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WALLS
23.27 FUNCTION OF WALLS
A building enclosure includes the roof, walls, openings, foundation, and below ground materials that separate the building
from its surrounding environment. It adds a thermal barrier, a vapor barrier, drainage planes, and an air barrier.
Walls function to control, support, distribute utilities and other services, and beautify (or "uglify") our homes.
The control function of the walls is quite important. They actually control:
Rain and snow penetration Pollution, dust, other contaminants or environmental hazards Heat flow Airflow, including wind Water vapor Groundwater Light Solar radiation Fire Noise Entrance of animals, bugs, vermin Entrance of unwanted humans
Introduction
The principle of the cavity wall is quite simple. The cavity prevents moisture passing
through the wall. As long as the cavity is kept clean and the wall ties are correctly
positioned the house should remain dry even if the external leaf becomes
saturated. Water is free to run down the inner face of the external leaf (and this is
quite likely in severe exposures) and is either ejected via weep-holes or drips safely
below the DPC.
Early walls were usually brick in both leaves. During the 1930s blockwork became
more popular for the internal leaf. Blocks contained all manner of aggregates
depending on what was available locally. Early cavity walls are usually 250mm thick
( 10 inches) with the cavity 50 mm (2 inches). In modern construction cavities are
often 75mm or more wide to accommodate insulation and allow a clear space
between insulation and outer leaf.
Of course, if cavities are bridged damp penetration can occur. The most common
cause of bridging is debris in the cavity but it can also be caused by ties which slope
towards the inner leaf or ties (some designs only) being laid upside down. This
endoscope picture clearly shows mortar on the tie. The purpose of the ties is to
bind the two halves of the wall together. Many early ties have failed prematurely.
This is usually because they had insufficient protection (usually in the form of
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galvanising).
Sometimes, where walls were rendered the wall could be built from 2 block leaves
or 2 leaves of common bricks. Spotting whether early walls are cavity or solid can
be quite difficult if properties are rendered. The depth of the reveals is one clue;
another can be found in the roof space at eaves level.
2 Early Cavity Walls
By the end of the Victorian period cavity walls were
not uncommon although most external walls were still
built as solid walls. London Building Regulations (and
many local by-laws) insisted that either the inner or
outer leaf of a cavity wall should be 1 brick thick. The
two leaves were held together by cast iron or wrought
iron ties (left), or, in some cases, special cavity bricks
(right). Clickherefor a drawing showing a Victorian
cavity wall (taken from an 1898 textbook).
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To see a short video clip
explaining the problems
and demise of the solid
wall, and the
introduction of the
cavity wallclick here
By the 1920s the Regulations had been relaxed and
most new houses were built with cavity walls, but with
both leaves half-brick thick. The wall was typically
250mm thick (10inches). However, solid walls
continued in some parts of the country for many years.
Many Victorian houses, for example, those re-built
after the Second World War (following bomb damage)
were built with solid walls.
The most common form of an early cavity wall is
shown on the left. An outer leaf of brickwork would be
built in facing bricks and usually in stretcher bond
(sometimes Flemish bond was used with 'snapped'
headers, ie, headers broken in two). The inner leaf was
usually formed in common bricks, ie, bricks intended to
be plastered or kept out of sight. As with solid walls
the internal plaster was usually lime based and applied
in two or, preferably, three coats. In the 1930s and 40s
this slowly gave way to gypsum plaster.
early cavity walls
In a solid wall headers bind the wall together. In a
cavity wall this is not possible and the two leaves are
tied together by wall ties. Early ties were sometimes
formed in wrought iron or mild steel. They weresometimes unprotected or possibly coated in bitumen
or zinc (galvanising). The ties were typically positioned
every sixth course vertically and about 900mm apart
horizontally. In practice these centres were often
'stretched' to save money. The tie on the right is from
a 1920s house. You can see some deterioration at the
bottom of the tie. The galvanised protection has
disappeared leaving the steel free to rust. The rusty
part was in the external leaf.
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Click hereto see an
example of modern
bricklayers in action.
The basic technique is
the same nowadays as itwas a hundred years
ago.
By the 1930s some developers were using concrete
blocks for the inner leaf. These were often made from
locally available aggregates, often industrial wastes.
