Detailed Design Consideration
Building Regulations
Typical U-value requirements for new build external
walls are shown below:
| |
U-value (W/m2K) |
| England, Wales & NI |
0.27 |
| Scotland |
0.25 |
| Ireland |
0.27 |
.
Thermal Bridging at Junctions and Openings
With improving U-values for walls and other elements, it has
become necessary to ensure that no cold bridges exist at junctions
and openings. In England and Wales special robust details have been
issued for housing. If these details are not suitable, the designer
may use other approved details or calculate heat loss through the
cold bridges.
The details at the top of page 7 demonstrate the methods prescribed
by the published Accredited Construction Details for applications
in masonry cavity walls. A full set of details is available in the
document "Accredited Construction Details" published by the
Planning Portal.
These details require a proprietary closer to be used at openings
for sills and jambs with a minimum thermal resistance of 0.45
m
2K/W. Polyfoam System Closer and Polyfoam Combi Closer
easily exceed this performance requirement.
Click
here for details of these products.
Wall Ties
Apart from structural considerations, which are obviously
pre-eminent, the correct specification of wall ties is crucial in
two additional respects. Firstly, it is necessary that the tie does
not compromise performance of the cavity wall insulation with
regard to liquid water penetration.
Secondly, that the U-value calculation method takes into
consideration the number of wall ties per square metre as well as
the cross sectional area, and thermal performance of the tie. Where
cavities are 100mm or less, stainless steel wire ties are
recommended as they have negligible effect on the wall U-value.
Where the cavity is over 100mm wide, a more robust tie is required,
which can have a significant effect on the wall U-value.
Masonry Cavity Walls and Fire Performance
Open cavities must be stopped to prevent the passage of fire.
This is required at specific intervals and the cavity stop has to
provide at least 30 minutes fire resistance. If the cavity is
fully or partially filled and is built in accordance with
Diagram 13 of Approved Document B, fire barriers are not
required.
Prevention of Liquid Water Penetration
Prevention of liquid water penetration from the outer to inner
leaf is one of the major considerations when designing cavity
walls. The selection of appropriate materials and pointing methods
for the outer leaf are crucial.
Cavity Trays
Cavity trays should be provided:
- at all interruptions of the cavity, such as lintels and sleeved
vents and ducts
- above insulation that stops short of the top of the wall
Cavity trays should rise at least 140mm within the cavity, be
self-supporting or fully supported with joints lapped and sealed.
Stop ends should be provided to the ends of all cavity trays.
Weepholes should be provided at not more than 900mm centres to
drain each cavity tray.
Condensation Risk
In cavity wall construction it is necessary to ensure that
there is no danger of condensation forming within the inner
leaf.
Condensation may have a detrimental effect on the thermal
performance of a structure or cause damp on the inside. Unfaced
mineral wool products, being 'vapour open' offer virtually no
resistance to the passage of water vapour. The Knauf Insulation
Academy Technical Advisory Centre are able to carry out
condensation risk calculations if further reassurance is
required.
Typical "Accredited Construction Details"
Methods for insulating masonry cavity walls
The recommended masonry cavity wall system is full fill,
either injected (Supafil) or built in slabs using Crown
DriTherm Cavity Slab.
These systems not only provide the best U-value to wall width ratio
but are also the lowest in cost. Even with dense concrete blocks it
is possible to achieve very high thermal performance in a
manageable wall width. The systems shown require no fire stopping
and have
BBA
Certificate clearance to the same height as partial fill systems.
With formal guarantees against liquid water penetration and a long
history of use in housing construction, they offer peace of mind
for the specifier, builder and client alike.
If it is not possible to fully fill a cavity owing to
insurance/guarantee company restrictions, the recommended system is
Polyfoam Cavityboard. The robust nature of Polyfoam Cavityboard
allows the formation of a shiplap detail on all edges of the board,
offering a further guarantee against water penetration and cold
bridging that might occur with plain square edge cellular plastic
boards.
Typical Specification Clauses
1) Partial fill: Crown DriTherm Cavity
Slab
Crown DriTherm Cavity Slab, ......mm thick to be located in the
cavity of all external masonry cavity walls. The residual
cavity width to be 25mm/ 50mm. Insulation to be retained against
the cavity face of the inner leaf of masonry by means of insulation
retaining wall ties as manufactured by ..... .
