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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.