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In-situ
U-values (W/m2K)
BR443 U-values (W/m2K)
In-situ U-values
Linear (BuildDesk U-values)
All U-value results 2009–2011 – BR 443/In-situ U-value comparison
solid-walled buildings. One of these is the
EU-funded SUSREF Project (Sustainable
Refurbishment of Building Facades and
External Walls). This project includes
modelling of buildings in Wales by BRE
Wales and Cardiff University to examine the
distribution of moisture in a solid wall and the
changes that occur when improvements are
made to the wall’s thermal performance.
Another project is being co-ordinated
by the Society for the Protection of Ancient
Buildings (SPAB). The results were reported at
a meeting held in June 2011. An example of the
equipment used is shown in Figure 1 and one
of the outputs can be seen in Figure 2, which
shows changes in temperature, water content
and relative humidity.
Understanding the distribution of
moisture and temperature in a wall is
important if changes such as the addition
of internal insulation are to be made to it.
Such changes may alter the point at which
interstitial condensation occurs or the
minimum winter temperature. (Interstitial
condensation occurs when relatively warm
moisture-laden air diffuses into a vapour-
permeable material – if it is relatively
warm on one side and below the dew point
temperature on the other, the moisture-
laden air can reach ‘dew point’ within the
material and deposit water there). These
changes can affect the long term durability of
the wall materials and of any timber or steel
frame materials, which may be particularly
vulnerable. The research undertaken by
SPAB has also shown that moisture content
can affect the measured U-value of the
wall by between 10 and 40 per cent.
Thermal conductivity
Most thermal performance calculations rely
on a series of assumed values, often based on
measured values and then extrapolated to
cover a wider range of building materials. In
the case of natural stone the values used are
often based on the density of the material and
assume that the wall is solid stone whereas, of
course, stone walls are rarely solid and include
varying numbers of voids which may or may
not be filled with rubble and/or mortar.
Two recent studies, The SPAB Research
Report 1: U-value Report (2010) and Historic
Scotland’s U-values and Traditional
Buildings (2011) have focused on U-values
as an indicator of thermal performance
and involved the comparison of in situ
measurements with U-values calculated
with software programs and often-used
‘default’ U-values. A key objective was
to help construction professionals and
building energy performance assessors
make more informed and balanced decisions
when assessing and improving the energy
performance of traditional buildings.
U-values are normally calculated with
computer programs developed with modern
non-traditional construction in mind which
follow the conventions set out in BR 443:
Conventions for U-value Calculations. For
the studies, in situ U-value measurements
were carried out, mostly of uninsulated
solid walls but, for comparison, some cavity
walls and building elements retrofitted with
insulation were also measured. The non-
invasive measurements were generally taken
of building elements with their internal and
external finishes intact.
The studies then compared the U-values
measured in situ with their calculated
equivalents using the software program
BuildDesk U v3.4. SPAB’s research suggests
that 73 per cent of the traditionally built walls
sampled (including timber, cob, limestone,
slate and granite construction, 59 walls in
total) actually performed better than expected
(see Figure 3). A particular focus of the
Historic Scotland comparison was the impact
of the lime and stone core of a traditional solid
stone wall on thermal performance.
This research is not a criticism of
the calculation methodology or U-value
modelling software, but it does highlight the
difficulty of modelling and calculating the
thermal performance of traditional walls
using conventional techniques. These studies
demonstrate that software programs for
U-value calculations tend to overestimate the
U-values of traditional building elements. In
other words, traditional building elements
tend to perform better thermally than would
be expected from the U-value calculations.
Furthermore, it is suggested that the in situ
measurement of U-values is a useful tool
which can aid in the assessment of the thermal
performance of traditional building elements.
Potential pitfalls
Although historic buildings are often
too precious to alter by adding solid wall
insulation, it is suitable for many unlisted
or converted pre-1919 buildings. In these
cases it is important to first establish that the
alteration will be of significant benefit and
then to understand its probable impact.
Overall, the recent work on moisture and
on thermal values has shown that in order to
make good and reliable plans for the thermal
improvement of solid walls we need to have a
good understand how the walls of a building
are performing now: how the moisture is
distributed, how and how much the walls are
breathing, and how good the current thermal
performance is. Only then is it possible to
make good, low-risk plans for improving the
thermal performance of walls, which will
improve the building’s thermal efficiency
without threatening its fabric.
Acknowledgements
This article draws on the work of many at BRE
and in the wider research community. The
author is very grateful for access to their work
and their ideas on thermal improvements for
solid-wall houses.
Useful Information
Building Research Establishment
www.bre.co.uk
Climate Change and Your Home (English
Heritage)
www.climatechangeandyourhome.org.uk/live
The Energy Saving Trust
www.energysavingtrust.org.uk
Historic Scotland technical papers
www.historic-scotland.gov.uk/
technicalpapers
The National Refurbishment Centre
www.rethinkingrefurbishment.com
The Refurbishment Portal
www.rethinkingrefurbishment.com/portal
The Society for the Protection of Ancient
Buildings research reports
www.spab.org.uk/advice/energy-efficiency
Sustainable Refurbishment of Building
Facades and External Walls
http://cic.vtt.fi/susref
Tim Yates
PhD is technical director of the
Building Research Establishment Ltd (see
page 38) with responsibility for projects on
heritage buildings. He has been involved in
the built heritage for more than 35 years and
is currently chairman of the British Standards
Committee on Cultural Heritage.
Figure 3 Plot of calculated U-values (using BR 443) versus measured U-values for a range of solid walls from
The SPAB
Research Report 1: U-value Report
(‘BR 443’ refers to the Building Research Establishment document
BR 443:
Conventions for U-value Calculations
, which describes the calculation methods for determining the U-values of building
elements based on British Standards)