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Step 2 – Specifying the Glass part 2

Glass is typically measured and described in the following format: [outer pane width] – [air gap] – inner pane width] – [special treatments]

For example 4 – 18 – 4  would be double glazing with two panes, both 4mm thick and an 18mm air gap between them.

The glass we want for this project will be:

4 – 20 – 4 – 20 – 4  toughened, all panes low iron, spaces Argon filled, warm spacer bars, 2X low emissivity hard coatings.

This configuration gives us a final width of 52mm, which is an important figure to know when designing the door frame.  20mm is around the optimum gap to have between panes.  This might seem unintuitive, but if you go much larger than 20mm eddy currents begin to form in the gas occupying the space and this actually starts decreasing its insulative properties of the unit.

The spacer bars are the bits round the outside that separate the two (or three) panes of glass. They are typically made from aluminium and are are weak spot in terms of thermal transfer. This has led to all sorts of confusion, with some companies measuring and quoting u-values for their units that are taken from the centre of a large pane and exclude spacers, while others come up with a ‘whole window’ calculation. All of which makes direct comparison for the consumer quite tricky. Anyway, to reduce this potential thermal bridge, there are now ‘warm edge’ or ‘soft spacers’ available, that have much better insulative characteristics (though getting hold of this in the UK is an interesting experience – ours were Swiss).

Another consideration is the ‘coating’ the glass panes have.

Although it does not have such an attractive g-value (solar heat gain), so called ‘soft coated’ glass has a better u-value than hard coated glass.  Debates about the merits of one over the other are endless (just do a google search).  We went for hard coating in the end, primarily because it was actually possible to get hold of it here in the UK!

When it comes to coatings, they are applied to a face – and which particular face makes a difference.  So you do have to remember to put the glass in the right way round.

We are skipping ahead a bit here, but when the glass does arrive, it is very exciting, and big!

Specifying the glass (part2)

As you can see Pilkington’s branding of low iron is known as OptiWhite (often shortened to OW), and the low emissivity coating or ‘hard coating’ is called ‘k glass’. These babies are quite thick and very heavy. The lorry driver who delivered them was confused by the triple’ness and wondered why there were “four units instead of the two he had on his delivery manifest”. I had to point out it was actually just two monster big triple ones ;)

Specifying the glass (part2)

So yeah, the point is that you don’t really want these arriving before you have finished making the oak frames, unless you have oodles of room to store them safely.

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