Hi Peter, I installed Clear Comfort and other similar films on a few windows a couple of years ago. It works well, is easy to install, is virtually invisible and surprisingly durable. The Australian sourced material was about 5 - 10 times the price of buying it from UK via eBay, where its use must be more common. The materials cost next to nothing, so the price in Oz is hard to justify.
The film is attached to double sided tape that is placed around the window pane. Therefore you must have a continuous flat frame at least 15 mm wide with paintwork in good condition for the tape to adhere to. The film is rather like plastic lunch wrap to handle, but fortunately doesn't stick to itself like lunch wrap. You don't have to get it perfectly straight and it can be repositioned if handled carefully.
Once you have the film in place, it is very easily shrunk with a hairdryer to become near invisible. If necessary, you can reapply the hairdryer months or even years later. Windows covered with the film do not mist up in cold damp weather like untreated single glazed glass so I would say it is reasonably effective and definitely worthwhile. I guess if the film is eventually damaged, the adhesive could be removed with a solvent and a new tape and film applied.
However there are some limitations to consider. The tape edge will not be entirely invisible. You can over apply heat and shrink the film to the extent that it pulls off the adhesive. Also the tension on the film is a function of span, so large windows will be a problem and may require a double width of tape to cope. I personally would not use it where one of the dimensions is over 600 mm. On sash windows with central muntins (timbers), the muntins will probably be too thin for the adhesive tape.
In my house I have chosen to use it on casement and awning windows that have internal insect screens, so there is absolutely no visible indication that the film is there either from inside or outside. The screens also protect the film from accidental damage.
Of course with opening windows it is also important to have functioning seals to prevent drafts, or you may as well not bother with the film. On casement and awning windows I have found the Raven RP 48 and RP 59 rubber strips very effective.
I would not suggest shrink film in preference to double glazing windows or modules - it is purely a retrofit solution that is very cost effective. Small windows are disproportionately expensive to retrofit double glazing for the small gains to be had, but film is very cheap, especially if sourced from overseas.
The film also has minimal acoustic effect, so if sound reduction is important, you should consider double glazing or secondary glass or acrylic glazing. Airtight seals are also very important for sound reduction.
Curtains and pelmets, the way they are installed in Australia, are not great. Putting a pelmet on the top of a blind or curtain does not do much to stop the "reverse chimney" effect created by the parallel surfaces of the blind and glass, because the pelmet is not sealed and only impedes the draft slightly. Air can still leak into the sides. In Europe, blinds are fitted with a seal across the top and sides of the blind to exclude drafts.
There are pocketed concertina blinds readily available in Australia that have very good thermal performance. Some have foil on internal surfaces if you have a problem with solar gain or want good light exclusion. These blinds will much perform better than curtains if fitted properly.
With double glazing, regardless of the type, if good acoustic performance is important (i.e. good outdoor sound attenuation) then the gap between the two glazing panels should be at least 100 mm and up to 300 mm if you are on a highway or busy road. It also helps if the inner and outer panes are significantly different thicknesses, say 3 mm single and 6 mm. This is because glass has a "critical frequency" that passes though it and the frequency is a function of density and thickness. It you have the same thickness for both panes, the attenuation at the critical frequency will be compromised, and sound around this frequency will still pass through. One pane can be e glass too if that is useful. Having one pane of laminated glass also provides security.
The best thermal performance is with small gaps around 10 mm, but the loss in thermal performance of going to a 100 mm gap or even greater is generally not significant in the scheme of things.
Existing windows can be upgraded by replacing the glass with custom made double glazed modules, argon filled for best thermal performance, which are slim enough to replace the original glass in many cases. Alternatively it is often possible to fit secondary glazing to an existing frame, especially older wooden type frames. We have one wall of near floor to ceiling glass where we fitted laminated glass on the inside of existing frames. The glass company had a timber moulding made to suit the frame to finish off and you would never tell the double glazing was an afterthought. This worked out much cheaper than replacing the windows, gives great thermal and acoustic performance (the panes are silicon sealed for acoustics) and was even cheaper than secondary magnetic acrylic glazing by a considerable margin as well as looking much better.
There are also other alternatives to the well advertised acrylic secondary glazing company which are cheaper, with one DIY supplier less than half the price, so if budget is important, do some Googling. There is nothing between the three companies I have investigated in the performance of the product except possibly the standard of fit and finish. Bear in mind that acrylic sheet will eventually yellow and get surface marks. This is a property of the base material itself that the secondary glazing manufacturers do not control, as they do not make the acrylic sheet, despite any claims to their particular material being superior...
There are some advantages in some cases to magnetic secondary glazing. For example, it can exclude drafts by sealing an opening window completely. This will also improve the acoustic performance of the window. The downside is that you have to remove and store the acrylic glazing panel when you want to open the window. It also does not enhance security because it is easily removed.
I have fitted secondary double hung aluminium sash windows inside original 100+ year old wooden sash windows and found that very effective. The new window can be made to match the original so it isn't visible from outside of the house. This method also provides acoustic benefits and security too if laminated glass is used. In my case, aluminium frames will not degrade thermal performance because the original wooden frames are still in place.
There is also a company that specialises in restoring old sash windows, fitting draft seals and double glazing units. This is probably more expensive than either of the previous methods, but may be the only option if heritage looks are important, or in some cases where the architecture simply doesn't have space for secondary glazing. There are thermal and acoustic benefits over a standard window, but performance will not be quite as good as a secondary window as described above. I am sure the same company makes traditional timber double glazed double hung sash windows.
If you are replacing windows and frames, it is important that the new frames are insulated as well. A great deal of the thermal performance of double glazing is wasted if aluminium frames are used, unless the frames have a built in thermal break. This is where a PVC or timber section in the middle of the frame keeps the internal and external aluminium surfaces separated. Wooden frames would be even better thermally. There are UV stabilised PVC frames with good thermal performance and low maintenance too, if you haven't kicked the oil habit yet (PVC is made from non-renewable fossil oil supplies).
In the colder climate regions of Central Europe, dual action PVC, aluminium or wooden framed double glazed tilt or turn windows are all the rage and are fantastic at catching breezes in summer and letting in a little ventilation in winter without a freezing draft. On a recent visit I noticed there were so many old apartments with new windows of this type fitted that there must be a government incentive scheme happening. I think I have seen one supplier in Australia of this type of window.