However, the use of blockwork was slow to catch on
and even as late as the 1950s bricks were still used forinternal leaves of cavity walls and internal partitions.
By the 1920s most walls included DPCs. Nowadays two
separate DPCs are used (see left), one for each leaf. In
some early cavity walls large pieces of slate were used
which actually bridged the cavity. Other materials
included lead, copper, asphalt and lead cored bitumen
felt. Today, most DPCs are made from polythene.
The inner and outer leaf
of a cavity wall should
never touch - they
should always be
separated by a DPC.
3 Early Cavity Walls - Head, Sill and Jamb
Openings in early cavity walls could take many forms.
This page shows one or two of them for cavity walls
built in the first half of the 20th century. Many aspects
of good practice were ignored and the consequences
of this sometimes manifested themselves in problems
of damp penetration. However, many of these walls
are still functioning quite adequately.
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As shown in the top right photo the lintel was often formed in precast or insitu
concrete. It could be in two halves, one for each leaf, or as a single deep lintel as
shown in the left-hand examples. It should be clear that with a solid lintel damp
penetration is a possibility due to mortar droppings bridging the cavity, or to water
running across the top of the lintel. The example on the right of the graphic
includes a cavity tray - designed to prevent the above problems. However,
condensation is still a risk and is easily confused with damp penetration. This is
explained in more detail below.
Cavity trays were not always used at the heads of openings. On many older houses
they have been added subsequently. Look for tell tale signs, usually a few new
courses of brickwork over the lintel. In this photo the cavity tray appears to be
made from copper - you can just see the front 'strip' of the tray projecting from the
wall.
The construction on the left, known as a boot lintel, alleviated the problems shown
above. The top of the lintel was usually coated in tar (from coal) or bitumen (from
oil) to prevent water soaking into the lintel itself. However, even where these exist
there is still another potential problem; condensation. This can occur on the inside
face of the lintel because it is cold. The phenomenon is sometimes referred to as
cold bridging. Moist air in a room comes into contact with the cold inner face of the
lintel and condenses. The problem is often confused with damp penetration and, as
a result, the diagnosis is often wrong. This can lead to expensive repairs which do
nothing to alleviate the underlying problem.
Some windows have soldier arches above them. This is
a row of bricks on end, usually only in the outer leaf.
The inner leaf often comprises a concrete lintel. In
narrow openings the soldier arch stays in place due to
the adhesive affects of the mortar. In wider openings
you can sometimes find a steel or wrought iron bar as
shown on the left. In a few cases the bricks may have
steel reinforcing rods running through the holes in the
bricks.
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At the jambs, ie, the sides of the opening, cavities were
often closed to provide a good fixing for the frame and
a good base for the internal plaster. The example on
the left shows a cavity wall with the inner leaf returned
to the outer leaf. This creates a path for damp
penetration but there are still lots of examples of this
construction, some of them damp free. Sometimes the
cavity was left open as shown on the right. This does
not provide a very good fixing for the window.
These examples both show a cavity closed but with the
addition of a DPC. In the left hand example the DPC is a
sheet material, possibly bitumen felt or even lead. The
right hand example shows a cavity closed with plain
tiles bedded in mortar. This principle, ie of providing a
DPC, remains today.
There were a variety of sill details. Two examples are
shown here. In the left hand example the timber sill is
bedded in mortar on the external leaf. A drip at the
end of the sill prevents water from running back under
the sill. In the right hand example a smaller sill section
sits deeper in the jamb. A sub sill formed from plain
tiles is bedded in mortar on a DPC. In this example thecavity is closed with a three quarter brick.
4 Modern Cavity Walls - Generally
Below ground level it is common to find blockwork.
Dense blocks and most aerated blocks are suitable for
use below the ground. Holes in the blockwork, with
lintels over for top for support, can be left for building
services (water will be lower).
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Modern brick/block cavity walls vary in thickness
depending on the nature of the cavity and the nature
of the inner leaf. The cavity will normally be 50mm,
75mm or even 100mm wide. The thickness of the inner
leaf depends on the type of blocks; 125mm aerated
blocks are quite common. The use of thicker
lightweight internal leaves or special aerated blocks
can preclude the need for additional insulation. See
the section on Building Regulations or Insulation for
current requirements. Click here for another example
of a modern cavity wall.
