26
BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
these U-values have been measured
rather than derived from the standard
calculating method, which has been
shown to have limitations when used
to estimate heat loss for solid walls (see
Further Information: Baker and BRE).
The wall at Drewsteignton is quite
different, being a north-west-facing,
600mm thick granite construction. In
this wall we find higher moisture levels
(in terms of both %RH and AH g/m
3
) and
narrower saturation margins. We also find,
over the past three years, a trend of rising
RH in the centre of the wall which, year
on year, moves this part of the wall closer
to saturation conditions. As this trend
has continued over a number of years, we
conclude that the high RH within the wall
is not solely a response to atmospheric
conditions but is also a function of certain
qualities of the construction that might
limit or inhibit drying.
This may be, in part, down to the
heavyweight nature of the wall and its
aspect, but vapour profiles have climbed
since the wall was insulated and have
not returned to pre-insulation levels.
This suggests that the insulation itself
may be having some impact on the
wall’s performance, although it is not
clear whether this is primarily due to its
thickness or its impermeability.
The wall at Drewsteignton has been
insulated with a greater quantity of more
thermally resistive insulation and this
reduced the measured in situ U-value
from 1.20 W/m2K to 0.16 W/m2K. This
ensures that less heat passes into the cold
side of the masonry during the winter
period, thus saturation margins are
lower and air is more likely to become
saturated and remain saturated for
longer periods, limiting the wall’s ability
to dry. Furthermore, the foil-facing of
the PIR board acts as a barrier to the
movement of moisture from the core
of the wall, which can no longer access
the potential evaporative surface of the
interior wall face.
In conclusion, we find that the
performance of these walls is in part
conditioned by their individual material
components, including changes made to
the fabric to improve energy efficiency.
Interstitial condensation has been a
particular concern, yet the internally
insulated brick wall at Shrewsbury, which
uses a limited quantity of insulation and
does not incorporate a VCL, has stable
vapour responses that operate within
safe limits. In contrast, at Drewsteignton,
where insulation has reduced the U-value
of the wall to a fraction of its previous
heat loss and a VCL limits the movement
of vapour towards the internal side of the
wall, vapour conditions are deteriorating.
The measurements from the BPS
tell us that, rather than internally
generated moisture, the influence of the
external environment in combination
with the individual circumstances of
the walls – their materials, aspect and
condition – has the greatest impact on
their moisture performance. These walls
are solid, there is no capillary break
in the form of a cavity or damp-proof
course to prevent moisture, particularly
wind-driven rain, penetrating
deep into the core of the wall.
Many solid walls are thick, built with
heavyweight materials and they can be
shaded and/or sheltered. This means that
the ability of heat and air movement to
dry these walls may be limited. While
this may not, prior to insulation, create a
moisture problem in the wall, the method
by which a wall is retrofitted must take
into account all the factors which might
impinge upon its performance.
The decision as to what type and
what thickness of wall insulation might
be suitable for a solid wall cannot be
answered by looking at heat loss reduction
alone. Those charged with improving the
energy profiles of these buildings must
view the building as a whole, looking at
how it may perform in its specific context
including individual wall aspects and what
the effect of its constituent materials,
condition and finishes may be.
The wall at Drewsteignton shows that
the use of a relatively large quantity of
higher performing close-cell insulation,
incorporating an impermeable VCL,
can result in a risky vapour profile.
This is not to say that the application of
similar material to the internal wall at
Shrewsbury would have produced the
same results. Indeed, at this location the
wall’s performance may have been more
satisfactory as this wall is able to dry
more readily. However, the BPS shows
how complex and multifactorial the
hygrothermal performance of walls can
be. It is an interplay between materials,
condition and context, and the exact
effect of all these upon the long-term
performance of the building may remain
unknown or difficult to predict.
Given this uncertainty, we need
to acknowledge the limits of our
understanding and adopt a precautionary
principle. This would ensure that
elements are not deprived of all internally
generated heat by excessive amounts
of internal insulation because it may
be that it is the contribution of this
heat, in combination with external
solar radiation, that allows the wall to
moderate its moisture load over time. In
addition, materials that are vapour-open
and capillary-active and thus have some
ability to move moisture through the
structure to surfaces from which it can
evaporate are also more likely to be a safer
option for the insulation of a solid wall.
This study demonstrates that it
is possible to make positive changes
to the energy efficiency of solid walls
through the application of insulation
but that an approach that favours
limited improvements to heat loss
and materials that promote moisture
movement may introduce less risk
than alternative strategies.
Further Information
H Altamirano-Medina et al, ‘Guidelines to
Avoid Mould Growth in Buildings’,
Advances
in Building Energy Research
, Vol 3:1, 2009
P Baker,
Technical Paper 10: U-values and
Traditional Buildings
, Historic Scotland,
Edinburgh, 2011
(http://bc-url.com/trad-buildings)Building Research Establishment,
In-situ
Measurements of Wall U-values in English
Housing
, BRE/Department for Energy and
Climate Change, Watford, 2014
(http://bc-url.com/wall-uvalues)
Department for Communities and Local
Government,
Approved document F:
Ventilation
, NBS, London, 2010
CAROLINE RYE
and
CAMERON SCOTT
founded the research company
ArchiMetrics Ltd in 2011, with the aim of
increasing the understanding of building
performance via the measurement of
real buildings. ArchiMetrics carries out
research using bespoke methods of
measurement and analysis to ensure an
integrated and thorough approach, and
the partnership specialises in older or non-
standard buildings throughout the UK.
Monitoring a south-facing brick wall at Shrewsbury