BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
23
INTERNALLY INSULATED
SOLID WALLS
The SPAB building performance survey
CAROLINE RYE and CAMERON SCOTT
T
HE SOCIETY for the Protection
of Ancient Buildings Building
Performance Survey (SPAB BPS)
was first established in 2011 to address
the dearth of information on energy
efficiency and traditional buildings.
In particular, there was an absence
of measured evidence showing how
traditional buildings performed before
alteration, and a lack of understanding as
to what constituted effective and risk-free
energy saving interventions. Of specific
concern was the potential for damage to
fabric and occupants’ wellbeing over the
long-term as a result of the application of
insulation and reductions in ventilation/
air infiltration in older buildings.
The BPS measured various aspects of
performance in solid-wall, traditionally
constructed properties before and
after energy efficiency retrofitting. The
survey looked at fabric heat loss, air
leakage, indoor air quality, wall moisture
behaviour, room comfort and fabric
risk conditions in seven houses.
A central part of the study looked at
the impact of insulation on solid walls.
Measurements of four of the buildings
were made again after refurbishment,
and the analyses of three are ongoing,
with findings published annually online
at
www.spab.org.uk/advice/energy-efficiency. One wall in each of the
three buildings chosen – two internally
insulated and one an externally insulated
cob wall – were subject to extended
interstitial hygrothermal monitoring.
In particular, the internal insulation of
a wall is seen as a risk because fabric
on the external side of the wall, outside
the insulating layer, no longer benefits
from the heat inside the building
and in the winter months becomes
cooler. The effect of this is to lower
the dew point, meaning the air within
the wall may more frequently reach
saturation – 100% relative humidity
(RH) – leading to condensation. High
levels of fabric moisture could give rise
to uncomfortable living conditions and
increased heat loss. They could also
have serious consequences in the form
of mould growth and rot, which can be
harmful both to human health and to the
structural integrity of the building.
Over the past four years, as part
of the BPS, moisture profiles (in the
form of vapour, measured as RH)
and temperature profiles have been
monitored continuously at four points
through and either side of insulated solid
walls. (This element of the BPS work
was extended in 2014 due to a grant
provided by English Heritage.) This
method of moisture monitoring, which
relies on high quality instrumentation
and careful installation, has been
developed specifically for this purpose.
The measurement of water vapour in
North-west facing granite wall in Drewsteignton, Devon: one of two internally insulated solid walls featured
in the Building Performance Survey, this 600mm granite wall had been internally insulated with 100mm of
polyisocyanurate board with an air gap and a plasterboard and gypsum skim finish.