BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
11
the interspace, and temperature changes.
There was a wide variation in the windows
tested in terms of the geometry of the window,
the size of the air vents and the depth of the
interspace. VIDRIO reported that the size
of the interspace and dimensions of the air
vent were fundamental elements for the
ventilation of protective glazing. However, there
were no firm guidelines on what sizes were
critical when designing protective glazing.
The VIDRIO research also showed that glass
corrosion is accelerated by prolonged exposure
to liquid moisture from condensation, as well as
by high levels of gaseous pollution. For unstable
glasses these effects are increased as the glass
develops cracks allowing pollutants and moisture
to continue to penetrate beneath the surface.
ENGLISH HERITAGE RESEARCH
Because of the limited research data available
on specific design details, environmental
effects and the aesthetic impact of protective
glazing, there is considerable difficulty for
conservators, advisory bodies and clients in
evaluating the benefits or drawbacks for specific
installations, in particular in parish churches.
In order to provide the necessary
information to allow informed discussions
to take place, in 2012 English Heritage
commissioned a research programme to look
at both the technical and aesthetic issues
involved and, in particular, questions relating
to energy efficiency. This included a detailed
literature study to establish the current state
of understanding (including translation of
key materials), computational fluid dynamic
modelling to assess how specific design
details affect functionality, and environmental
monitoring and evaluation of existing and
new protective glazing systems, to provide a
better understanding of actual installations.
The research programme is expected to
be completed in 2015 with the results
The ventilation gap at the base of an internally ventilated system: in this example the stained glass has been left in the original glazing grooves and the base of the glass tilted
forward to allow airflow, with a similar gap at the top. (Photo: Holy Well Glass)
Poorly designed and badly maintained grilles obscure
the exterior of a stained glass window, and (below)
shadows cast by a grille disfigure the image inside
(Both photos: Tobit Curteis Associates)
published both in academic journals and as
an English Heritage advisory document.
PRELIMINARY RESULTS
The preliminary results of the research
have demonstrated that protective glazing
provides significant benefits to vulnerable
stained glass. In almost all cases it provides
protection against physical damage, wind-
loading and direct rainfall.
For internally ventilated systems, the
range of temperature fluctuations can be
significantly reduced due to the thermal
buffering provided by the protective glazing.
The lowest temperature values on the
historic glass are generally maintained at a
higher level than on the unprotected glass;
this reduces the dew point temperature and
so minimises condensation. In most well-
designed installations, condensation is entirely
prevented on the historic glass. Condensation
continues to occur on the modern glass but
New protective glazing in the south oculus window
at Canterbury Cathedral, partially obscured by the
heavy structural armature and grille (Photo: Tobit
Curteis Associates)