The wall ties to have one of their drip centres half-way
across the residual cavity. The first run of wall ties to be
located at 600mm centres horizontally (at a level to be decided by
the specifier). Ties thereafter to be at 450mm centres vertically
and a maximum of 600mm horizontally.
All work under construction must be protected overnight and
during adverse weather conditions in accordance with BS 5628: Part
3: 1985.
Alternatively, refer to NBS clause: F30/12,151
2) Partial fill: Polyfoam Cavityboard
Polyfoam Cavityboard, ......mm thick, to be located in the cavity
of all external masonry cavity walls; residual cavity width to be
....mm. Insulation to be retained against the cavity face of the
inner leaf of masonry by means of insulation retaining wall ties as
manufactured by ....... .
The wall ties to have one of their drip centres half-way across the
residual cavity. The residual cavity width to be 25mm/50mm. The
first run of wall ties to be located at the bottom level of the
insulation (to be decided by the specifier). Ties to be at 450mm
centres vertically and a maximum of 900mm horizontally.
All work under construction must be protected overnight and during
adverse weather conditions in accordance with BS 5628: Part 3:
1985.
All work to be in strict accordance with this specification and the
recommendations contained in Knauf Insulation's literature.
Alternatively, refer to NBS clause: F30/12,151
3) Crown Supafil
Crown Supafil Cavity Wall Insulation glass mineral wool to be
injected into the cavity by a Knauf Insulation/BBA Approved
Installer.
New cavity walls to receive Crown Supafil Cavity Wall Insulation by
injecting insulation into the cavity should be constructed so that
insulation cannot penetrate ventilation ducts or pass through to
cavities in adjoining buildings or compartments which are not to be
insulated.
Close fitting ducts or sleeves should be installed across the
cavity to serve air bricks and other ventilation openings. Cavity
barriers should be installed at junctions with other properties and
compartments, and as required by the Building Regulations.
Existing cavity walls to be inspected by the Approved Installer and
all necessary builder's work carried out prior to Crown Supafil
Cavity Wall Insulation being injected.
All work to be in strict accordance with the procedures laid out in
the relevant BBA Certificate and the Knauf Insulation "Operators"
and "Survey & Assessment" manuals.
Alternatively, refer to NBS clause: P11/40,220
4) Full fill: Crown DriTherm Cavity Slab
All external walls to be insulated during construction by
completely filling the cavities with Crown DriTherm Cavity Slab
......mm thick.
The first run of wall ties to be located at 600mm centres
horizontally (at a level to be decided by the specifier).
Subsequent runs of wall ties to be at not more than 900mm centres
horizontally, or as otherwise required by the structure, and at
450mm vertically.
All work under construction must be protected overnight and during
adverse weather conditions in accordance with BS 5628: Part 3:
1985.
Alternatively, refer to NBS clause: F30/10,150
5) Polyfoam System Closer
Install Polyfoam System Closers, 25mm thick x ....mm wide (to equal
cavity width) at jambs and sills of all openings in masonry cavity
walls. Secure with the proprietary ties at maximum 500mm
centres.
Ensure that closers fit tightly to each other and to the
lintel. The closer at the jamb must completely lap the closer at
the sill.
Alternatively, refer to NBS clause: F30/18,180
6) Polyfoam Combi Closer
Install Polyfoam Combi Closers, 25mm thick x ....mm wide (to be
cut, where necessary, to equal cavity width) at jambs and sills of
all openings in masonry cavity walls. Secure with the proprietary
ties at maximum 500mm centres.
Ensure that closers fit tightly to each other and to the
lintel. The closer at the jamb must completely lap the closer at
the sill.
Alternatively, refer to NBS clause: F30/18,180
7) Polyfoam Linerboard
Polyfoam Linerboard of insulation thickness .....mm with integral
9.5mm plasterboard to be fixed directly to inner masonry leaf by
adhesive*/ plaster dabs*/ mechanical fixing* (*delete as required)
using the method and materials recommended in Knauf Insulation
literature.
When the adhesive has set, secure with two .....
(manufacturer/reference) mechanical fixings along the top edge of
each board (for adhesive/plaster dabs).
Alternatively, refer to NBS clause: K10/175,185,205