Obviously we need windows for daylight, but even taking into account heat gains from the sun, UK houses typically lose 10% of their heat through their windows. This is lost by heat radiation travelling through the glass, conduction through the glazing spacer bars and window frame, and air leakage.
 
The largest losses for new windows will be through the glass, whereas the largest losses for old rattley windows will undoubtedly be through air leakage – especially old sash windows.
 

Glass

Glazing has a ‘G-value’, which measures the degree to which the glass blocks out heat from sunlight – this is useful in a commercial situation, but in a domestic situation heat from the sun is to be encouraged, especially during the winter (during the summer it is best to shade the windows from the sun).
 
‘Low-E’ or low emissivity glazing has its inner pane of glass coated with metal or metal oxide, which allows shortwave radiation from the sun to enter, but reflects the longwave radiation from inside back into the room – it essentially amplifies the greenhouse effect.
 
In most situations good double glazing should suffice, unless you live in a highly insulated house in which case you would be better off installing triple glazing in order to combat condensation issues.
 

Frames - uPVC vs Timber 

If sourced properly and locally, timber is inherently a sustainable material, and good wood window frames are very durable – even more so if made from acetylated wood. It is attractive, has good thermal properties, and is reusable, recyclable, or compostable. It is however comparably dear.
 
As detailed elsewhere on this website, PVC is an oil-based volatile organic compound, toxic to the environment during production and disposal, consisting of persistent, bio accumulative chemicals, of hormone disrupters and of heavy metals. It doesn’t look very good, but it is easily extruded and it is cheap….or is it?
 
Over time ultra-violet light will cause uPVC to deteriorate, meaning that the window will only last 20 years perhaps, and the seals will probably have gone before then. uPVC windows cannot be mended and they cannot be recycled.
 
Timber windows may cost 3 times as much, but they are fully maintainable and should last at least 5 times longer, look better, perform better, and can be recycled or composted at the end of their life – they also sequester CO2 and are not an oil-derived product.
 
Depending upon your point of view, it could be argued that uPVC window frames are a lot more expensive than timber.
 

Certification

The British Fenestration Rating Council has launched a rating system that grades windows from A (best) to G depending upon their overall u-value – taking into account the frame and glass.
 
Alternative certification must be achieved by manufacturers wanting their products to be passed for use in PassivHaus buildings, which requires triple glazing with an overall u-value of 0.8 at most.
 

Acrylic (Perspex/Plexiglass) Secondary Glazing

Methyl methacrylate is a non-oil based organic compound produced to make Poly(methyl methacrylate) (PMMA) or acrylic glass. PMMA is a strong and lightweight material, with a higher impact strength than glass, is able to transmit 92% of visible light, and has an environmental stability superior to most other plastics.
 
Mounting acrylic panels on your window frames, using purpose-made magnetic strips, is a very simple and inexpensive method for secondary (or tertiary) glazing, which dramatically reduces energy losses through the windows and greatly improves comfort.
 
Because of its simplicity, acrylic secondary glazing is arguably the most attractive, most elegant, least disruptive, and least intrusive of all secondary glazing options – it’s certainly the least expensive.
 
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