Heritage Retrofit

9 / 54

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 9
data from the interface between the
insulation and the wall.
During the winter months the room
is heated and humidified to simulate
occupation. Internal air temperature and
relative humidity are monitored while a
weather station records meteorological
data. In addition, there are gauges to
measure the amounts of wind-driven rain
striking the elevations of the building.
A little over a year’s worth of data has
been gathered and is currently being
analysed. However, differences in the
hygrothermal behaviour of insulated and
uninsulated walls, and variations resulting
from the differing orientations of the walls
are already apparent.
Interestingly, the largest differences
between summer and winter temperature
and humidity levels were observed in
the exterior sections of the insulated
walls. In contrast, the scale of the
fluctuations in the exterior sections of the
uninsulated walls was generally smaller.
Further investigation will be needed to
determine whether the magnitude of
these fluctuations would be sufficient to
increase the vulnerability of fragile brick
surfaces to deterioration over time.
WUFI
Measured data obtained from New
Bolsover and Shrewsbury Flaxmill
Maltings is also being used to investigate
factors affecting the accuracy of heat
and moisture transfer simulations using
WUFI software (Wärme Und Feuchte
Instationär or ‘heat and moisture
transiency’) in a project being carried out
on behalf of Historic England by Dr Paul
Baker at Glasgow Caledonian University.
The ability to predict the hygrothermal
behaviour of building components is
important in assessing and managing
moisture risks.
WUFI software has been developed
by the Fraunhofer Institute for Building
Physics, Germany. It complies with
BS EN 15026:2007 which sets out minimum
requirements for simulation software
use to predict one-dimensional transient
heat and moisture transfer in multi-layer
building components exposed on both
sides to transient climate conditions.
In the first phase of the project, WUFI
Pro 5 software was used to simulate
the hygrothermal behaviour of brick
walls, both insulated and uninsulated,
at Shrewsbury Flaxmill Maltings over
a period of 30 years (1960–1990) using
historical meteorological data recorded
at a nearby weather station. The
simulations were carried out using the
material properties of two bricks from
the WUFI database (‘hand-formed brick’
and ‘historical brick’) plus the measured
material properties of a third brick from
the main mill building. Four types of
insulation system were modelled:
• wood fibre
• mineral wool without an air and
vapour control layer (AVCL)
• mineral wool with AVCL
• PIR.
Results of the various simulations were
then compared.
There were significant differences
between the results obtained when the
measured properties of the Flaxmill
brick were used instead of the WUFI
database values. Clearly, it is better
to use the measured properties of
traditional building materials for
hygrothermal simulations – there will
always be uncertainties when using
alternatives from the WUFI database.
(This observation has also been made by
other researchers.) It is a drawback of the
application that the database contains no
traditional UK building materials.
After comparing the simulation
results obtained for the different types
of insulation, wood fibre – which is
hygroscopic and has some vapour
diffusion resistance – appeared to be the
best of the four systems. Mineral wool,
although it has very low vapour resistance,
is non-hygroscopic and therefore unable
to buffer moisture. The insulation systems
with higher vapour diffusion resistance
– mineral wool with AVCL and PIR –
appeared to cause moisture to accumulate
within the walls in the long term.
Altering the rain adherence factor
in the model had a significant effect
on simulation results. The actual rain
adherence factor at the Flaxmill is
unknown, and may vary depending on
the intensity of wind-driven rain. It may
be possible to ‘calibrate’ the model by
adjusting the rain adherence factor based
on the site measurements of wind-
driven rain. These, and other unknown
boundary conditions led to a high level of
uncertainty about the simulation results.
The next stages of the WUFI project
will include sensitivity analysis of input
parameters and the modelling of the
walls at New Bolsover so that a direct
comparison can be made between the
simulation results and the measured data
gathered over the past five years. In due
course it will also be possible to compare
the simulation results with measured data
from Shrewsbury Flaxmill Maltings to
provide further validation of the model.
(A research report on the first phase of
the hygrothermal modelling project can
be downloaded from Historic England’s
website, see Further Information.)
LOOKING AHEAD
In addition to the research described
above, Historic England’s Building
Conservation and Research Team
is also investigating the effects of
added insulation on the hygrothermal
behaviour of roofs and suspended timber
ground floors, including the role of
ventilation in maintaining moisture at
safe levels.
Building physics is complicated.
Dr Paul Baker observed recently ‘It’s not
rocket science – it’s harder!’ And there
are still many gaps in our knowledge and
understanding. Therefore, a very welcome
and timely development has been the
launch earlier this year of the UK Centre
for Moisture in Buildings (UKCMB). This
not-for-profit organisation will work with
partners from academia, government,
industry and the public to substantially
improve the way moisture risk is
understood and managed in the UK.
Watch this space.
Further Information
Historic England, A Retrofit of a Victorian
Terrace House in New Bolsover: A Whole
House Thermal Performance Assessment, 2015
)
Historic England, Ditherington Flax Mill:
Hygrothermal Modelling, 2015 (http://bc-url.
com/ditherington)
Historic England Research Report, External
Wall Insulation in Traditional Buildings: Case
studies of three large-scale project in the North
of England, 2014 )
Historic England technical guides on energy
efficiency and historic buildings are available
at
technical-advice/energy-efficiency-and-
historic-buildings
Historic Environment Scotland guidance on
saving energy in traditional buildings is
available at
scot/advice-and-support/your-property/
saving-energy-in-traditional-buildings/saving-
energy-guidance
Sustainable Traditional Buildings Alliance,
A Bristolian’s Guide to Solid Wall Insulation,
BCC, 2015
)
Sustainable Traditional Buildings Alliance,
Planning for Responsible Retrofit of
Traditional Buildings, 2015
)
UK Centre for Moisture in Buildings
IAIN McCAIG is senior architectural
conservator at Historic England. He
studied architecture before specialising
in building conservation and has many
years of experience in both statutory
conservation bodies and private practice.