Technical Paper 9 Slim-profile double glazing - Historic Scotland

Introduction
The in‐situ centre‐of‐pane U‐values of various ‘slimline’ double‐glazed replacement units,
fitted into Georgian sash and casement windows, were measured in 2009/2010 by the
Centre for Research on Indoor Climate & Health, School of the Built & Natural Environment,
Glasgow Caledonian University (GCU). These measurements were part of the Double Glazing
In Listed Buildings research project by Changeworks published in July 2010 [1]. This paper
has also been published as part of Historic Scotland Technical Paper 9 [2].
In addition to these centre‐of‐pane U‐value measurements, whole‐window U‐values were
estimated using a basic area‐weighted calculation method. (For details, please refer to
page 8 of the Changeworks report.) These calculations were based on assumptions from a
previous research project in 2008, Performance of Traditional Windows [3], in which a
similar window design had been measured.
This lead to three concerns: Firstly, to estimate the whole‐window U‐values for the windows
measured in 2009/2010 more precisely, accurate dimensions of the actually measured
windows should be used rather than an assumption from a previous research project.
Secondly, the windows tested were Georgian style windows (with ‘6 over 6’ glass panes, see
Figure 3). It was felt that is would be beneficial to also calculate windows of the same size
but of two Victorian window designs (‘2 over 2’ and ‘1 over 1’).
And thirdly, the application of an area‐weighted calculation method could be improved by
using the software programme FRAME 3.1 which is a 2‐D finite element model and has
specifically been designed window calculations [4].
In addition to this, it was thought beneficial to also provide a comparison between slimprofile
double glazing and secondary glazing, and indeed both options combined.
The above listed amendments to the calculation method resulted in improved wholewindow
U‐values which are reported in this paper, together with a comparison of the
impact of the three window style designs, and a comparison with the use of secondary
glazing.
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