Mc01 - Masonry cavity walls - Full-fill with built-in
glasswool
Advantages
- Low system width
- Low cost
- Readily available from local stockists
- All products have BBA Certification for all exposure zones
- Product guaranteed to resist moisture penetration
- 35 year performance heritage
- Compression fit between slabs prevents moisture penetration and
cold bridging at joints
- Dritherm Cavity Slabs are Zero ODP and Zero GWP insulation
products
- The manufacture of Dritherm Cavity Slab products has a very low
environmental impact
|
  |
Products
DriTherm Cavity Slab and DriTherm
Cavity Slab 34 and 32 are a range of lightweight semi
rigid or rigid slabs of glass mineral wool with a water repellent
additive. It is dimensioned to fit between wall ties at standard
spacings. Its manufacture has a very low impact on the environment
and is classified as Zero ODP and Zero GWP*
* Ecohomes and Code for Sustainable Homes
classification
Typical construction
Brick or block outer leaf (which may be rendered), brick or
block inner leaf with cavity fully filled with DriTherm Cavity
Slab. Internal finish of plasterboard on dabs. For buildings from
12m to 25m high, BBA Certificate No 95/3212 imposes additional
requirements. For additional information see the BBA Certificate or
contact our Technical Advisory Centre on 01744 766666.
Guarantee
All products in the DriTherm Cavity Slab range are formally
guaranteed for 50 years to resist the transmission of liquid water
from the outer masonry leaf to the inner masonry leaf in new
external cavity walls and to maintain the original manufactured
thermal conductivity at normal building temperatures. Masonry
includes bricks, blocks and dressed and reconstituted stone. The
guarantee is available by application from Knauf Insulation.
Performance
Thermal performance
- DriTherm Cavity Slab has a thermal conductivity of either 0.035
W/mK or 0.037 W/mK depending on thickness (Table 35)
- DriTherm Cavity Slab 34 has a thermal conductivity of 0.034
W/mK (Table 34)
- DriTherm Cavity Slab 32 has a thermal conductivity of 0.032
W/mK (Table 34)
Table 34: Typical U-values for
masonry cavity walls fully insulated with DriTherm Cavity
Slab
| |
U-values (W/m2K) for brick outer
leaf/cavity/100mm block inner leaf, plasterboard on
dabs
|
|
Insulation
Thickness
(mm)
|
Dense
block
( λ=1.13)
|
Medium
block
( λ= 0.51)
|
Lightweight
aggregate
( λ=0.34)
|
Standard
aircrete
( λ=0.16)
|
Lightweight
aircrete
( λ=0.11)
|
| DriTherm Cavity Slab 32 |
| 300 (3x100) |
0.12 |
0.12 |
0.11 |
0.11 |
0.11 |
| 200 (2x100) |
0.15 |
0.15 |
0.15 |
0.14 |
0.14 |
| 170 (2x85) |
0.17 |
0.17 |
0.17 |
0.16 |
0.15 |
| 150 (2x75) |
0.19 |
0.19 |
0.18 |
0.18 |
0.17 |
| 130 (2x65) |
0.22 |
0.21 |
0.21 |
0.20 |
0.20 |
| 100 |
0.27 |
0.26 |
0.26 |
0.24 |
0.23 |
| 85 |
0.31 |
0.30 |
0.29 |
0.27 |
0.26 |
| 75 |
0.34 |
0.33 |
0.32 |
0.30 |
0.28 |
| 65 |
0.38 |
0.37 |
0.36 |
0.33 |
0.32 |
| DriTherm Cavity Slab 34 |
| 300 (3x100) |
0.12 |
0.12 |
0.12 |
0.12 |
0.12 |
| 200 (2x100) |
0.17 |
0.17 |
0.17 |
0.16 |
0.16 |
| 170 (2x85) |
0.18 |
0.18 |
0.17 |
0.17 |
0.16 |
| 150 (2x75) |
0.20 |
0.20 |
0.19 |
0.18 |
0.18 |
| 100 |
0.28 |
0.28 |
0.27 |
0.25 |
0.24 |
| 85 |
0.32 |
0.31 |
0.31 |
0.28 |
0.28 |
| 75 |
0.36 |
0.35 |
0.34 |
0.31 |
0.29 |
Notes: The U-values have been calculated assuming that all
walls are lined with plasterboard on dabs. Block sizes assumed to
be 440 x 215mm, mortar joints assumed to be 10mm wide, wall ties
assumed to be stainless steel with a cross-sectional area of no
more than 12.5mm2 for structural cavities up to 175mm wide. Above
175mm use two part wall ties, these typically are stainless steel
and have a cross sectional area of 60mm2. The U-values have been
calculated to BS EN ISO 6946: 1997 and BR 443.
