DIY fake double-glazing

Perhaps you have heard of a heat-shrinkable plastic film, called "Clear Comfort", that can be used instead of a second layer of glass to trap a layer of air between it and the glass in your windows. I'm not connected with that company in any way, I'm merely a customer, but I'm sufficiently impressed with the difference it has made to the temperature of our house that I want to write about it here. This is the first winter since installing this film so I don't have any figures to quote yet, but at the end of winter I'll report whatever difference there ends up being in the cost of us keeping warm.

 Windows "double-glazed" with Clear Comfort on supplementary frames

Details of how to install the plastic film with various window types are available at the Clear Comfort website. Very briefly, one uses the double-sided tape that comes with a Clear Comfort kit to stick the plastic film directly on the window frames if you have sash windows, or to stick the film on removable supplementary frames if you have windows that open by sliding to the side. Either way, the film traps a 2-3cm layer of air between the glass and the film, which stops heat transfer between the outside glass and the inside of your house, keeping your house cooler in summer and warmer in winter than it would otherwise be.

What I've done...

Our house has windows that open by sliding sideways, so I needed to make up supplementary frames that fit snugly inside the existing window frames. This almost doubled the cost of the project compared with the sash window scenario, and involved about five times as much work. While I was making the supplementary frames, I often wished that I had sash windows - I had to make 26 frames to do the entire house! However, I eventually discovered that it is much, much easier to stick the film evenly on to a removable frame than it is to stick it directly to something fixed. We do have one small window (in a door) that did not need a supplementary frame.

 Clear Comfort stuck directly to door (no frame needed)

Before committing myself to doing the entire house, I bought the $20 sample kit to try on one window. I bought $1 per meter pine (42 x 18cm) to make the frame, and screwed on small metal brackets at the backs of the corners to hold the frame together.

Back of a supplementary frame (not the test one)

I then painted it, waited for the paint to cure, prepped the surface and stuck on the double-sided tape. When it came time to stick on the plastic film, I simply leaned the frame against a wall and bit-by-bit peeled off the backing tape and stuck on the film, starting from the top and working down both sides.

Perhaps one improves with practice, but I did not manage to stick the film on as evenly as I'd hoped. The tension was rather uneven, and there were a number of wrinkles at the edges. Even so, the heat-shrinking process was quite amazing - it smoothed out the wrinkles and made the film look almost exactly like glass. I was quite pleased, and relieved, until I noticed that my supplementary wooden frame had become slightly twisted during the shrinking process - possibly because I should have used better quality wood for the frame, or possibly because I did not manage sufficiently even tension when I stuck on the film.

In any case, having learned of the potential pitfalls, I thought the result looked promising enough for me decide to go ahead with "double-glazing" the entire house. This time I bought 42 x 18cm  pre-primed cypress fir (about $2 per meter). I don't know much about timber, but the cypress fir felt sturdier and more "solid" than the pine. I made and prepared the frames in the same way as before except that, for the taller windows (180cm high) and glass sliding doors, I added in a cross piece partway up because the long side pieces started to bow in slightly before I did that. For most of my windows I made a pair of supplementary frames so that, when I want to open the window, one of the pair can be removed and can sit in front of the other frame on the window ledge. This gives you a convenient and safe place to store it when not in use.

One frame moved over the other to open the window

When I was ready to stick the plastic film to the frames, I followed a very helpful tip from the Clear Comfort salesperson. I laid out the length of plastic film on the floor (after thoroughly cleaning the floor) and stuck the edges to the floor with masking tape, ensuring that the film was perfectly flat and evenly tensioned. I then removed all the backing tape from the double-sided tape on one frame and carefully put it down onto the film (a friend held one end of the frame and I the other). Sticking the frame to the film worked much, much better than trying to stick the film to a vertical frame. The film was a little loose (which is fine), but it had very little wrinkling at the edges and the tension was quite even.

This time I inserted each pair of finished frames into their particular window before heat shrinking the plastic film so that they couldn't be pulled out of square by the heat shrinking process if there was any unevenness in the tension. This time the frames did not twist at all - but I still don't know if that was due to using better wood or due to sticking the film on with much more even tension!

Fitting the supplementary frames...

I've not written much about how to handle the plastic film because those details are on the Clear Comfort website, but there are a few challenges to solve concerning the frames.

Firstly, the supplementary frames need to fit very snugly inside your existing window frames in order to trap a layer of air between the plastic film and the glass in your windows. I'm not sure how other people would achieve that. Perhaps accomplished carpenters could do it just by making perfectly-sized frames, but I opted to make the frames about 4mm shorter and narrower than the insides of the window frames to leave a 2mm gap all round. I then stuck self-adhesive weather seal foam strips around all edges of each frame except for the centre edge of one of each pair of frames.

Update: So far the foam-style weather seal strips I used are working fine, but since writing this post I've been advised that the brush-style weather seal strips are much more durable and worth the extra expense.

