

Hi there. We just got the lounge fitted out with vertical venetian blinds . Have you done any tests on these.

Consumer staff
27 May 2019
Vertical blinds

Hi Paul,
Unfortunately we didn't test them this time around but we will try and add them in the future. We'd say that their performance would be pretty similar to the horizontal venetians.
Kind regards
James le Page - Technical Writer

Gwnneth P.
26 May 2019
honeycomb blind

What is a honeycomb blind?

Consumer staff
27 May 2019
Honeycomb blind

Hi Gwnneth,
We'll add in an image to the article so you can see what they look like. Honeycomb blinds are made up honeycomb shaped sections stacked on top of each other. The sections a flatten when the blind is raised. When they are lowered, they take on the honeycomb shape and trap air within the structure. It'll be much clearer when you see an image.

Kind regards
James le Page - Technical Writer.

David & Rosie H.
26 May 2019
Curtains and double glazing

Do curtains add significant insulation over double glazed windows ?

Consumer staff
27 May 2019
Curtains and double glazing

Hi David & Rosie H.
Curtains should offer a significant boost over double-glazing. I don't have the exact figures since we didn't test it for this update. You'll still lose heat through double-glazing (though not as fast as single-glazing) so any window covering will be a bonus and will slow down this heat loss, make your house warmer and cheaper to heat.
Kind regards
James le Page - Technical Writer

Jen H.
26 May 2019
Jen

Surprisingly, the article does not appear to mention vertical blinds which are neither wood nor aluminium. Where do they stand in the overall results?

Consumer staff
27 May 2019
Re. Vertical blinds

Hi Jen,
Unfortunately we didn't test any vertical blinds. We'll look to add them to our test in the future.
Kind regards
James le Page - Technical Writer

Josie
25 May 2019
Blinds + thermal curtains

I have fine "sun-reducing" plasticised blinds on my north and west facing windows. They are rollers but set within the window frame. If I pulled these down to the floor each night on my floor to ceiling windows and drew my thermal floor length curtains, would/could this give me the still-air gap necessary to get the improved performance?
Anything to reduce the condensation overnight would also be a boon.

David & Rosie H.
26 May 2019
Still air gap behind roller blinds

I suspect that the air gap is only as effective as the lack of gap around the edges of the blind, especially top and bottom to avoid setting up the reverse chimney effect ( hot air entering at the top, cooling, descending and coming out of the bottom). Getting rid of this gap is a challenge...

Consumer staff
27 May 2019
Blinds + thermal curtains

Hi Josie,
That setup sounds ideal for creating that still air gap. I would just pull them down so they are sitting on the sill and pull the curtains over them. They'll be doing a good job working in tandem.
Unfortunately the curtains and blinds won't stop the condensation issues. You might want to check out our article on moisture in the home for tips on how you might be able to reduce condensation - https://www.consumer.org.nz/articles/moisture-in-the-home

Kind Regards
James le Page - Technical Writer

Terry H.
25 May 2019
venetian blinds

Does it make any difference which way you close the blinds?

Consumer staff
27 May 2019
venetian blinds

Hi Terry,
It shouldn't make a difference. The main thing is that you actually close the slats.

Kind Regards
James le Page - Technical Writer

Lyall D.
24 May 2019
Are the "blanket" results correct?

You show that "Thermal curtains, sill length with blanket" provide greater heat retention than "Thermal curtains, floor length with blanket", which seems to be at variance with all of your other results showing floor length curtains give better heat retention than sill length curtains. Has there been a mistake?

Also, how would these results compare with heat loss for purpose-designed double-glazing of an equivalent size or retrofitted double-glazed panels in existing aluminium frames?

Consumer staff
27 May 2019
Are the "blanket" results correct?

Hi Lyall,
Yes the blanket results are correct. It shows that the blanket was the real hero here and almost appeared to have made the curtains redundant. The blanket made a very good seal around the window frame and the difference between sill and floor-length curtains was just a bit of natural variation in the testing.
Double-glazing and retrofit double-glazing would probably have similar results to our acrylic secondary-glazing in terms of effectiveness over a bare window.
Kind Regards
James le Page - Technical Writer

Lynnette J K.
12 Jun 2019
How long was the blanket?

After reading your test results I invested in some polar fleece and new cheap thermal curtains. I attached the polar fleece to the top of the curtains - both curtains and fleece are resting on the floor. I thought that this would have been the best configuration, but your results have left me confused.
Accordingly
1) "Thermal curtains, sill length with blanket' is 60%;
2) and "Thermal curtains, floor length with blanket" is less at 53%.
How long was the thermal blanket in your test?
Please help - I'm trying very hard to make the most of my window coverings!

Edited: Thank you so much for your reply Natalie

Consumer staff
12 Jun 2019
Re: How long was the blanket?

Hi Lynette,

The main thing with the blanket was to get a good seal around the window frame. We pinned it up behind the curtain so it was similar in size to the window frame itself. We didn’t think it was viable in the long run unless you can come up with a system to make it easy to put up each night (such as using Velcro dots on the walls).

The configuration you have gone with will get you good results, probably better than the heavy lined curtains at floor length. However, to achieve the same performance as our testing, the blanket needs to be fitting tightly against the window frame.

Kind regards,

Natalie - Consumer NZ staff

James G.
23 Nov 2019
Results are conflicting

I agree with the statement of others and Consumer needs to look at their chart.

Consumer indicates it is better to have curtains/blanket to the floor for best results in their paragraphs, yet their chart indicates it is better to only fit to the sill.
A simple typo error in the graph I suspect but it does leave the consumer confused.