Extracts from The Learning
Curve insert within Master Builder 2004
Closing Cavity Walls
At one time forming reveals in cavity
walls was a very straightforward undertaking. One need only cut the block work
of the inner skin and return it at 90 degrees to ‘close’ the cavity. A length
of dpc was required to be sandwiched between the block work and the external
masonry. This separated the abutting masses so dampness could not permeate
inwardly. That was the theory.
The practice was
different. The exercise was time consuming and subject to errors and faults.
The block work return was often of a dubious standard, being irregular and
poorly aligned. Cold bridging was frequently experienced and the detail
suffered condensation and pickling. The roll dpc used to prevent damp
transmission would tear, dislodge or simply be bridged with excess mortar.
Dampness of the reveal was the accompanying legacy. Steps needed to be taken
to improve the detail.
Over the years
changes in the Building Regulations have prompted a rethink:
How can one
minimise cold bridging around openings – the majority of traditional methods
are generally now non-compliant. How can one ensure the dpc does not dislodge?
To what extent can mortar bridging be reduced or even eliminated? If the structure
is of a classification that requires fire integrity measures to openings, what
is the easiest way to achieve this also? Finally, if the external skin is built
with material of an unusual depth or perhaps is constructed against a backing
block, are there any additional measures one should take to ensure the
structure is built correctly to meet the latest regulation demands?
Caviclosers are
the answer and can eliminate problems. There are now caviclosers of all shapes
to fit perfectly the design criteria. Caviclosers are insulated and create a
thermal zone to the reveal, thus cold bridging need not be a problem. Closers
also act as a vertical dpc, so there is no need for conventional roll damp
course to be cut and built in vertically. Closers also eliminate the need to
cut and return the inside skin of block work, so construction can be more
speedy. And if you require closers with fire integrity, there are now models to
provide that also. But closers today are not limited to reveals only. The
examples describe caviclosers for all manner of applications, whilst focusing
on some specific everyday details you will undoubtedly encounter*.
*Traditional returning of the block work inner skin is
still allowed if using walling materials with appropriate insulation values. In
such circumstances, the Type D profiled DPC with insulation attached may be
used. Being preformed in profile, unlike roll DPC, it will not distort or
misplace. And it benefits an insulation layer.
Type V
cavicloser is available to suit a wide range of cavity widths. It is also
available with a Cavi 120 prefix which signifies
120 minutes fire integrity rating
The
Quickcloser insulator is within
a tube
that can be compressed to suit various cavity widths
Cavi
240 type PWIB Party
wall integrity barrier closer with four hours fire integrity rating.
Type D
insulated vertical DPC closer
permits a
traditional approach whilst
benefiting
a preformed dpc section
Examples
courtesy of cavity trays ltd from the range of sixteen closers
Wide Cavities
Type WCA.
Cavities are
getting wider and wider, and now there is a specific cavicloser to literally
‘fill the gap’. The Type WCA is for Wide Cavity Applications. It consists of a
robust solid extrusion in which is incorporated a large insulating core. The
Type WCA is available to suit any cavity width from 50mm up to 200mm maximum.
One simply states the cavity width required. There are three useful lengths:
2.1metres, 3.0 metres and 3.6 metres. Stainless steel securing ties accompany
all deliveries.
And if you
require a closer with a fire integrity rating also, there is the Cavi 60 Type WCA. This is an up rated version with a non-combustible rock mineral
wool insulation core. The Cavi 60 prefix means when tested the closer provided
60 minutes fire integrity. Maximum cavity width in this format is 150mm.
Type WCA and the fire-rated version Cavi 60 Type WCA. Ideal for wide
cavities.
Cavities from 50mm to
100mm
Type V Cavicloser
One of the most economical ways of
closing the typical cavity wall reveal is to use a Type V cavicloser. This lightweight
easy-to-handle model has a polystyrene insulator secured within a simple jaw.
Cutting to length is and fitting is rapidly accomplished, and the result is a
reveal face without protrusions or projections. This permits the door or window
frame to be installed in whatever position you require – subject to compliancy.
Cavities Require Closing in Many
Places.
Design analysis by James Long.
Definition.
In traditional
construction there are requirements to incorporate fire stops within the cavity
to reduce the risk of fire spreading should a structure be ablaze. The fire
stops effectively close the cavity and need to be of a medium that will contain
or restrict the fire for a determined amount of time.
Background.
Timber frame construction provides a straightforward
example in which fire stop barriers or closers are required to isolate and
compartment the cavity. They divide the structural void into individual
compartments and are commonly incorporated both horizontally and vertically. In
the event of fire, the flames are prevented from freely travelling via the
cavity from one part of the building to another. Thus in structures of multi
occupancy, every unit / accommodation benefits individual protection.
Four hour rated cavity stop/closer Cavi240 CFIS
The Cavi 240 Type CFIS is a typical
example. The Cavi 240 prefix indicates 240 minutes fire integrity rating whilst
the CFIS stands for Cavicheck Fire Integrity Stop. Manufactured from rock
mineral wool that has a non-combustible classification, this model by Cavity
Trays of Yeovil is enveloped within a polythene sleeve. Compressing and placing
within the cavity secures the Cavi240 Type CFIS in position. A minimum
compression of 10mm is required to adequately friction fit the integrity stop
and provide a functional relationship between surfaces.
