The wind speed is affected significantly
by local features such as the spacing and height of trees and buildings
and whether the ground is flat or steeply rising.
It follows that a site may be 'Severe' category within an area generally
categorised as 'Very Severe'.
The quantity of rain falling at any point on a wall surface depends
on both the intensity of the rainfall and the direction and speed
of the wind. Rainfall varies
considerably across the country and is largely unaffected by local
features.
Exposure categories defined in terms of wall spell indices, calculated
using the local spell index method, are given below the map in Fig
12.
The indices, derived as they are from inherently variable meteorological
data, should not be regarded as precise.
‘BS 8104 Assessing exposure of walls to wind driven rain’
describes a method of establishing the local spell index for walls
subject to wind driven rain.
This takes account of meteorological data and expresses the worst
expected conditions prevailing in a spell of bad weather during any
three year period.
Where assessment produces an index near the borderline between categories
on the map, the designer should decide which is the most appropriate
category for the particular case, using local knowledge and experience
and the local spell index method specified in BS 8104 Assessing exposure
of walls to wind driven rain.
An assessment of local wall spell indices should be made for each
type of wall and the most exposed part should be given particular
attention.
4.6.2 Resistance to rain
The designer should select a construction appropriate to the local
wall spell index.
By their nature masonry walls are not waterproof. To provide appropriate
resistance to rain penetration the specification, design, detailing
and construction of the total wall element should take account of
local exposure conditions.
Aircrete has an inherent high resistance to the passage of moisture
due to its closed cell internal structure but this should only be
regarded as a bonus of using Aircrete where moisture resistance
is required.
The resistance to rain penetration of Aircrete blockwork depends
on its thickness and its protection if any.
4.6.2.1 Single leaf walls
Rendered Aircrete blockwork is suitable for external solid walls
or external leafs of cavity wall construction to provide thermally
efficient buildings in Severe, Moderate,
and Sheltered exposure zones.
In Very Severe exposure zones the walls should be protected by some
form of impervious cladding e.g. tile hanging or weather boarding.
Rendered 215mm Aircrete blocks in single leaf construction are suitable
in areas of Severe exposure conditions, but cladding is necessary
for Very Severe
exposure zones.
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4.6.2.2 Cavity walls
Some water may penetrate the outer masonry leaf in prolonged periods
of wind– driven rain, but proper design and positioning of dpc’s,
trays and of any insulation will minimise the risk of penetration
further into the building.
In cavity walls the 50mm space between the two leaves is intended
to prevent any water from passing from the outer leaf to the inner
one. In conditions of Severe or Very Severe exposure, consideration
may be given to the use of wider cavities.
Where the cavity is unavoidably
bridged, e.g. at window and door openings,
careful detailing is required. The inner leaf of a cavity wall should
not be relied upon to resist water penetration.
A cavity wall with a minimum outer leaf thickness of 90mm of unrendered
brick masonry and a 50mm minimum clear cavity with an Aircrete inner
leaf of any thickness can perform acceptably in all four areas of
exposure. Outer leaves of unrendered Aircrete are not recommended.
A cavity wall with a partially filled cavity should maintain a minimum
residual clear cavity width of 50mm.
A cavity wall with a fully filled cavity can increase the risk of
rain penetration of the wall.
The cavity should be 50mm minimum, but the risk of rain penetration
will be reduced by specifying a wider cavity.
4.6.3 Rising damp
Moisture in the ground tends to be ‘drawn’ upward into
the structure by capillary action in the form of rising damp unless
it is prevented by a barrier.
4.6.4 Resistance to rising
damp
A dpc in a building is intended to provide a barrier to the passage
of water from the exterior of the building to the interior, or from
the ground to the structure, or from one part of the structure to
another.
In every external wall, a horizontal dpc should be provided at least
150mm above the finished level of the external ground or paving.
To prevent the transfer of moisture from external walls into solid
floors, the damp proof membrane in the floor, and the dpc in the
wall, should overlap a minimum of 100mm or be sealed.
Dpc’s should not be bridged by rendering and should extend
through the full
thickness of the wall or leaf, and preferably project slightly beyond
the external face.
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