Table 35: Typical U-values for
masonry cavity walls fully insulated with DriTherm Cavity
Slab
| |
U-values (W/m2K) for brick outer leaf/cavity/100mm
block inner leaf, plasterboard on dabs
|
|
Insulation
Thickness
(mm)
|
Dense
block
( λ=1.13)
|
Medium
block
( λ= 0.51)
|
Lightweight
aggregate
( λ=0.34)
|
Standard
aircrete
( λ=0.16)
|
Lightweight
aircrete
( λ=0.11)
|
| 300 (3x100) |
0.13 |
0.13 |
0.13 |
0.13 |
0.12 |
| 200 (2x100) |
0.18 |
0.18 |
0.18 |
0.17 |
0.17 |
| 170 (2x85) |
0.19 |
0.19 |
0.19 |
0.19 |
0.17 |
| 150 (2x75) |
0.22 |
0.21 |
0.21 |
0.20 |
0.20 |
| 130 (2x65) |
0.25 |
0.24 |
0.23 |
0.22 |
0.21 |
| 125 (75+50) |
0.25 |
0.24 |
0.24 |
0.22 |
0.21 |
| 115 (65+50) |
0.26 |
0.26 |
0.25 |
0.23 |
0.22 |
| 100 |
0.30 |
0.29 |
0.29 |
0.27 |
0.25 |
| 85 |
0.35 |
0.34 |
0.33 |
0.30 |
0.28 |
| 75 |
0.38 |
0.37 |
0.36 |
0.33 |
0.31 |
Notes: The U-values have been calculated assuming that all
walls are lined with plasterboard on dabs. Block sizes assumed to
be 440 x 215mm, mortar joints assumed to be 10mm wide, wall ties
assumed to be stainless steel with a cross-sectional area of no
more than 12.5mm2for structural cavities up to 175mm wide.
Above 175mm use two part wall ties, these typically are stainless
steel and have a cross sectional area of 60mm2. The U-values have
been calculated to BS EN ISO 6946: 1997 and BR 443.
Fire performance
All products in the DriTherm Cavity Slab range are classified as
Euroclass A1 to BS EN ISO 13501-1.
Moisture resistance
All products in the DriTherm Cavity Slabs range contain a
water-repellent silicone additive to ensure that no liquid water is
able to pass through the slab and reach the inner leaf of
masonry.
Tests by the British Board of Agrément confirm that the range of
DriTherm Cavity Slabs will not transmit water to the inner leaf.
Nor will they transmit moisture by capillary action across the
cavity or from below DPC level.
Vapour resistance
All products in the DriTherm Cavity Slabs range have negligible
water vapour resistance, allowing water vapour to pass freely
through the slab.
Installation
The thickness of DriTherm Cavity Slab and the cavity width
should be designed within the tolerances given in Table 35 (right).
It is not possible to compress DriTherm Cavity Slab during
installation because its resilience will be enough to dislodge
bricks before the mortar has set. DriTherm Cavity Slab should be
kept clean and free from mortar droppings. All joints should be
closely butted. Any cutting and fitting should be neatly done and
not distort the layers of glasswool which comprise the material –
see ‘Problems to avoid’ on opposite page. DPCs should be installed
to ensure that penetrating water is directed only to the outer
leaf.
The illustrations above outline technique only, and do not imply
that the outer leaf must be built first.
Construction practice will vary from site to site. Where design
details differ from those illustrated please do not hesitate to
contact Knauf Insulation for any clarification required.
Wall ties
DriTherm Cavity Slab is supplied in 1200 x 455mm slabs for use
between wall ties at 450mm vertical centres. Standard butterfly,
stainless steel wire ties and vertical twist ties are suitable, as
are all ties with a positive drip which will penetrate the top edge
of the DriTherm Cavity Slab halfway across its width. The use of
any other type of tie should be referred to Knauf Insulation.
Advice should also be sought from the wall tie manufacturer as to
the maximum cavity width for which the use of a specific tie is
approved.
Generally, rows of wall ties should be at 450mm vertical spacing
and at horizontal spacings of not more than 900mm or as otherwise
required by the structure. Where whole rows of ties are at
different vertical spacing, DriTherm Cavity Slab should be cut to
course, allowing an extra 5mm for compression to form close butt
joints. Where extra ties are required, e.g. at the side of
openings, DriTherm Cavity Slab should be cut and fitted carefully
around them. When off-cuts of DriTherm Cavity Slab are needed, the
slabs can be cut with a long bladed knife or bricklayer’s
trowel.
