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BCD SPECIAL REPORT ON
HERITAGE RETROFIT
FIRST ANNUAL EDITION
Wood-wool insulation boards ready for lime rendering
and daub as possible. The theoretically
poorer U-value may not be as bad in
practice and the greatest reduction in
heat-loss is often achieved simply by
creating a dry, draught-free structure.
A modern material similar in concept
to daub, but with more durability and
better U-value, is a hydraulic lime/hemp
mix that can be cast in-situ
to form a
homogenous breathable infill.
If the frame and/or the panels are
in poor condition and repairs would
involve the loss of a high proportion of
historically significant fabric, there would
be a strong case for protecting the wall
behind a shelter coat of lime render or
other regionally appropriate material. This
is usually preferable to creating a crude
modern replica of the wall in band-sawn
timber, and may provide the opportunity
to insulate outside the wall line.
INSIDE THE WALL LINE
If the timber frame and infill are in
sufficiently good condition, and are robust
enough to cope with continuing exposure
with limited interventions, insulation
can be fitted to the inside face, either
directly to the wall or with an air gap.
However, this will have a serious impact
on the appearance of the room, obscuring
features such as window surrounds,
skirtings and adjacent ceiling mouldings,
and it will reduce the internal floor area.
More significantly, there is an increased
risk that moisture entering the wall will
become trapped, even if all the materials
used in the new lining (insulation, plaster
and paint finish) are vapour permeable.
If problems do occur, they are unlikely to
become apparent until significant damage
has occurred. The risk of driven rain
penetration can be reduced by careful
gap-stopping and the reinstatement of
overhangs, but any intervention that
restricts the passage of water vapour
through the wall significantly increases
the risk of condensation and/or water
entrapment. For this reason, non-
breathable rigid insulation such as PIR
(polyisocyanurate) boards should not be
used, even though they can achieve better
U-values at relatively small thicknesses.
Insulating inside the wall line also
greatly increases the risk of condensation
due to cold-bridging in those areas which,
for various reasons, cannot be insulated.
In particular, the ends of floor beams and
joists built into the external wall are at
greater risk of increased degradation.
OUTSIDE THE WALL LINE
For many reasons, fitting insulation to the
outside face of a timber-framed wall is
often the best solution, both in terms of
hygrothermal performance and building
conservation.
• The wall is fully protected (assuming
materials and detailing are correct)
• Necessary repairs can be kept to the
minimum structurally required, and
can usually take the form of additional
surface-fixed straps, etc. These repairs
are reversible and involve no loss of
historic fabric.
• Air penetration through the wall can
be fully controlled
• Insulation can be continuous with
all original fabric on the warm side,
reducing the risk of cold-bridging and
condensation
• Keeping what thermal mass there is in
the wall on the warm side also helps to
balance diurnal variations
• The historic significance and
appearance of the interior is not
compromised
• The intervention is reversible.
External insulation will alter the external
appearance: the additional thickness
requires changes to window reveals and
other features, and conceals the timber
frame. This often meets with resistance,
both professional and public. However,
there is a strong historical precedent and
the benefits are considerable.
Historically, render was usually
applied direct to lath nailed to the frame,
and it is widely held that this must offer
good protection to the frame, simply
because it is breathable. However, it is
quite common to find widespread active
Deathwatch beetle attack in timbers
immediately behind lime renders, but
rare to find it in exposed external timbers,
suggesting that sometimes moisture
content of a lime-rendered frame can be
high enough to sustain fungal and beetle
attack. When applying new or replacing
old render, a vapour permeable membrane
should be used and the lath set off the
frame on counter-battens if possible.
The recent development of relatively
high-performance breathable multi-layer
insulation quilts, effectively insulated
breather membranes, has great potential
as they increase wall thickness far less
than most other breathable insulation
materials. Although designed for use in
roofs, these quilts have been successfully
used to insulate timber-framed walls
behind render or weatherboard. New
materials need to be used cautiously until
their long-term performance is better
understood, but equally, they should not
be dismissed out of hand. Furthermore,
imported materials that perform well in
cold dry climates may not work in wetter
UK conditions. Perhaps the best advice is
to question everything.
In a surprising number of cases, what
appears to be a timber frame is actually
an agglomeration of paint, mastic and
cementitious render repair concealing
a severely degraded and structurally
compromised frame. Sooner or later
this will require such extensive repair/
replacement that protection with a lime
render or other cladding would almost
certainly provide a more effective and
conservative solution while avoiding
further loss.
If the appearance of a
timber-framed building is deemed
desirable, this can always be applied
to the face of the new render – there
is a long tradition of what many now
consider ‘fakery’. At least what remains
of the frame and surrounding fabric
is retained for future generations.
RELATED REPAIRS
If the timber frame is to remain exposed,
the essential first step in improving
the thermal performance is to ensure
that the frame and surrounding fabric
are in good condition, and consist of
materials that allow the wall to breathe.
A conflict arises where an alteration
regarded as part of the building’s history
is demonstrably causing damage. Brick
infill for example, does not always
cause problems, but can significantly
increase the rate of degradation of the
frame, particularly when bedded in
cementitious mortar, where frames
are relatively light, poorly constructed
or weakened by decay, or where the
bricks project outside the face of the
frame, creating ledges that trap water.
The use of inappropriate materials is
not the only problem. The introduction
of impermeable materials was usually
prompted by the failure of earlier or
original wattle and daub infill, which
usually began to fail once the protection
of big overhangs was lost. Although