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BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
cooler, potentially inviting interstitial
condensation. When combined with a
decrease in air movement due to draft
exclusion, there is a real risk that dry
exterior walls above ground floor level
could become damp. Proposals for the
retrofit were therefore preceded by a
three-year programme of investigation,
monitoring and modelling to develop
a clear picture of the hygrothermal
performance of the spaces and fabric that
could be most affected, and to provide
a benchmark for assessing subsequent
performance, from one season to another.
A modified form of WUFI software
was used by building physics engineers
Max Fordham to explore how the
materials would be affected. WUFI (an
acronym of
wärme und feuchte instationär
– heat and moisture transiency) tends to
underestimate the thermal performance
of traditional materials. Old bricks,
for example, tend to be less well fired
than modern ones and the clays are less
uniform so they do not conduct heat
as well. Nevertheless, WUFI provides a
useful model for assessing the relative
performance of insulation measures and
the effects of cold bridging, particularly
when combined with real data from
monitoring the performance of the existing
structures, and by material analysis.
Samples of brick, stone and render were
therefore sent for testing by Glasgow
Caledonian University, and probes were
installed by Archimetrics in 2011 to
record real time variations in moisture
and temperature at four depths through
walls of different orientation and material.
A weather station was also installed so
the WUFI model could be calibrated
according to the local environmental
conditions and the U-values of the walls
recorded by Archimetrics.
In addition to the technical impact
on the performance of historic fabric, the
insulation of walls and windows has a
substantial design impact, and all aspects
of the retrofit would affect the historic and
architectural significance of the building.
Before any proposals were put forward,
the building was thoroughly surveyed by
the architects and Beacon Planning to
identify how the building had evolved,
what alterations had been made in the
past, and what fabric was original. At New
Court, exterior insulation was clearly out
of the question. Interiors, on the other
hand, were generally quite plain and had
been affected by alterations over the
course of 185 years of student occupation,
particularly in the 1970s when extensive
repairs were required for dry rot.
From a design perspective, phenolic
foam insulation offers the least intrusive
solution as it gives the highest insulation
levels for the least thickness, but the
material is impermeable. The WUFI
modelling indicated that this could
cause problems on those elevations most
exposed to driving rain, and a permeable
solution which allowed evaporation from
both interior and exterior surfaces would
be necessary, particularly on north- and
west-facing walls. The exception was
in rooms where high levels of humidity
would be expected, such as bathrooms.
The solution was to locate en suite showers
and bathrooms away from external walls
and ventilate them thoroughly. Only two
bathrooms could not easily be moved. In
these cases the design of the ventilation
was particularly important to ensure that
the interior vapour pressure remains
within acceptable levels, and moisture
levels in these walls will be monitored
carefully for years to come.
INTERIOR WALL INSULATION
Most rooms had been subjected to
extensive repairs in the past, particularly
the exterior walls, due to defective
gutters and outbreaks of dry rot. Few
retained original plasterwork. The
exterior walls were stripped of their
plaster finishes and refinished with a
lime plaster base coat to ensure that all
gaps were sealed, particularly where
penetrated by structural timbers and
joinery. As well as being essential for
air-tightness, this would also help to
draw moisture away from joist ends
and other vulnerable timbers. The
Originally the façades facing the courtyard were all rendered with Roman cement, later repairs were executed in
cement, and they have now been re-rendered using a more permeable hydraulic lime painted with limewash (above).
One of the windows facing the courtyard (left) at the start of the project, and (right), the same image modified
to show the architect’s proposals for re-colouring the walls and window frames, following surviving evidence of
the original colour scheme (All photos: Tim Soar)