Green Oversite

logo









Home

My House

Curtains and Other Window Treatments

(On the cheap!)

Bruce Barbour - December 2019

I have recently been doing some work installing curtains and pelmets on the windows in my house. There are three reasons for doing this: ascetics, privacy and energy efficiency. I will concentrate mainly the benefits of curtains for house energy efficiency. Firstly some background and theory.

Windows are one of the weak points in the building fabric for the loss (in winter) and gain (in summer) of heat energy. The other main weak point is air tightness - the rate at which air enters and leaves your house.

The thermal resistance of a typical double glazed window - without curtains - could be R = 0.3 to 0.4 (Units = Km2/W = Kelvin (temperature) * metre squared per Watt - SI units not Imperial). The window industry tends to use a measurement of U or thermal conductance instead of R. U is the reciprocal of R - i.e. 1/R. I will use R because it is easier to compare. The thermal resistance of walls in a newer house should be a minimum of R = 2.8 (for Melbourne). Consequently the rate of heat transfer through a window can range from 7 to 10 times the rate of heat transfer through walls.

If curtains and other treatments are applied to windows the treatment could add, say, R = 0.15 to 0.25 to the thermal resistance of the window, and perhaps more. The rate of heat transfer would still be many times the heat flow rate through an insulated wall but it will be an improvement, a significant improvement.
Pelmet Diagram showing air flow change
                          due to pelmet installation

Pelmet Diagram showing air flow change due to pelmet installation. (Diagram from Energycut.com.au

As well as curtains the windows need to have pelmets to prevent air flow between the curtain and the window, as per the diagram. If not installed this air flow would result in cooling of the air in winter - and warming in summer.

If the window is full height the curtain should touch the floor in preference to leaving a gap. 

I have installed pelmets and curtains on a number of windows and will complete the rest over the coming month.

It is preferable to have not just single thickness curtains or even single lined curtains. For greater energy efficiency it is best to have what can be described as heavy drapes. I want the drapes to have multiple layers and a reasonable thickness - too much thickness would make them too bulky when open.

I looked for ways of obtaining and installing the drape economically. To get good quality curtains made up is expensive in Australia (due I imagine to our relatively high cost of labour). To get the heavy drapes ideal for energy efficient housing would be even more expensive. I doubt whether many usual curtain manufacturers would ever make them.

Firstly I owned a couple of curtains that had been used in my previous house. However these curtains could not be describes as drapes. They were either single thickness material or lined - so to meet my requirements they would have to be added to in some fashion to convert them to my desired drape. And I only had enough of these for two windows - I needed more.

As discussed curtains made specifically for the house would be expensive. However ready made curtains (undoubtedly made in Asia somewhere) are not that expensive  - especially when on sale. I purchased the ready made curtains for half normal price at a Spotlight sale. I then improved the curtains to make them like heavy drapes by adding detachable linings.

Detachable linings are effectively additional curtains that are installed behind the curtain facing the room (the front curtain) by hanging off the same hooks as the front curtain. You could engage a curtain manufacturing company to sew them up - for a significant cost - or do it your self. I decided to do it myself - or more correctly most of it as will be explained.

I did not have the skill of sewing. I probably could have taught myself but I didn't have a sewing machine. I decided to use the services of one of the clothing alteration/repair shops that you see in shopping centres for any sewing that I may need. The alterations shop that I used charged $10 per meter of stitching, so there was incentive to minimise the amount of sewing needed as much as possible.

The first item needed was linings tape. This can be purchased at Spotlight (though they were out of stock when I tried to buy some) or Lincraft. This is sewed on the top of the detachable curtain and provides the means of hooking the detachable curtain to the curtain rail hooks behind the front curtain.

For the detachable curtain material itself I used two layers of bed sheets with a layer of thin wadding in between the layers.  I purchased the wadding from Spotlight - again it was on sale for half price - $4.30 per sqm.) This would give be a total thickness of four layers when combined with the front curtain. Why did I use bed sheets? I had a number spare at the time so they might as well be used. However I didn't have enough for the whole house so I had to buy a number of new flat sheets. The second reason for the use of bed sheets rather than a more traditional linings material was that on bed sheets the edges are already hemmed (so I did not have to pay for hemming at $10 per metre). I bought the sheets I needed from Target so they were relatively cheap ($15 to $22 for a double bed flat sheet. Hemming along just one edge of the sheet would have cost that much.) The size of the sheet purchased depended on the size of the window to be covered.

On reflection I perhaps should have used blockout lining material which would have worked out nearly as cheap as the sheets - Spotlight were selling it on sale for $12 per metre - providing I did not want to have the edges hemmed. It may actually work better in terms of insulation. The blockout linings material is coated with a backing material (rubberised?) consequently when the material is cut the edges do not fray so providing you cut straight (rule a line as a guide) and carefully you can get away without hemming the material - remember it will be behind the front curtain so will not be readily seen. Or you can get them hemmed - it would ad $50 to $70 per curtain set per window, so still not expensive. This would still allow the use of multiple layers - blockout | wadding | blockout - with the blockout is wrapped around the wadding material to protect it - as per the instructions below. However, while I think that this would work fine  I don't know how bulky the linings would be if made like this. If trying this suggest you try it on one window to see how the curtains hang.

New Zealand Spotlight was selling a ready made linings curtain - hemmed blockout with linings tape attached. I asked locally and they weren't available from Spotlight Australia. Import?

