Vacuum glazing
An alternative option which is now beginning
to attract interest in the heritage sector
is ‘vacuum glazing’, where all the air is
extracted to form a vacuum cavity between
a pane of low emissivity (low-e) glass and
a pane of clear float glass. With no air or
gas between the panes, there is nothing to
transfer heat, so the energy efficiency is
much greater. The same applies to sound.
Originally developed in Japan twenty
years ago as a lightweight, energy efficient
solution for buildings in earthquake zones,
Pilkington Spacia™ is the first commercially
available vacuum glazing in the UK, which
offers U-values of 1.1 W/m2K on its
standard units and as low as 0.9W/m2K on
its higher performance Spacia™ Cool units.
With a total thickness of 6.2mm, (the
vacuum cavity being just 0.2mm), it is
roughly a quarter of the thickness of a
conventional double-glazed unit and half the
thickness of a typical slimline double-glazed
unit. It is also approximately two thirds
of the weight, which makes it narrow and
light enough to fit into most existing timber
window frames without any discernible
double reflection.
However, the vacuum process can only be
achieved by creating a hole in the inner pane,
which is located 50 mm from the edge of the
glass and covered by a permanent 12 mm
black plastic cap. For some conservation
officers, this can be a distraction, but for
many others it is barely noticeable behind
the curtain sweep. Similarly, the 0.25 mm
micro-spacers, which sit at 20mm intervals
to keep the two panes a fixed distance apart,
are visible on close inspection, but not in day
to day use.
The sustainability of a glazed unit imported
from Japan could also potentially be
challenged, until one considers that the
embodied energy required in the production
of inert gasses used in the manufacture
of most conventional double glazing is far
higher than the energy involved in global
shipping. 2
Despite its imperfections, vacuum glazing
technology is now opening up opportunities
for architects and specifiers looking to
replace the glazing on old windows, whilst
preserving as much of the original joinery
as possible. It is also being adopted by
specialist joinery companies wanting to
create traditional looking windows with
modern performance benefits for those
period homes where the original frames
cannot be salvaged.
A modern period window
As modern methods of fabrication can so
easily destroy the overall aesthetic that
conservation officers are trying to preserve,
it is vitally important that any thin glazing
solution is combined with traditional
joinery and hardware, and that any modern
performance enhancing features are as un-
intrusive as possible.
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