I tested the fit of the supplementary frames before sticking on the plastic film. The foam strips compress a little and make for a very snug firm fit. In fact the fit is sufficiently snug that I did not need to use anything to stop the supplementary frames falling forward out of the existing window frames. The frames were easy enough to position correctly at this stage because I could put my hands around the wood of the frames, but bear in mind that, once the film is attached, you will only be able to push on the frames to insert them and you will need something to pull on to remove them (when you want to open a window).

Before I stuck the weather seal strips onto the supplementary frames, I used drawing pins to attach loops of ribbon to pull on to remove the frames. I used wood glue to stick small rectangles of balsa wood (from a craft kit) inside the sides and tops of the existing window frames. These stop the supplementary frames going in too close to the glass when I push on them to insert them and maintain a 20mm gap between the glass and the film.


After I'd stuck the film on the frames, I discovered that I needed to push quite firmly on the central edges of each frame pair to get them to sit flush with each other, and that I therefore needed something quite solid to push against. At that point I raided the collection of kids wooden blocks and found some that were just the right size to place between the bottoms of the supplementary frames and the bottom of the window frames. These just sit at the halfway point on the window ledge without being stuck on in any way.

Detail showing ribbon loop, kids block, and small balsa block

These blocks solved the problem, but even so, inserting and removing the pairs of frames is still a bit tricky. I have one window that is sufficiently narrow to only have one supplementary frame, and that is much easier to insert and remove. Assuming you use weather seal strips or something else that compresses to achieve a snug fit, I suggest you should consider using only a single supplementary frame where possible, but you will need somewhere safe to keep the frame when you remove it.. In my case I have another window of the same height nearby where I store the single supplementary frame when I want to open that window.

I used Clear Comfort with supplementary frames for all our windows, and also for the sliding doors shown below. However, for the sake of convenience, I opted not to use it for the glass sliding door that I go in and out of numerous times a day.

 Glass sliding doors with supplementary frames

What might have been better...

I used 42 x 18cm cypress fir for my frames, but think I should have used wider timber to prevent any bowing of the longer sides of frames. The side pieces of the 120cm high frames bowed almost not at all and the weather seal strips took care of the slight bowing, but the 150cm high frames did bow enough for there to be a slight air gap between the central edges. I originally had weather seal strip only on one of the central edges, but for the window shown below I ended up having to add another strip on the other frame to close that gap. The 180cm high frames all have cross pieces partway up the frames and so did not bow at all.

 You can see the wider gap halfway up the bowed sides of the frame

I considered attaching some sort of cupboard door handles to the frames to grasp when removing the frames, but I thought the loops of ribbon would be less conspicuous. The ribbon might ultimately not be durable enough for the frames I remove often though, so maybe some frames will end up having handles eventually!

Other alternatives...

 A cheaper alternative for temporary window insulation during mid-winter is bubble-wrap. I don't think you'd want this on all your windows, even temporarily, but used strategically it sounds quite effective. This is what Denise had to say:

"I put it on bubble side towards the window, and used the minimum amount of sticky tape on the window frame to hold it on - I'd get a better seal if I used tape around all the edges, no doubt, but I want to minimise the tape residue clean up when I take it all off!

The view from the window needs blocking in any case - a bare fence, weeds, and noisy neighbours. It just nicely blurs things :

While it doesn't stop all the cold air, I must say it's pretty bloody good. First thing in the mornings (even after -4ºC (24ºF) overnight) my office sits at around 17ºC (63ºF) without any extra heating."

Update: Yesterday I decided to follow Denise's example and stuck bubble-wrap to the frequently-used sliding door that does not have the Clear Comfort treatment. This sliding door was the weak link in my house insulation, but since I use that door extremely frequently, I felt it was not practical to remove and insert supplementary frames each time I needed to use the door. I thought that having just one expanse of glass without insulation would not make a huge difference, but I've been surprised! Bubble-wrapping that door has indeed made the house noticeably warmer.

The bubble-wrap lets in plenty of light, but it does blur the view through that door, so I plan to leave it in place only during really cold weather, and then remove it and store it for use again next winter.

Measured results...finally (added December 2010)

I compared this winter's energy consumption for heating with last winter's consumption. This year, it was 92.7% of what it was last year (before the Clear Comfort was installed). At first glance that does not seem like a lot of improvement, but then I compared the average daily maximum temperatures for the 3 winter months for both years. This year the average maximum for those months was 15.43 C, compared with 16.83 C last year.

Now, they say that raising or lowering the thermostat by 1 degree affects energy consumption by up to 10%. So, I guess I can say that, if it weren't for the Clear Comfort, my energy consumption for heating should have been "up to" 14% higher this year, but in fact it was 7.3% lower. (The "up to 10%" figure is much more vague than I'd like, but I'm not sure how else to factor in the colder than usual winter.)

Also, the bubble-wrap I mentioned above fairly quickly started coming unstuck so, in practice, those wide glass doors were bare almost all winter. Now I'm curious to know how much difference it would make if those doors also were effectively insulated. Next winter I hope to find tape that will stick the bubble-wrap on securely, and keep it in place all winter. I'll then compare next winter's energy consumption to see how much the energy consumption for heating was affected by having one uninsulated expanse of glass, and post the results here.