Warmer Bath

Bath Preservation Trust and the Centre for Sustainable Energy
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then drying out through the action of the sun and the
movement of internal air.
If you install wall insulation over the surface of a
traditional wall, you will compromise its ability to sweat
out any moisture within it. Moisture trapped within a
wall will increase heat losses across the wall, encourage
mould growth and potentially cause damage to timbers
within the walls such as the ends of floor joists. These
risks can be minimised with good design, appropriate
materials and adequate ventilation. However, this is a
complex issue and a careful assessment of the risks of
damp within your walls must be made first. If you are
considering installing wall insulation, seek professional
advice.
Exterior wall insulation
Of the two wall insulation options, exterior insulation
is the most effective and the least risky, as long as a
comprehensive approach is taken. This is because the
entire wall can be covered up so there are fewer routes
for the heat to escape. However the job is complex and
expensive. The wall has to be built out and a new wall
finish created. Everything on the outside of the building
has to be replaced or adjusted: window reveals and sills,
door architraves, roof overhangs, decorative features,
gutters and downpipes, soil and vent pipes and other
services.
Although exterior insulation is obviously not appropriate
for the prominent facades of the buildings of Bath,
there are many secondary facades that are either
hidden or of little importance to the character of the
city. Furthermore, some of the rear facades of Georgian
buildings in Bath were originally rendered, so the
installation of a new rendered facade might actually be
in keeping with the building’s history.
When installing exterior insulation on top of a traditional
wall, it is important to preserve the permeability of the
wall by using a moisture-permeable insulation and
finish. This ensures that any moisture within the wall
does not get trapped. This is especially important for
the outside face of the wall because of the high risk of
rainwater getting into the wall and the importance of the
action of the sun in drying out the wall.
Interior wall insulation
Like exterior wall insulation, the installation of interior
wall insulation typically involves building out the wall,
creating a new wall finish and making adjustments to
all the windows, doors and features affected. In many
of the traditional buildings of Bath, the list of important
features that could be affected is considerable and
includes panelling, dados, skirting boards, cornices,
plaster work and reveals around architraves. Internal wall
insulation is unlikely to be viable if important historic
features are in place.
The installation of internal insulation is a complex
process, best done as part of a major renovation.
However, this depends on how much insulation you
want to install and how much room space you are willing
to lose.
The thinnest interior wall insulation is a proprietary
product, Sempatap, which can be installed with relatively
little impact on existing features as it is only 10mm thick.
It is effectively a thermal wall-paper. Although thin, it
cuts heat losses, raises the wall surface temperature and
so makes a room feel warmer. It is relatively inexpensive
and easy to install.
If you want to achieve more meaningful reductions
in your heat losses, you will have to build out the wall
and make adjustments to all the wall details including
window reveals, architraves and mouldings. You can
build out either by fixing an insulating board directly
to the wall or, for deeper insulation, building out the
wall with timber battens, infilling with insulation and
boarding over. Either way, a plaster finish completes the
job and helps to make the wall airtight.
The big problem with interior wall insulation is ‘cold
bridging’. This happens at all the points where the
insulation stops and the heat can get round the edges
such as the junctions with windows, floors and ceilings
and around sockets which have been cut into the
insulation. It can also happen at a junction with an
internal wall unless the insulation is taken round the
corner for at least a couple of feet. At all these points,
heat and water vapour escape, the temperature drops
and the risk of condensation increases. This can lead
to long-term problems, especially if the condensation
is hidden, for example within a floor void between
ground and first floor. These risks can be reduced, for
example by treating timbers that could be affected,
installing insulation in floor voids, and keeping indoor
humidity low through controlled ventilation (see case
study opposite). However, given the risks involved, you
should seek professional advice to work out the best
approach. Great care is needed in detailing all the joins
and junctions of the insulation with the existing building
fabric.
Most internal insulation jobs in traditional buildings use
synthetic insulation materials that are impermeable to
moisture. They focus on keeping moisture out of the
walls, including using a vapour barrier on the inside of
the plasterboard. Most of the products on the market,
such as insulation-backed plasterboard, are made with
impermeable insulation. This approach is reasonable,
given the problems that can arise when moisture does
get into a wall, and is supported by English Heritage*.
Nonetheless, there is a case for using natural moisturepermeable
materials for internal insulation as well as
external insulation. If condensation does occur in your
wall after the insulation has been installed, moisturepermeable
materials will allow the moisture to escape to
the inside of the building. We do not yet know enough
about how traditional buildings perform with solid wall
insulation to give the final word on this issue.