Technical Paper 9 Slim-profile double glazing - Historic Scotland

This is further illustrated in Figure 5 showing U‐values of double glazing (with a build‐up of
4mm‐Xmm‐4mm with the inner pane having a low‐emissivity coating) for varying cavity fills
and widths calculated in accordance with ISO 15099:2003 [5]. This shows an optimum cavity
width for krypton at about 10mm, and for xenon at about 7mm. The procedure to calculate
the properties of gas mixture is more complicated but the optimum cavity width for a
krypton‐xenon mix might be estimated to be about 8mm.
Figure 5: U‐values calculations in accordance with ISO 15099:2003 [5] for double glazing
(with a built‐up of 4mm‐Xmm‐4mm with the outer pane having a low‐emissive coating)
The vacuum glazing is the most effective option offering good thermal performance with a
slim profile. The other double glazed options, whilst giving a significant improvement, are
not optimised for thermal performance. This is sacrificed in order to produce slimmer units
suitable for conservation‐grade properties.
The window type has less effect on the performance of adding secondary glazing, since it
covers most of the window with the result that the calculated values for each of the zones
used by the FRAME program are similar. The results show that single glazing with secondary
glazing is almost as effective as vacuum glazing and generally out performs the other double
glazing options, except for the Slenderglaze system in the Type C window. Using secondary
glazing in conjunction with double glazing is comparable with the vacuum glazing.
The results indicate that secondary glazing is an effective option where the use of
replacement double glazed panes is to be avoided.
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