The challenge of replacing windows
in listed properties
By Andrew Madge, Managing Director of Gowercroft Joinery
Finding appropriate replacements for
old and degraded single-glazed windows
in heritage and listed properties can be
a challenge, particularly when trying to
combine building conservation requirements
with modern lifestyle aspirations.
When the original wooden window frames
are too rotten to be repaired, the traditional
approach has usually been to specify like-
for-like handmade single-glazed timber
replacements, which usually means having
to accept a relatively poor level of thermal
and acoustic performance or opt for
secondary glazing solutions to compensate.
In an ideal world, most of today’s owners
of old and period properties would like
to benefit from modern performance
standards, including durability, sustainability
and a good level of energy efficiency, in
order to create a pleasant living environment
but without spoiling the original aesthetic.
Energy efficient double-glazing
Although superior energy efficiency can be
achieved through standard factory fitted
double-glazed units (typically with a 16 –
20mm Argon filled cavity), the giveaway
‘double shadow’ on the glazing and the
thicker profiles required to accommodate
heavy double-glazed units are almost
invariably unacceptable for period
properties.
This has led to the popularity of ‘low
sightline’ slim double-glazing, which aims
to combine some of the features of a slim
window with better thermal performance.
This is achieved by using a heavier inert
gas such as Krypton or Xenon to reduce the
thermal conductivity sufficiently to create
an effective smaller cavity in between the
panes. The sightline, (which is the area
from the edge of the glass to the top of
the spacer bar), is typically reduced to just
5-6mm to allow thinner window sections.
However, this type of window is now the
subject of debate, not least within the Glass
and Glazing Federation (GGF), because in
order to achieve such a slim slightline it is
necessary to reduce the amount of sealant
and desiccant used in the perimeter of the
unit. This can cause instability and increase
the likelihood of unit failure. In fact, there
is an ongoing discussion as to whether
some of these units even comply with the
Construction Products Regulations (CPR) 1 .
As a result of these still unresolved
matters, striking the right balance between
performance and aesthetics has become
even more challenging and complicated for
specifiers.
1. https://www.ggf.org.uk/low-sightline-insulated-glass-units/. 2. https://www.changeworks.org.uk/sites/default/files/Double_Glazing_in_Listed_Building.pdf
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