Civil Engineering Reference
In-Depth Information
provide adequate resistance to rain
penetration if proper attention is paid
to design and workmanship and if
certain constructions are avoided in
areas of the country with the highest
exposure to wind-driven rain.
Guidance is given in the BRE report
Thermal insulation: avoiding risks
(36)
and BS 5628-3. Cavity wall
insulation, either as insulation batts
built-in during construction, or blown
into the cavity post-construction,
should have been installed in
accordance with a relevant British
Board of Agrément Certificate or, for
foam, with
BS 5618.
A description of the methods for
tracing the causes of dampness and to
discuss the relative merits of the
various remedial options is to be
found in
Rain penetration through
masonry walls: diagnosis and
remedial measures
.
Aerated concrete blocks or panels
clearly must be coated to resist rain
penetration. A range of coating
types has been investigated by BRE
for water penetration, vapour
resistance and durability. Results
from test buildings show the
influence of different coatings on
moisture content gradients in the wall
cross-section: in certain conditions
persistent high levels or even
penetration to the inner
surface occurred
(105)
.
finished floor. With solid ground
floors an overlapping link between
the floor DPM and the wall DPC is a
key detail in preventing rising damp;
often there is no effective linkage and
in older properties there may be no
floor DPM.
Rising damp can also occur if a
cavity wall DPC is bridged by mortar
droppings which had accumulated
during construction at the base of the
wall (Figure 2.28).
Bridging of DPCs is commonly
caused by external render, internal
plaster, external ground, raised steps,
solid floors and external works
(such as screen or boundary walls)
bonded to or abutting the cavity
wall. Accumulation of dropped
building mortar at the base of the wall
cavity can cause bridging. Previously
installed remedial DPC systems may
be ineffective, particularly osmosis
types, those relying on evaporative
tubes, and poorly installed chemical
systems.
Case study
The condition of wall cavities in an
office block
The BRE Advisory Service was asked to
inspect and comment on the condition of
some external wall cavities to resolve a
contractual dispute. The building had been
constructed with a reinforced concrete
frame, and clad with a blockwork inner leaf
and a brick outer leaf, with some glazing
between the columns with masonry panels
beneath. The cavity was 50 mm in width and
unfilled. The walls at roof level finished as a
cavity parapet wall with a brick coping. The
development was in a relatively sheltered
position from the south and westerly
directions, though the east elevation was
more exposed.
A general inspection of the external
brickwork showed that it had been built to a
reasonably good standard. The mortar
joints were hard and well tooled to a 'bucket
handle' profile. The perpends were seen to
rise vertically and remained reasonably
plumb. The interior was finished with
plasterboard dry lining fixed to the
blockwork by plaster dabs (observed when
drilling into the cavity) and decorated by
wallpaper or hessian wall lining.
Holes were drilled into the cavity through
the dry lining and internal blockwork leaf.
The optical probe showed the presence of
both vertical twist ties and butterfly ties and
their approximate position. Sufficient ties
had been installed, and the majority of ties
were sufficiently bedded into the mortar
joints. The standard of the workmanship for
the construction of the cavities was
reasonable. There were, however, examples
of mortar deposits on the ties, mortar
extrusions projecting from the cavity face of
the outer leaf, and drips of the butterfly ties
positioned near or touching the inner leaf.
These defects could divert the water across
to the inner leaf to cause dampness to the
internal finishes.
In the opinion of the Advisory Officer, the
cavity walls were structurally sound.
Although the cleanliness of the cavity could
have been to a higher standard, it was
unlikely that any significant rain ingress
would cross the cavity; if it did then it would
occur at is
o
lated positions.
The opening up of the cavities to remove
mortar deposits and repair or improve cavity
tray details and the subsequent disruption to
the walling could not be justified, especially
as no reports of rain ingress had been
recorded.
Condensation
Masonry cavity walls are not usually
affected by interstitial condensation,
but problems could arise if
impermeable materials are used on
the wall. Any external wall can be
affected by surface condensation, but
cavity walls are most vulnerable
where the cavity is bridged by
materials with a high thermal
conductivity. Where this thermal
bridging is causing a surface
Rising damp
Few buildings constructed with
cavity external walls will have been
constructed without a DPC of some
kind. Where rising dampness is
suspected, the cause is therefore
likely to be breakdown or bridging of
the DPC rather than its omission.
Bridging is much more common than
failure as most DPC materials will
have a very long life in the protected
inner leaf of a wall. The outer leaf
should also have an effective DPC,
but localised failure here is unlikely
to cause dampness internally in the
dwelling.
Where timber ground floors are
used, the DPC in the wall would
normally be below the level of any
timbers but with solid floors the DPC
may be at any level below the
Figure 2.28
A common path for rising
damp
