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Step 1 – Specifying the Glass part 1

One of the most important design criteria is that the doors keep the place warm while still letting in loads of natural light. This is traditionally a tricky challenge, because in terms of insulative value, glass is usually one of the weakest links of a buildings exterior envelope. When you are making your own windows you can address this problem (and in many instances you will now be obliged to by local or national building regulations).

In this case we wanted to get the best we could (within a reasonable budget), and this means specifying triple glazing, with multiple ‘soft coatings’ or ‘hard coatings’. See the table below taken from an Energy savings Trust document (link) on the subject. We also had the option of Krypton fill too, which turned out to be a lot more expensive than Argon fill for very little difference in performance – a warning.

I have yet to find a comprehensive comparison table

This gives you an idea of the U-value and estimated energy loss (or gain) the window will make to the building.  The performance of a window, and the choice to go triple is not as simple as tables like this may imply though, because, depending upon a number of factors, the heat loss through windows can be offset by solar gains (the heat energy that comes through when the sun shines) – also known as the ‘g-value’. In some instances this has lead people to argue that double glazing could actually outperform triple, as it lets through more solar energy. Our opening is South facing, and we know from the experience of upstairs windows that the solar gain can be really useful in warming the place up.

From what I have read though, such an argument is overstated: here in the UK, in winter the sun is not out for the majority of each 24hour period. It is obscured by clouds, does not have its hat on, and is just not coming out to play as much in general, especially when it is cold and the heat is needed the most. In almost all the calculations I have seen that consider the ‘net energy loss indicator’, as in the table below, triple glazing comes out on top.

Nevertheless, we want as much light coming in as possible to make it a nice place to tinker about in. Low-iron glass is a bit more expensive but helps here. Because of its low iron content – what gives glass its greenish tinge – it is clearer and has a good solar transmittance, typically 90 percent compared to 85 percent for normal ‘clear’ float glass.

Balancing the different factors we chose triple glazing for a few reasons:

  • Contain and conserve heat better (high insulative value, or low u-value);
  • Keep out/in sound better;
  • More secure;
  • Condensation is less likely to form on the inner surface;
  • Cost wise, it is only a little bit more than a similar sized double glazed unit.

So, onwards with triple glazing in mind!

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