It is worth
qualifying that any medium placed horizontally within a cavity should have
accompanying measures to prevent penetrating water from using the top surface
as a bridge to the inner leaf. There are various types of cavitray available
for this purpose, including a new style launched during Interbuild week called
the Type CD.
Manufacturers
There are in excess of eight manufacturers
in the UK.
Standards
Whilst the type of material used to create
a fire stop varies slightly from one manufacturer to another, all are tested to
the requirements of a British Standard
that is generally BS 476:Pt 20. However,
not all fire stops have identical ratings, so look for the fire integrity
rating duration normally indicated as minutes or hours.
Which proprietary products can one
purchase that permit easy compliance?
Cavity Trays of Yeovil has launched a new
up rated range of closers providing up to 240 minutes fire integrity rating.
See new technical manual released by the company in April.
Timber frame
construction requires barriers to isolate and compartment the cavity.
Check This Out
In a checked reveal detail the frame sits
behind rather than against the external skin. Thus part of the frame face
extends behind the external skin. This detail is popular in many parts of the
country that experience severe weather conditions. One way of closing the
cavity of a checked reveal is to use a Quickcloser. The insulation medium is
contained within a tubular bag, which can be ‘squeezed’ to optimum shape to
suit the cavity dimension. The tubular bag is attached to the extruded body of
the closer, the front section of which is hinged.
If you are
building a checked reveal, the front section is simply turned 90 degrees.
Alternatively,
if you are building a straight reveal, you leave it as it is.
The Quickcloser insulating tube shapes to
suit the cavity width.
Monthly Tip for Builders.
To maintain fire resistance, separating /
party walls should stop about 25mm below the top of the adjacent roof trusses
and appropriate fire-resistant soft packing introduced above and below the
roofing underlay. This method closes the detail and provides continuity of fire
integrity whilst allowing for timber roof movement.
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Party Walls
There are Conflicting Requirements.
Party walls separating one property from another
connect to the exterior skin. There is a requirement to construct in such a
manner that dampness is prevented from tracking inwardly into the building and
a predetermined level of fire integrity protection is also provided.
One solution is to use a party wall integrity
barrier. The one illustrated is the Cavi
240 Type PWIB Party Wall
Integrity Barrier. (The prefix of Cavi 240 indicates it has a 240 minute (4
hour) rating). Supplied in long lengths the Cavi240 Type PWIB consists of
semi-rigid profiled solid DPC to which is bonded an alkaline silicate fibre
insulation layer. The profiled design ensures no mortar bridging or capillary
attraction can function where the abutting party wall joins the external skin.
The insulating layer is in slight compression when built in, and additionally
promotes excellent acoustic restraint, addressing flanking transmission (sound
attenuation).
This product was tested to BS 476: pt 20 with the
maximum furnace level reaching 1136 Deg C. Cavity Trays of Yeovil supplies this
product with whatever depth of bonded alkaline fibre is required to suit the
specified cavity width.
Fours hours fire integrity
rating with the Cavi240 Type PWIB.
Closing, Closing, Closing - and a Problem.
When raising a cavity wall and constructing an
opening, one is faced with a total of three ‘closing’ requirements.
The first as already covered, is the closing of the
reveal brickwork using an appropriate cavicloser.
The second closing requirement occurs where the
vertical closer meets the lintel. The illustration shows how the lintel is
bedded down so the underside of the lintel closes tightly with the top of the
vertical closer.
Finally, once the DPC or cavitray has been
positioned to provide protection of whatever lintel style is used, a stopend is
required to close the end to prevent collected water discharging into the cavity.
And it is at this point problems can occur.
The British Standard calls for any DPC or tray to
oversail the lintel by only 25mm. So where do you position the stopend?
If a lintel
has only 150mm bearing and the DPC/tray overhang is 25mm, the combined total of
175mm is short of the length of a standard brick of 225mm. With no vertical
perp free to receive the stopend you have to break bond.
The better practice is to have a longer DPC or tray,
so you are always sure of being able to build into an ideally positioned
masonry perp. Type C cavitrays for use with lintels are supplied as standard
with such a greater oversail – regardless of whether the lintels have 150mm or
225mm bearings. This way you know the detail will always work and the
functionality is assured.
The illustration shows how vertical caviclosers,
lintel cavitrays and stopends readily unite to create a robust detail.
Questions and Answers
I supply and fit replacement windows in
domestic properties. The majority of homes in my town were built prior to 1960.
When the existing windows are removed, an open cavity is exposed. I fit the
replacement windows in exactly the same mid-cavity position, but should I
really leave the cavity as it is?
It was a common practice in the 1950’s and
early 60’s for builders not to close the cavity but to rely on the window frame
to cover it. Ideally you should now improve the build detail before you
introduce the replacement window.
Consider use of Type O caviclosers. These
are flexible insulation-filled DPC sections that can be pushed into open
cavities. Their soft body adapts to the cavity width encountered.
Many replacement window companies use
type O caviclosers. They find keeping some in the back of the van regularly
comes in handy! It also enables them to demonstrate to the house owner that
they are doing a good job by introducing a thermal and DPC enhancement.
(Footnote: it is also possible to use the Quickcloser in certain existing
applications if you require a rigid faceplate finish).
You refer to closing and protecting the walls, but what
about the ceiling?
There is a Cavi60 Loft hatch available that
is ready to screw into a trimmed opening.
If is self-finished, requires no painting,
and has a one hour fire integrity rating.
END OF EXTRACT
2004.