If bricks are used in the
outer leaf its thickness is
normally 102.5mm;
often referred to as
100mm or half-brick.
Where insulation is
required it can either be
in the cavity or in the
form of dry lining.
External insulation is
rare in new cavity walls.
Where cavities are filled
or partially filled goodworkmanship is vital to
prevent rain
penetration. Follow the
tips in the bullet points
below.
Ensure the cavity is kept clean Make sure all joints are well filled with mortar Use a tooled joint which compresses mortar
and forms a good seal with the brick
Ensure wall ties are level (or slope outwards)and keep ties clean Provide cavity trays (with stop ends if
necessary) over lintels etc. which bridge the
cavity
Make sure DPC details are correct Make sure trapped water can escape through
weep-holes
Ensure cavity insulation is fitted in accordancewith manufacturer's recommendations.
Although cavity walls are formed in two leaves they
should be regarded as a single structural unit. Neither
leaf should normally be built over 1.35metres high
(about 6 courses of blockwork) on its own; single
leaves are more likely to suffer wind damage. Where
rigid ties are used this difference should not exceed 2
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ties are usually only suitable for cavities up to 75mm
wide. Vertical twist ties are suitable for cavities from
50mm to 175mm (but cavities greater than 100mm are
still rare).
The spacing of the ties should be 900mm horizontally and 450mm vertically (for
cavity widths of 50-75mm). Where cavities are 76-100mm wide the spacing should
be 750 horizontally and 450 vertically. Additional ties are required within 225mm
(Code of Practice - B. Regs requires 150mm but will soon be altered) of the jambs
of any openings. They should be no more than 300mm apart vertically which
effectively means every three courses of brickwork (1 course of blockwork). Extra
ties may be required to support insulation boards.
The ties should ideally slope slightly outwards to help
prevent water reaching the inner leaf. Where partial
cavity fill is specified a special tie incorporating a large
plastic retaining washer should be used. It is preferable
to leave a cavity of 50mm although 25mm may be
acceptable in sheltered situations. The tie should be
bedded at least 50mm in each leaf.
Remember: ties sloping inwards may encourage water
penetration, and mortar droppings may also be a
problem. Good practice should avoid both these
potential defects but they are not uncommon. Poor
work can be expensive to resolve.
These photos show partial insulation. On the left the
ties has been positioned but not the retaining washer -
this can be seen on the right. When using partial
insulation extra ties are required to support the
boards.
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6 Movement Joints
Most materials expand and contract throughout their
lives depending on their temperature and moisture
content. In addition, new clay brickwork will expand
slightly for several months as it slowly absorbsmoisture in the air until equilibrium is reached.
Calcium silicate and concrete bricks will shrink slightly
(like all cement or hydraulic-lime based materials).
Long runs of brickwork therefore need to be divided
into shorter panels to prevent this movement from
causing unsightly cracking.
Movement joints are generally every 12 to 15 metres
although they should be more frequent if concrete or
calcium silicate bricks are used. In free-standing walls
the spacing should be about half the above figures (ie
about 6 to 8 metres). The joints can be hidden in a
variety of ways or may even form part of the overall
design. The one on the right is quite noticeable, the
one on the left less so. It is quite common to find them
hidden behind down pipes (below right) although the
pipe clips must not be fixed across the joint.
Movement joints are filled with a soft, compressible
material and then sealed against water penetration.
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NB. The sealer should
not stick to the filler -
this would reduce its
ability to flex.
There are a few key factors to ensure success:
Keep movement joints free from mortar Make sure brick face against movement joint
has properly filled joints.
Keep the joint width constant Keep the joint vertical Keep the bricks either side of the joint to the
same course heights.
Joint filler must be compressible The joint width in mm should be at least the
joint spacing in metres plus 30%. So joints at
12m equals a joint width of16mm.
Spacing from a corner of a building to the firstmovement joint should be 50% of the normal
joint spacing.
One method is to build-in a joint filler as work proceeds. The joint filler must be
semi-rigid material with the right amount of 'compressibility'. Hemp, fibre board
and cork will not allow enough movement. The filler should be the full depth of the
wall and should be cut back when the wall is complete to allow space for the
flexible sealant which will weatherproof the joint. The width of the gap depends on
a number of factors including the nature of the bricks, the size of the panel and the
qualities of the sealer. Sealants are usually polysulphides or silicones. They are
usually applied by 'gun' and 'tooled' to compact the sealant and ensure it adheres
to the sides of the bricks. The sealer should not stick to the filler - this would
reduce its ability to flex.
