Environmental control of damp
Most of the damp situations that we encounter during our
surveys are usually remedied without the need for any
form of damp-proofing works. If the control of damp can
be addressed by balancing the ingress of moisture and
the evaporation rate of dampness through moisture
reservoirs and moisture sinks ( see diagrams ) then this
should be sufficient to prevent any internal damage to
plaster, skirtings and sub-floor timbers.
Before considering the concept of moisture reservoirs
and moisture sinks the external fabric of the
property should be inspected and any obvious defects
repaired. This may seem obvious but we get numerous
calls to remedy rising or penetrating dampness and the
source of the moisture is often a leaking downpipe or
cistern overflow.
Internal pipe leaks are also often a cause of dampness
that can be misinterpreted as rising damp and many of
our clients have been recommended to have damp-proofing
works where the cause of damp was from a leaking shower
unit and was easily rectified with a tube of silicon
mastic.
The most common moisture reservoirs that create rising
damp are undrained ground and raised ground levels
bridging existing damp-proof courses. This allows
low-level moisture ingress which then migrates through
the wall by capillary action and can appear as 'rising
damp' internally affecting plaster, skirtings and other
timbers abutting the damp walls. Reducing ground levels
and installing drainage or ventilating channels at the
base of the affected walls will remove the source of
moisture and create a moisture sink by allowing
evaporation from the exposed wall.
Usually no plastering is required as the walls should
dry out gradually but if the plaster needs to be renewed
we find that the best option is to use a combined
damp-proofing and insulating system with a damp-proof
membrane such as Newlath 500 or Delta Plaster-Lath
fitted to the damp walls which is then over-boarded with
Larfarge Thermacheck plasterboard or Walltransform
Insulating Plaster. As well as providing a damp-free
internal finish ready fro decorating the U-value ( on a
standard 9 inch/225mm thickness wall ) will be reduced
in accordance with Building Regulations to around 0.3
which results in less heat loss through walls and lower
energy bills.
Another major moisture sink is sub-floor ventilation. If
existing air bricks are cleaned out and additional
sub-floor vents installed then the improved ventilation
under the floor will provide lower humidity levels and
help to purge excess moisture from the sub-floor void.
The lack of effective ventilation often causes water
vapour to condense on the exposed masonry in the floor
void which can then travel up the wall by capillary
action and appear as 'rising damp' The increased air
flow prevents this from happening and also reduces the
moisture content of floor timbers such as joists,
wall-plates and floorboards etc thereby reducing the
risk of fungal decay and insect infestation, providing
less justification for the use of chemical treatments to
prevent woodworm or wet and dry rot.
Installing extra air bricks also has the benefit of
reducing damp in the solid walls above them and the
effect is similar to that described by G & I Massari in
their book 'Damp Buildings, Old and New'. They showed
how the water absorbed by a porous material is slowed by
reducing the area of the base in contact with the
moisture source. Three bricks were inserted into a water
source. One with the whole cross section of 14cm intact,
another with a 5cm section cut out to leave 9cm in
contact with the water and a third brick with 9cm cut
out leaving 5cm available to water. The different rates
of absorption in each brick can be seen in the figure
below with the rising wet front reaching the top of the
brick with times ranging from 5 hours for the full brick
to 33 hours for the brick with 9cm removed. Full
saturation of the full brick as a result of capillary
absorption and air diffusion would take approximately
two years.
The experiment demonstrated that reducing the inflow
area greatly extends the time it takes for water to rise
to the top of the brick: by a factor of 7 in the brick
shown on the right, where the area of the header face in
contact with the water is reduced by around two-thirds.
The perforated profile of air bricks acts in the same
way as the reduced section bricks and helps to slow down
the upward passage of moisture in walls and when extra
air bricks are installed in conjunction with ground
reduction/drainage channels then these are usually
sufficient along with the original physical damp-proof
course to prevent any rising dampness occurring
internally.
The Massari effect is also beneficial to chemical
damp-proofing and the siphon tube system where the
drilling of holes at regular intervals reduces the
surface area available to moisture and therefore reduces
the capillary effect of any rising damp and we have on
occasion been instructed to install siphon systems
internally behind skirtings to gain some of these
advantages without having the unsightly holes on the
outside walls
Further reading:
Water transport in Brick, Stone and Concrete by
Christopher Hall and William D Hoff
Capillary flow in Building Elements by Christopher Hall
Damp Buildings , Old and New by Giovanni and Ippolito
Massari
Treatment of rising damp in historical buildings:
wall base ventilation Building and Environment, Volume
42, Issue 1, January 2007, Pages 424-435 M. Isabel M.
Torres and Vasco Peixoto de Freitas
Moisture beneath suspended timber floors
Structural Survey, Volume 13, Number 3, 1995 , pp. 11-15
Harris D.J.
Also recommended is the DVD by Mike Parrett, star of the
BBC 2 series Raising the Roof, entitled Building
Pathology which offers a holistic approach into the
diagnosis of building failure together with effective
remedies for common defects.
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