12
BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
the flow of air through the interspace ensures
that any condensation rapidly evaporates.
The reduction in condensation on the
internal face of the historic glass reduces
deterioration associated with both dissolution
of the soluble components of the glass and
failure of the paint and enamel layers. In
addition it has a direct and significant effect
in reducing microbiological growth.
Research on grisaille glass has shown
that reducing the range of rapid temperature
change is particularly important for glass
with applied layers of paint or enamel (in
grisaille work, designs are generally painted
onto the face of predominantly ‘white’ glass
in blacks and greys). In these types of glass,
large and rapid temperature changes can
lead to stresses developing at the boundary
between the applied layers and the body
glass, resulting in delamination and flaking.
Many of these benefits were also observed
on externally ventilated systems, although they
were found to be less efficient, providing a
lower level of protection. Externally ventilated
systems have the additional disadvantage that
they can allow wind-driven rain to enter the
interspace through the ventilation openings.
One much-repeated concern regarding
protective glazing has been that it reduces
rainwater washing of the historic glass
surface, leading to the build up of dirt and
pollutants, including particles that retain water.
While surface washing will be prevented,
protective glazing has been shown to lead
to lower levels of particulates and gaseous
pollution. This indicates that the loss of the
washing effect is unlikely to be critical.
Data collected from many UK sites has
demonstrated that even an inefficient system
of secondary glazing can offer significant
protection, minimising the impact of
external factors such as wind and rain and
greatly reducing internal condensation.
MATHEMATICAL MODELLING
With the aid of computational fluid dynamics,
a computer simulation of protective glazing
systems has been created to identify the
key thermal performance features and
phenomena. This has shown that the air vents
should ideally be placed at the very top and
bottom of the glazing to maximise airflow.
However, air vents on the front face can
still be effective if they are properly sized.
To promote optimal airflow the air vents at
the top and bottom should also be equally sized
and distributed as evenly as possible across
the width of the glazing. While larger vents
improve interspace airflow – and therefore
thermal buffering – the improvement slows
when the ratio of vent area to interspace area
(measured horizontally) exceeds 30 per cent
as turbulence increases. The use of fine wire
mesh and other materials covering vents also
decreases efficiency of airflow so an unrestricted
vent size of 25–30 per cent is desirable.
Further modelling of realistic geometries
for stained glass windows with protective
glazing, such as small and large lancet
windows and tracery lights, is planned. This
should also further test different air vent
geometries and different interspace depths.
ENERGY EFFICIENCY
Previous research into the energy efficiency
benefits of ventilated protective glazing has
been limited. Studies in the USA on the use of
storm windows with clear glass have shown a
significant reduction in heat loss, even when
the units have some air infiltration. While these
systems do not achieve the same levels of heat-
loss reduction as sealed secondary glazing or
double glazing units, an increase in efficiency
of up to 80 per cent is commonly observed.
The use of low-emissivity glass can also have
a significant effect in reducing heat loss.
However, the amount of glazing in a church
or cathedral relative to the amount of masonry,
is likely to remain the critical parameter in
terms of how much heat is lost via the envelope
and therefore the extent to which protective
glazing can improve overall thermal efficiency.
Nevertheless, in a church with extensive
glazing, the impact could be significant.
OUTCOMES
The extensive environmental monitoring
data from protective glazing systems in a
number of UK case study sites confirmed
previous published findings and provided a
more detailed understanding of how different
systems function. The research outcomes
will draw together these findings with the
results from previous studies in Europe and
will provide detailed information on the
benefits of protective glazing. Together with
the mathematical modelling, the analysis
will enable an overview of the critical design
features for protective glazing systems
and will help to identify which features of
the design are most significant in building
efficient systems. The aesthetic impact
and functional performance of protective
glazing can then be evaluated against
the deterioration of the stained glass.
The information will be available as a
guidance document from English Heritage
for conservation practitioners and advisors as
well as those charged with the care of historic
stained glass.
Recommended Reading
D Anderson, ‘Stained Glass and Its Decay’,
The Conservation and Repair of
Ecclesiastical Buildings, Cathedral
Communications Limited, Tisbury, 1996
(available online at: bc-url.com/glass-decay
M Bambrough, ‘Aesthetic Protective
Glazing’, Historic Churches, Cathedral
Communications Limited, Tisbury,
2005 (available online at: bc-url.com/
bambrough)
F Becherini et al, ‘Thermal Stress as a Possible
Cause of Paintwork Loss in Medieval
Stained Glass Windows’, Studies in
Conservation, 53, 2008
A Bernardi et al, ‘Conservation of Stained Glass
Windows with Protective Glazing’, Journal
of Cultural Heritage, 14, 2013
Corpus Vitrearum Medii Aevi: Medieval
Stained Glass in Great Britain,
conservation section:
/
conserv/index.html
RHM Godoi et al, ‘The Shielding Effect of the
Protective Glazing of Historical Stained
Glass Windows’, Atmospheric Environment,
40, 2006
T Husband et al (eds), The Art of Collaboration:
Stained Glass Conservation in the 21st
Century, Harvey Miller, Washington, 2010
VIDRIO project, for further information see:
TOBIT CURTEIS
trained in the conservation of
wall paintings at the Courtauld Institute of Art in
conjunction with the Getty Conservation Institute.
Since 1992 he has run a practice specialising in
architectural conservation and the diagnosis and
control of environmental deterioration in historic
buildings (
. He works in
the UK and abroad for private and institutional
clients, is an external consultant for the Building
Conservation and Research Team at English Heritage
and is the National Trust’s advisor on wall paintings.
NAOMI LUXFORD
is associate architectural
conservator at Tobit Curteis Associates. As a
research fellow at UCL she studied the effects
of climate on historic furniture. She completed
a PhD in 2009 at the Textile Conservation
Centre, University of Southampton. In 2006 she
graduated from the RCA/V&A Conservation MA
programme having specialised in conservation
science in the care of historic interiors.
Thermographic image showing heat loss though a
stained glass window and the thermal buffering effect
of the protective glazing fitted to the upper centre light
(Photo: Tobit Curteis Associates)
Computational fluid dynamics modelling to assess
how design options affect air flow and performance
(Photo: Element Energy)