Cautionary Note
Experience shows that using this method can lead to
problems. As mortar is squeezed out of the brick joints
either side of the filler it compresses the filler and may
reduce the width of the joint. This obviously limits the
ability of the panels to expand. The picture on the right
shows how mortar, squeezed against the filler, has
actually reduced the effective width of the movement
joint.
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Alternative Method
Another method uses a piece of timber temporarily
built into the wall. The timber should be the full depth
of the wall. The timber can be raised with the
brickwork as shown on the right. When the wall is
complete the timber can be removed, the gap cleaned,
out and the filler inserted. It should be inserted so that
the applied sealant will be of the correct depth. Again,
the sealant should not stick to the filler. In both
methods care must be taken when applying the
sealant. Masking tape can sometimes be fixed to the
brick face either side of the joint to keep it clean. If
sealant spreads onto the bricks it can be difficult to
remove.
7 Cavity Wall Tie Failure
clickherefor video clip
Perhaps the most significant defect associated with
cavity walls is wall tie failure. Failure usually occurs
through rusting of the ties. Rust can fracture the ties
and this can result in redistributed loading.
Unrestrained brickwork can bow or bulge. Rusting ties
can also cause expansion of the joints which further
encourages damp penetration. Expanding joints can
ultimately lift the edge of the roof. Houses most at risk
are those built in the 1920s and 1930s and those builtduring the 1970s (when standards of galvanising were
temporarily reduced).
In the 1920s mild steel ties were generally
unprotected, bitumen coated or galvanised. This
galvanised, vertical twist tie has corroded leaving the
steel free to rust. When the ties corrode they are likely
to expand and force the bed joints apart. However,
wire ties (below right), because of their small cross
section, can sometimes fail without the associated 'tell
tale' expansion of the horizontal bed joints.
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Modern galvanised ties are based on a British Standard
set in 1981. Ties manufactured to this standard should
have at least a 60 year life. Galvanised ties are coated
in zinc. The thicker the zinc the better the protection.
In damp conditions, even if the coating is scratched,
sacrificial corrosion of the zinc protects the steel. Over
time, the area of exposed steel increases to a point
where it can't be protected and it starts to rust. As rust
forms the volume of the tie increases. The speed at
which rust occurs depends on the type of tie,
saturation of the wall, nature of the mortar and
proximity to marine climates.
Corrosion of the galvanised layer is aggravated by the
use of black ash mortars with a high sulfur content.
When wet for long periods a weak sulfuric acid is
created which attacks the zinc coating. Once the zinc
has worn away the tie can fail quite quickly,
particularly the end in the outer leaf of brickwork. The
need to remove the existing ties will depend on the
nature of the mortar, the level of corrosion, and the
type of tie. On these houses the use of thick twist ties
and aggressive ash mortar dictates complete removal.
Wire ties can often be left in place.
clickherefor video clip
of wall tie repair
There are several types of replacement tie. A stainless
steel tie with expanding metal sleeves is shown on the
right. Expanding neoprene sleeves are also common.
The one on the left has a twisted inner section for resin
fixing in the inner leaf (expanding ties can damage light
weight blocks). Tie choice, therefore, depends on a
number of factors including nature of the inner and
outer leaf, tie strength and stiffness, experience, and
cost.
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On this housing estate the inner leaf is a soft brick,
often unsuitable for expanding ties, so the inner part
of the tie is designed for resin fixing (see photo above
left). Once drilled the hole in the inner leaf needs
cleaning out to ensure a good bond for the resin. A two
part resin is then applied to the inner leaf and the tie
inserted. The outer, expanding part, of the tie will be
tightened several hours later.
Typical cost for a 3 bed
house is between 3,000and 6,000.
Where wire ties have corroded without damaging the external leaf they can
sometimes be left in place. In these cases simpler solutions are possible. One
method developed by Halfen Unistrut involves drilling the bed joints and inserting
stainless steel Spiro ties (right). When the hole has been drilled and cleaned out
resin is applied to the inner leaf. The tie is then pushed home and resin is applied
to the outer leaf.