Further recommendations
Above, below and beside openings, where cut strips of DriTherm
Cavity Slab may be needed, particular care should be taken to fit
closely and ensure work is clean and free from debris. At the end
of the day’s work, or during rain, any exposed DriTherm Cavity Slab
should be covered. If DriTherm Cavity Slab is terminated vertically
at an open cavity, a vertical DPC must be fitted up the inside face
of the outer leaf to ensure that any mortar droppings on exposed
edges do not bridge the cavity.
Table 36: Permitted deviations in
cavity widths
|
DriTherm Cavity Slab size and nominal cavity width Thickness
(mm)
|
Permitted cavity deviation (mm)
|
| 150 or more |
-0 or +20 |
| 125 |
-0 or +15 |
| 100 |
-0 or +15 |
| 85 |
-0 or +15 |
| 75 |
-0 or +15 |
| 65 |
-0 or +10 |
| 50 |
-0 or +10 |
Installation sequence
Problems to avoid
 |
 |
| Do not push
slabs into the cavity. Mortar snots may be dislodged and bridge the
cavity. This can happen all too easily where a change in the
leading leaf occurs and care should be exercised at such positions
to ensure correct application. |
Do not tear or
impale DriTherm Cavity Slab. If there are protrusions into the
cavity, DriTherm Cavity Slab should be carefully cut to fit,
particularly where there are extra wall ties around openings. |
 |
 |
| Do not position
DriTherm Cavity Slab on slabs which have not been cleaned of mortar
droppings. |
When using small
off-cuts, the face of the slabs and not the edge, shall be
positioned against the wall surface. |
|
Typical specification
|
|
All external walls to be insulated during construction by
completely filling the cavities with DriTherm Cavity Slab/34*/32*
......mm thick. (*Delete as appropriate)
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
|
Typical section
Wider Cavities
With the move towards carbon neutral housing, the use of wider
cavities in masonry cavity walls is likely to become much more
common.
DriTherm Cavity Slab and Rocksilk DriTherm Cavity Slab can be
installed in multiple layers to fully fill cavities up to 300mm
wide. Proprietary cavity closers and folded steel lintels are
widely available for cavities up to 100mm wide. For cavities over
100mm wide, the choice of proprietary products is more limited. For
cavities over 150mm wide, the detailing of openings may need to be
altered radically.
Jambs
Cavities over 150mm wide present particular problems at the
jambs. One solution to this is to use a plywood liner to the
opening that can also double up as a former for the window frame.
The diagrams below indicate this option for a 200mm wide
cavity.
The plywood box answers a number of problems:
- It closes off the cavity
- It can be sealed against the inner and outer leaf to limit air
leakage
- It provides an accurate template for the window frame
- The temporary polythene front cover provides weather protection
until the window frame is installed
- The window frames can be installed after the brickwork is
complete –lessening the risk of damage from mortar, etc.
Lintels
The use of separate lintels for the inner and outer leaf becomes
more common and practical as the cavity width increases. Separate
lintels not only avoid the thermal bridging problems of one piece
steel lintels, but they also provide the designer with a greater
level of design freedom. A concrete or aircrete lintel for the
inner leaf is a simple, economic and firesafe option. The lintel in
the outer leaf can be anything from reconstituted stone to plain
brickwork supported on a steel angle.
Wall ties
DriTherm Cavity
Slab is supplied in 1200 x 455mm slabs for use between wall ties at
450mm vertical centres. For cavities up to 75mm wide, stainless
steel wire ties are recommended by Knauf Insulation because:
- Stainless steel has a thermal conductivity of 17 W/mK, compared
with 50 W/mK for mild steel
- Wire ties have a much smaller cross-sectional area than flat
metal ties For these two reasons, stainless steel wire ties present
a smaller thermal bridge through the cavity insulation than other
types of metal wall tie.
For cavities up to 175mm wide, double triangle stainless steel
wire ties are available up to 300mm long.
These ties conform to BS EN 845-1 as a Type 3 tie. It is
recommended that they are embedded 85mm into the inner leaf to help
keep the cantilevered section of the tie horizontal during the
build.
With the use of wider cavities, it is anticipated that other
designs of stainless steel wire ties will be developed for cavities
up to 175mm wide. For cavities over 175mm wide, Knauf Insulation
recommend the use of two part stainless steel ties. These are
available from Ancon and Wincro Metal Industries. These ties
overcome the problem of keeping a long tie horizontal when built
into the inner leaf. However, they have a much greater
cross-sectional area than wire ties and their thermalbridging
effect must be taken into account when calculating the wall
U-value. They are suitable for cavities up to 300mm wide.
