7 / 54
BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
7
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.
VICTORIAN END-OF-TERRACE,
NEW BOLSOVER
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.