This wall tie system involves minimum disruption to the occupiers and
causes minimum damage to the wall. In addition, repointing the drilled holes
is a simple exercise.
Once the house has been re-pointed the work is
completely hidden. It's not quite as easy to disguise a
rendered wall. Painting the render will help but it will
not completely hide the repairs - and painting render,
particularly rough-cast, is a life sentence!
8 Modern Cavity Walls -Jambs
At the jambs, ie the sides of the
opening, cavities are usually closed to
provide a good fixing for the frame
and a good base for the internal
plaster or dry lining. The example on
the left shows a cavity wall with the
inner leaf of dense blockwork
returned to the outer leaf of brick.
This detail was common until the
1980s - a better detail is to extend the
vertical DPC by 25mm into the cavity
(right). Note that by the late 1980s
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most internal leaves were being built
in lightweight rather than dense
blocks.
The DPC mentioned above is the mostcommon way to prevent water
crossing the cavity at the jamb. It's
normal practice to return the
blockwork to the brickwork rather
than the other way round - it's
cheaper and provides better weather
protection; water dripping off the side
of the DPC cannot touch the inner
leaf. The blocks can be 'specials' or
off-cuts (shown on the right). The
jamb DPC should be positioned to
overlap any horizontal DPC at the sill
and be overlapped by any cavity tray
at the head. Clickhereto see an
example of Ruberoid's DPC system for
openings in cavity walls.
The top examples are similar in that
the window is set forward in thefacework, this means that the external
reveal is quite narrow. This has
aesthetic, maintenance, and
environmental implications. Narrow
reveals make the walls look thin,
there are increased chances of rain
penetration and potentially a greater
likelihood of condensation caused by
cold bridging. These pictures also
show narrow external reveals; thewindows on the left are new
standard, metric units, those on the
left are replacement PVC.
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The risk of cold bridging should not be
ignored and a range of simple
precautions can prevent its
manifestation. The problem occurs
because of the cold bridge formed just
behind the frame (arrow on the left).
This may lower the blockwork
temperature below the dewpoint and
create a cold spot for condensation. If
the blocks are lightweight it is less
likely to occur. The temperature of
the inside surface can usually be
maintained above the dew point by
insulating the inner reveal with 25mm
or so of insulation board or by placinga strip of insulation between the
vertical DPC and the blockwork (both
shown right).
Another option is to position the
frame deeper inside the jamb. Just re-
locating it 25mm or so may eliminate
any cold bridging. Setting the frame
deeper inside the jamb will also
provide improved weather protection.
However, many windows have small
sill sections and moving the window
may require sub sills. Sub sills can be
quite expensive and are obviously
unlikely to be popular with the
volume house builders. However, long
term they may be cost effective. Look
at the sill pages for more information.
It is important to prevent any airleakage around the window. This
may require external sealants and
expanding foam or mineral woolpacking where the frame meets the
jamb (right).
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An even more radical approach (in
England - it's pretty much standard
practice in Scotland!) is to use check
reveals. In a check reveal a small
rebate is formed by running the
brickwork beyond the blockwork at
the jamb. This rebate protects the
frame and at the same time
eliminates cold bridging. Traditionally,
sash windows were always set in
rebates although, of course, they
would originally have been fixed to
solid, not cavity, walls.
An entirely different approach is use
extruded plastic sections. These avoid
the need for a separate vertical DPC
and preclude the need for messy
block cutting. 'Dacatie' (shown here) is
a common product and it's available
in a wide range of patterns, some of
which contain insulation. The sections
are nailed or screwed to the frame
and built in as work proceeds. These
could be used as an alternative to
check reveals for those parts of
England and Wales subject to severe
exposure.
A fairly recent product is shown here.
It's a foil backed 'bubble wrap'. It can
be used as a cavity insulation and as a
vertical DPC around jambs. It requires
special wall ties to hold the foilagainst the inner leaf and to make
sure that the upper sheets overlap
those below.
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The picture on the left shows another
method of forming the opening. The
white plastic extrusion helps the
bricklayers get the jambs plumb. The
extrusion is kept rigid by steel bar(s0.
These are eventually removed and the
window slotted into place inside the
extrusion. Click here for another
example.
