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Victorian end-of-terrace in New Bolsover, Derbyshire

before (above) and after (below) installation of loft

insulation, secondary glazing, internal wall insulation

and insulation of the suspended timber ground floor.

These thermal improvement measures increased the

building’s SAP rating from E (46) to C (73).

operate as efficiently as possible (‘Lean’).

The final consideration is supplying

energy requirements from renewable

sources to minimise greenhouse gas

emissions (‘Green’).

Although this philosophy can be

applied in principle to existing buildings,

a more nuanced approach is needed and

the priorities will differ. For example, while

the ‘fabric first’ approach (which focusses

on achieving a high performance building

envelope) makes perfect sense for a new

building, in a historic building this may be

neither practicable nor desirable. Instead,

effective, cost-efficient and less risky

measures that have minimal impact on

heritage significance might be identified.

Such measures include improving building

services and controls, changing the way a

building is occupied, used and managed,

and questioning current expectations

and standards to find out what is really

necessary. It is important to remember that

success cannot be achieved by technical

means alone – building owners, managers

and occupiers play a crucial role and

should be fully engaged in plans for saving

energy at every stage.

Where building fabric improvements

such as reducing uncontrolled air

infiltration or adding insulation are

considered desirable and feasible

as part of a whole building energy

strategy, careful consideration must

be given to minimising the risks of

unintended consequences. For example,

if adequate provision is not made for

ventilation, making a building more

airtight can result in poor indoor air

quality, with consequential health risks

for the occupants. And the failure to

remove excess moisture generated by

activities within the building can lead to

condensation and mould.

Similarly, the building may be

harmed if added insulation adversely

affects its benign ‘hygric balance’

(water in = water out) leading to a

build-up of moisture within the fabric.

Moisture problems caused by poorly

designed and badly installed external

wall insulation, or pre-existing building

defects which allow rain to penetrate and

become trapped are already becoming

evident in some retrofitted buildings.

In some of the worst cases buildings

have been rendered uninhabitable.

The interactions between a

building and the internal and external

environments are complex and dynamic.

It can be difficult, therefore, to fully

predict the effects of particular retrofit

measures and to assess the technical risks

with any degree of certainty. Although

the risk of moisture accumulation can

be assessed using numerical models – a

range of software applications of varying

degrees of sophistication exists for this

purpose – there is very little empirical

evidence to validate the models.

Concerns about the risk of moisture

accumulation associated with retrofitted

wall insulation have prompted Historic

England and others to obtain data

from systematic site- and laboratory-

based observations conducted over

extended periods. The aim is to

better understand the hygrothermal

behaviour (heat and moisture transfer)

of building elements and the effects of

energy efficiency retrofit measures.



New Bolsover is a model village built by

the Bolsover Collier Company in 1891 on

the outskirts of Bolsover, Derbyshire. It

comprises 206 two- and three-storey brick

houses (Grade II listed) arranged in double

terraces on three sides of a square village

green. In 2011 Historic England (then

English Heritage) leased an end-of-terrace

two-storey brick house and carried out a

package of measures to improve the energy

performance of the building envelope. This

included loft insulation, secondary glazing,

internal wall insulation and insulation of

the suspended timber ground floor. Two

types of wall insulation were used for

comparison: a non-hygroscopic, vapour-

closed system (polyisocyanurate or ‘PIR’),

and a hygroscopic, vapour-open system

(wood fibre).

Hygrothermal monitoring set-up at New Bolsover. Wooden blocks (or dowels) are a convenient method for

assessing moisture content in historic building materials. The moisture content of the timber equilibrates to the

relative humidity of its surroundings and can be determined (within limits) from measurements of electrical

resistance. The humidity of the surroundings can then be deduced.