8
BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
The thermal performance of the
building envelope was quantified before
and after the improvements using in situ
U-value measurements, co-heating and
air pressurisation tests. The improvements
were also modelled using the government
Standard Assessment Procedure (SAP)
for the energy rating of dwellings, and
the outputs compared with the measured
results (see Further Information for
details of the research report).
In addition, sensors were installed
behind the internal wall insulation in
16 locations to monitor heat and moisture
at the interface with the wall. In a heated
building the addition of internal wall
insulation will make the existing wall
colder because it gains less heat from
the interior of the building. As the wall
Measuring the moisture content of the dowels
manually, using a resistance moisture meter. The
dowels are connected to terminal blocks located to
suit the spacing of the pins on the moisture meter.
This enables repeated measurements to be made
quickly and easily.
Hygrothermal monitoring set-up at Shrewsbury Flaxmill Maltings: the dowels are cut into three sections
isolated by epoxy resin. The ends of the dowels are also sealed with epoxy resin. In each section there is a pair
of embedded electrodes which enable the electrical resistance to be measured remotely to provide a moisture
profile through the thickness of the wall.
The interior of the flax mill: the line of short cast iron columns once held the drive shaft to power the looms.
becomes colder, its humidity increases.
This prompts two questions: does the wall
remain sufficiently wet for long enough to
harm building fabric? And does moisture
tend to accumulate over time? The sensors
installed measure temperature, relative
humidity and the moisture content of a
small block of wood (page 7).
Moisture monitoring at New Bolsover
has been carried out continuously
since 2011. During this period seasonal
fluctuations have been observed, with
walls becoming wetter in winter and
drying out during the summer months.
So far, no conclusive evidence of moisture
accumulation behind either insulation
system has been observed, but monitoring
is continuing. (The results of the work
to date will be published in an interim
research report report which will be
available for download from Historic
England’s website).
GEORGIAN FLAX MILL,
SHREWSBURY
Historic England has also been
monitoring site trials to assess the
effects of internal wall insulation on the
hygrothermal behaviour of brickwork
at Shrewsbury Flaxmill Maltings,
Shropshire, (also known as Ditherington
Flax Mill). The internationally important
historic site comprises seven listed
buildings, including the main mill
(Grade I listed), built in 1797 to the
designs of Charles Bage. It is the world’s
first iron-framed building – a forerunner
to the modern skyscraper.
The site ceased trading in 1987
and became derelict. English Heritage
acquired it in 2005 and carried out
emergency works to halt the decline of its
buildings. Plans to bring the site back into
sustainable beneficial use as a mixed-use
commercial and residential development
are in preparation. In this context
questions have arisen about the extent
to which wall insulation might form part
of the energy strategy for the buildings,
providing an ideal opportunity for further
research into its effects.
Two systems of internal wall
insulation, similar to those used at New
Bolsover, have been installed in trial areas
on three exterior walls in a room in the
engine house adjoining the main mill. The
1½-brick thick walls face south, east and
west respectively. The monitoring set-up
is similar to that at New Bolsover, except
that wooden dowel moisture sensors have
been installed in holes drilled in the walls
to within 50mm of the external faces.
This allows moisture profiles through the
thickness of the wall to be obtained, in
addition to heat and moisture transfer