9 Modern Cavity Walls - Lintels
Nowadays most lintels are made from galvanised steel
(there may be additional protection systems in the
form of polyesters coatings and separate cavity trays).There are two basic patterns generally available for
cavity walls - box sections (left) and open backed (right
below). Lintels generally are available in lengths of up
to 4500mm and should normally have at least 150mm
bearing on the wall either side. They are designed to
carry the distributed load of brickwork - not point
loads. In fact, the load is not as high as one might
imagine because of the bonding nature of the
brickwork. Look at the graphic (right). The lintel is only
carrying a small triangle of brickwork directly as therest of the brickwork is supported by the corbelling
action of the bonding.
These two images show modern lintel details. Note
that modern lintels contain insulation, partly to
keep energy wastage low but also to eliminate coldbridging and condensation. This is explained at the
bottom of the page. Some lintels are made from
light-weight concrete. These are usually positioned
on the inner leaf and have a separate metal traycarrying the outer leaf. Some lintels require
separate cavity trays - the need for this depends on
manufacturing techniques and/or exposure.
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The lintel type shown in the top left picture was
popular throughout the 1960s and 1970s. It's a Catnic
box section lintel - made in a variety of lengths and
sizes to suit most openings. Catnic still make lintels
and, nowadays, most of them contain insulation. These
lintels are light, easy to carry and don't normally
require a separate cavity tray (these are normally only
required in areas of severe or very severe exposure -
west of Bristol, Cumbria, most of Scotland). Catnic also
produce other lintel patterns. If the cavity work
includes an arch slightly different construction is
required. The right-hand image shows a solution from
Ruberoid. It uses a pre-formed cavity tray to prevent
damp penetration - this is explained below.
In modern construction a cavity tray is required to
direct any water running down the cavity away
through the external leaf - via weepholes (left).
Weepholes, usually every 450mm or so, can be plastic
inserts or open perp joints. As stated above some
lintels need a separate tray - this is usually formed
from polythene. The cavity tray usually sits on top of
the lintel although some companies produce details
showing the cavity tray a few courses above it (left).
Stop ends to lintels (or the cavity tray over it) have
always been recommended if cavities are filled with
insulation. The latest Approved Document C (2004)
does not specifically mention stop ends (or weep holes
for that matter) over window openings. However, the
appropriate Code of Practice recommends their use.
Although lintels only generally require 150mm
bearing either side of the opening they should be
supported on a full concrete block (it's the inner
leaf which carries most load). Failure to do thismay result in unsightly cracking in the plaster
either side of the window. The lintel on the right issupported by a small piece of block.
Click here to see a simple graphic showing DPCdetail around window.
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The cold bridge (left) can often be avoided by using an
insulated lintel as shown on the right or by using a
lintel without a continuous lower web. Another option
is to move the window frame deeper into the opening.
If the frame sits directly under the cavity the cold
bridging effect is reduced although there will be
implications for the window sill - in most cases a
separate subsill will be required. Cold bridges can be
very difficult to identify and resolve. Condensation,
due to cold bridging, is often mistaken for damp
penetration - 'solutions' may be expensive and wholly
inappropriate if diagnosis is incorrect. Note that many
manufacturers of lintels show cavity insulation
stopping at the top of the lintel (below examples). The
BRE, however, in Thermal Insulation - Avoiding Risks,2002, suggest it should be cut to fit the lintel profile as
in the left and right-hand examples.
If the window cannot be moved deeper into the reveal
another option is to insulate the soffit. In this example
insulation board is fixed to the bottom of the lintel
thus eliminating the cold bridge. This is probably
cheaper than moving the window frame deeper into
the reveal although the extra protection offered to the
frame by moving it inwards may pay-off in the long
term. Most modern, box-section lintels are available
with an insulation fill - they will help avoid cold
bridging although consideration should still be given to
insulating the soffit. In addition, where lintels have a
continuous lower web, it is often perforated to help its
thermal efficiency. Note that lightweight concrete
lintels (which sit over the internal leaf) are also
available. These often have a steel tray to support the
outer leaf.
It's also important to note that in modern construction
steps must be taken to avoid air leakage. This may
require sealants around the window frame, inside and
out. Click here for a modern example of a window
opening showing suitable details at jambs, head and
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sill.
