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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.


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


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.)


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.


Historic England Research Report,


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




Historic Environment Scotland guidance on

saving energy in traditional buildings is

available at





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


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.