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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).



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