There are a couple of factors to consider when choosing the width of the linings. The lining does not have to be the same width as the front curtain. It only needs to be the width of the window plus say a minimum of 150mm. It can be the complete width but does not have to be for it to act effectively as a heat restricting lining. Any width shortness of the lining curtain cannot be seen behind the front curtain. This assumes that the curtain rail extends beyond the width of the window side architraves, which they should to allow the curtain to reveal the full window when the curtain is open. So the size depends on this plus the size of the bed sheet if used - it should not be an issue if blockout lining material is used. The height of the lining should be the height of the front curtain to the curtain rail hanging hooks (less a centimeter or two so it doesn't drag).

The steps are:

  • purchase linings tape for all curtains to be lined;
  • determine what size of sheet or block out, and wadding is needed and obtain those;
  • Cut the wadding to size;
  • Cut the blockout to size, allowing to wrap around, on both sides of the wadding, with a little bit of overhang, and also for the width of the hem if hems are to be used on one side and the bottom.
    (If you want to used sheets, the sheet has to be double the width of the wadding plus at least 100mm, preferably more, to allow overlap when wrapped around the wadding. For sheets the important thing is to maintain as much hemming as possible. The top of the lining will be covered by the lining tape so that can be a cut edge. The edge that is hidden under the overlap can also be a cut edge.)
  • Fold the blockout material around the lining so it is underneath and on top of the lining. (For sheets overlap);
  •  pin the linings tap to the top of the new lining, making sure that two of the arms of the linings tape go over the top of the three layers of the blockout (or sheet), wadding and blockout (or sheet). All must be securely pinned and held by the linings tape. The slits in the linings tape that the lining will hang on the curtain rail hooks must be accessible. Pin up the hems if using them;
  • take to alterations shop for them to sew the lining tape on - and hems;
  • Bring back and hang behind front curtain. Remember that you are not hanging it so the lining follows the folds of the curtain. When the curtain is shut the lining will be largely flat (with a few undulations so the drapes can be pulled a bit past the halfway point); and
  • Now that the curtain and lining is hanging in place you can put a few single stitches (with a couple of loops to reinforce the stitch) in a few places, joining the edge of the front curtain with the edge of the lining. This only needs to be done on the inside edge where the curtain is pulled closed. Also perhaps a single stitch or two to hold the overlap on the lining in place. (Alternatively you could use a some small safety pins instead of stitching). 

The arrangement outlined will provide superior thermal performance compared to simply lined curtains:

  • instead just two layers for a lined curtain there will be four layers, one of which is a layer of wadding which is thicker than normal material; and
  • The detachable lining will be essentially flat when the curtains are closed. This should give superior thermal performance compared to an attached lining that follows the folds of the curtain. (If the lining follows the folds of the curtain there is more surface area which in theory means a higher level of thermal conductivity).

The other advantages are the low initial cost (a bit over $100 for the lining curtains for a 1.8m wide 2.1 high window - add $70 if hemmed) and if it was ever required that the curtains be changed then just the front curtain could be changed. The detachable lining can be reused. Or if the detachable lining has degraded due to solar exposure it can be replaced or repaired separately to the front curtain.

If the curtain has to stop direct solar insolation, that is the sun shining through the window into the room then blockout material (and not sheets) needs to be used. Note it is preferable that internal curtains not be used to block out the sun in this way. If sun is shining directly through the window consideration needs to be given to how the window can be shaded from the outside so the sunlight does not directly hit the window in summer.

The outlined method can be used to upgrade existing unlined curtains. And even if the curtains are already lined, a detachable lining can be added to further improve thermal performance - though there might be a concern regarding how bulky the curtain and detachable lining combination is. If this was a concern perhaps leave out the wadding material - it would still make an improvement. Or improve them as per the next paragraph.

The other way to improve an existing lined curtain would be to simply slip a layer (or two!) of wadding up between the curtain and lining and stitch in place - it doesn't have to be super neat - it can't be seen easily. Not as good as adding a detachable lining but still an improvement.

One other thing that I tried was that I installed a spacer - a strip of a couple of layers of wadding - between the two of the layers of the lining. This was to try to create an air gap between the layers which adds to the effective insulation property of the lining. Don't know how effective this will be.

Anyway I hope this information is useful - as I had never before made curtain linings I went through a trial and error process, improving the design with each iteration, and then finally realising at the end that I should have perhaps used blockout linings material instead of sheets for a small cost increase. Lining of curtains is a relatively cost effective way to improve the overall thermal insulation of the house. So consider linings, even if you don't make the linings in the manner suggested in this article. Any extra layer over the windows will help.

Kitchen Window

I did not want to install curtains on the kitchen window. I was concerned that the curtains would be dirtied too easily being situated just above the kitchen sink. Instead I installed a concertina blind (sometimes called a honeycomb or cellular blind). This would certainly not be as good as curtains with a detachable lining, so it was a compromise. This window also has an aluminum roller shutter on the outside. This may assist a bit with slowing thermal conduction, but it doesn't seem to seal all that well so I don't know how much assistance it would provide. The roller shutter was primarily installed to stop direct solar gains (solar insolation) through this East facing window on hot summer days, and it seems quite effective in doing this.

Green Oversite Home Page




Top of Page
| Site Information | (C) |