Looking into, and out of, windows

Anyone who’s studied Medieval European History for A Level – and I can’t be the only one – will have learned two intriguing facts. One is that Game of Thrones is a severely watered down version of what really went on in the Middle Ages. The second is the existence of the Defenestration of Prague.

Actually the details of the latter escape me. It’s the sound of it that’s so intriguing: a mixture of the slightly comic and the wilfully obscure. It could be an unexpected trouser removal or so hideously depraved only case-hardened psychiatrists or priests should be allowed to learn the full details.

In fact, it simply means throwing something – in this case, several city councillors – out of a window.

Logically then, ‘fenestration’ should involve throwing something – or someone – into a window: a marginally less hostile act, unless, of course, the window is shut. But, confusingly, not so.

Fenestration, it turns out, is an architectural term for the arrangement of openings that admit daylight into a building, including doors but usually referring to windows. It enables architects to gaze at the facades of elegantly proportioned Georgian homes and impress clients by remarking sagely: “Excellent fenestration.”

Choosing the right windows for your project is one of the most important self build tasks, and not just because the ‘wrong’ size or positioning can ruin the look of a facade, but because it can also have a significant effect on the interior space.

Complicating matters even further are the current energy efficiency requirements for windows, as detailed in Part L of the Building Regulations. This is because glazing leaks heat at a much greater rate than an insulated wall or roof, which is why double glazing has been standard for many years.

Here, the glazing panels are between 10mm and 20mm apart, the intervening gap typically filled by argon or krypton, dense gases which circulate heat more slowly than air. Inward facing panes can also have invisible low-emissivity (low-e) coatings which reflect heat back into the house.

Until recently the regulations only took into account heat lost through the glazing and ignored the frame. Not any more. Now the ‘frame factor’ has to be included to provide an overall heat loss figure. As a result, so-called ‘thermal breaks’ – typically, lengths of solid insulating material – are often incorporated into the frame to reduce the heat passing through it.

And if all this isn’t enough, there’s a further factor to take into account: the ‘g-value’. This is a measure of solar gain, the heat provided by the sun’s rays passing through the window. In winter it can be a useful boost to internal comfort. But in summer it can lead to overheating, especially in highly insulated homes.

The good news, however, is that the British Fenestration Rating Council (BFRC) provides an independent assessment of the overall energy efficiency of windows, using a colour-coded system ranging from high-performance A++ to E.

The better news is that the variety of windows is wider today than ever before, not just in terms of shapes and styles but also materials. So whether you’re building an ultra-energy-efficient eco home or renovating a Victorian terrace in a Conservation Area your ideal window will be out there.

Tips and facts

In the meantime here are seven random window tips and facts which may help in your quest.

1  Try to give main living areas a source of daylight on at least two sides. Two contrasting light sources reduce glare, provide more even illumination throughout the room and for longer throughout the day.

2  Where this isn’t possible, consider increasing the light from a single source by increasing the window height, fitting splayed reveals, french windows or a bay window.

3  Narrow frames and fewer frame members in a window are generally more thermally efficient – and provide more daylight – than thicker ones. They also tend to be more elegant in appearance.

4  Triple glazing is only likely to justify its extra cost over double glazing in fuel-saving terms with an ultra-energy-efficient home, such as a passive house. Its real value is in increased comfort – the same argument previously used for double glazing. That never paid for itself in fuel-saving, either.

5  In rising order of cost, window materials are broadly as follows: uPVC, standard-sized softwood timber, hardwood timber, steel/aluminium/fibreglass and composites – which are generally timber but with an external covering of aluminium.

6  In terms of maintenance uPVC, aluminium and galvanised steel lead the field, requiring little more than occasional wiping down. But uPVC will only last between 25 and 35 years, aluminium around 45 and steel even longer. Timber requires regular repainting or re-staining, but properly maintained can last the lifetime of a building. However, new forms of modified timber such as Accoya and Thermowood promise minimal maintenance for at least as long as uPVC.

7  Georgian and other classically designed windows use the so-called ‘golden ratio’, where the proportion of the sides is 1:1.618. In other words, the height is 1.618 times the width, a ratio the human eye finds deeply satisfying. Unfortunately, for slightly bizarre historical reasons, it isn’t a ratio used for the standard window sizes available from main UK manufacturers.

Achieving it, then, requires either bespoke windows, which are significantly more expensive, or opting for a more contemporary use of glazing – typically the large expanses, up to complete wall sized, which were beyond the technical abilities of our predecessors.

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