10 Modern Cavity Walls - Sills
Sill details depend on a number of factors. Perhaps themost common detail in housing is the integral sill -
where the sill forms part of the window frame. The sill,
which can be hardwood or softwood, is bedded in
mortar onto the external leaf of brickwork. A groove in
the sill helps secure it in position. It is not normally
fixed (windows are normally fixed at the jambs only). A
typical detail from the 70s and 80s is shown on the
right. This form of construction is not regarded as good
practice today because of the risk of cold bridging (see
below)
Sections through windows look, on first sight, fairly
complex. Consider the design of the on the left. The
groove in the back (far left) is to take the window
board, the groove in the bottom is the bedding groove
and the groove at the front is a drip - to help prevent
water running back under the sill. The drip encourages
a build-up of water. The weight of the drip eventually
overcomes the water's surface tension and it drops to
the ground. The drip must therefore be positioned well
clear of the wall. In addition, sills often have additional,
small, inverted 'V' shaped grooves underneath to help
prevent the timber from twisting.
To provide better support for tiling (kitchens andbathrooms) it was common practice in the 1960s to1980s to close the cavity by using a cut block
(right). This obviously bridges the cavity and, to
prevent damp penetration, a DPC was required.
This construction could also be used below awindow board.
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In modern construction, with well insulated cavities,
there are risks of cold bridging where cavities are
closed. The graphics (left) show a range of modern
options. A vertical DPC, ie between the brick and block,
is required if the cavity is closed; some authorities say
that a DPC under the window only needs to be used
where sills are permeable or composed of sections.
A sub sill may be required with the windowsshown on the left if they are set deeper into the
jamb (the drip will not clear the wall). Sub sills can
be formed from special bricks, stone, concrete or
plain tiles (see photos below).
In exposed situations, particularly where cavities
are filled, it is nowadays recognised as goodpractice to provide stop ends to sill DPCs and
cavity trays. Where there is a sill DPC it should belapped with the jamb DPC as shown here. Where
there is no sill DPC the jamb DPCs should be
continued 150mm below sill level.
Mastic beading is not normally necessary around
the window for weather protection. The NHBC
recommend it in very exposed situations. However,it is important to prevent any air leakage around
the window. This may require internal and externalsealants and expanding foam or mineral woolpacking where the frame meets the surround.
Clickhere for a modern window opening showing
jambs, sill and head.
These two photographs show examples of sub sills.
They are typical of some of the speculative
construction that took place in the mid 1980s. The
work is not a very high standard but the nature of the
construction is clear enough. Click here to see a
modern window with a concrete sub sill.
Clickhereto see an example of Ruberoid's DPC
system for windows in cavity walls.
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11 Modern Cavity Walls - Parapets
Parapet walls require careful detailing. At the top of the wall it is good
practice to provide a weathered coping. The coping can be once or twice
weathered - in other words it can slope in one direction as shown in the
picture (below left) or in both directions. A once weathered copingnormally directs the water onto the roof to avoid water running down the
external face. In some designs brick copings are used although careful
detailing may be required if the copings don't have drips (sometimes
called throatings).
A good coping stone will overhang the wall either side and will incorporate
small drips to prevent water running back under the coping. A full-width
DPC should be bedded in mortar to prevent water penetrating the coping
through the coping joints. Unlike the example on the left the DPC shouldbe laid on a rigid support to prevent it sagging into the cavity and allowing
water to pond where it may freeze and expand in cold weather. Any
sagging may also form a trough and allow water to penetrate the cavity
where the DPC is lapped.
Because parapet walls are exposed on both sides a cavity tray is required
to prevent water running down the cavity face of the inner leaf and
penetrating the building. The example on the left shows the tray stepping
down to the outer face; water escapes through weepholes. Although this
may cause minor staining of the wall it is sometimes preferable to sloping
the tray inwards. This is because sloping the tray inwards may allow
rainwater to run along the underside of the tray and reach the internal
leaf. Click on the picture for an example. In moderate or sheltered
exposures this is not normally an issue. However, if the cavity contains
cavity batts the tray should always slope outwards to protect the top of
the insulation. If it slopes inwards there is the risk of water running down
the cavity face of the external leaf and crossing the cavity on top of the
batts.
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Flashings should be in the same joint as the cavity tray (DPC) and always
under them to prevent the risk of water ingress.
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