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	Brickwork outer leaf
	Crown Dritherm Cavity Slab or Rocksilk Dritherm Cavity Slab
	Suitable wall tie
	Blockwork inner leaf
	Internal plaster finish

Brick or block outer leaf (which may be rendered), brick or block inner leaf with cavity fully filled with Crown or Rocksilk DriTherm Cavity Slabs. Internal finish of plaster.

 

For buildings from 12m to 25m high, the British Board of Agrement Certificates impose additional requirements. Also the certification relates only to buildings where Knauf Insulation has given written approval for the use of Crown or Rocksilk DriTherm Cavity Slabs in the specified building

Crown DriTherm Cavity Slabs 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 have the thermal conductivity specified at normal building temperatures.
Masonry includes bricks, blocks and dressed and reconstituted stone. The guarantee is available by application from Knauf Insulation.

Crown DriTherm Cavity Slab has a thermal conductivity of between 0.035 or 0.037 W/mK depending on thickness.

Crown DriTherm Cavity Slab 32 has a thermal conductivity of 0.032 W/mK.
Crown DriTherm Cavity Slab 34 has a thermal conductivity of 0.034 W/mK.

Rocksilk DriTherm Cavity Slab 34 has a thermal conductivity of 0.037 W/mK.

	Brick outer leaf/cavity/100mm block inner leaf, plaster
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)
150 (75+75)	0.22	0.21	0.21	0.20	0.20
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
65	0.42	0.40	0.39	0.35	0.33

 

Typical U-values (W/m²K) for masonry cavity walls fully insulated with Crown DriTherm Cavity Slab 34

Typical U-values (W/m²K) for masonry cavity walls fully insulated with Crown DriTherm Cavity Slab 32

 

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 12.5mm2. The U-values have been calculated to BS EN ISO 6946: 1997.

All Crown and Rocksilk DriTherm Cavity Slabs are classified as non-combustible to BS 476: Part 4: 1970 (1984) and Euroclass A1, Class 1 surface spread of flame to BS 476: Part 7: 1997 and Class 'O' to the Building Regulations.

All Crown and Rocksilk DriTherm Cavity Slabs contain a water-repellent silicone binder 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 all Crown and Rocksilk 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.

All Crown and Rocksilk DriTherm Cavity Slabs offer virtually no resistance water vapour, allowing the water vapour to pass freely through the slab.

All Crown and Rocksilk DriTherm Cavity Slabs are third party certified by the British Board of Agrément.

 

 

The thickness of Crown or Rocksilk DriTherm Cavity Slabs and the cavity width should be designed within the tolerances given in the table opposite. Where walls go out of tolerance, the appropriate thickness of Crown or Rocksilk DriTherm Cavity Slab should be chosen from the table.

It is not usually possible to compress Crown or Rocksilk DriTherm Cavity Slabs during installation because its resilience will be enough to dislodge bricks before the mortar has set.

Crown and Rocksilk DriTherm Cavity Slabs 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 glass wool which comprise the material - see page 24. DPCs should be installed to ensure that penetrating water is directed only to the outer leaf.

The illustrations 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.

 

1. Build up the first stage of one leaf of masonry to include the first row of ties above the commencement of the Crown or Rocksilk DriTherm Cavity Slabs. Clean mortar squeeze from the masonry and snots from any ties or cavity tray.

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Position the Crown or Rocksilk DriTherm Cavity Slab against the masonry, so that the wall tie drips are halfway across the top edge of the slabs. The Crown or Rocksilk DriTherm Cavity Slabs should be cut to course if necessary. Crown or Rocksilk DriTherm Cavity Slabs should be taken below DPC level (preferably by at least 150mm) to provide edge insulation, with no risk of capillary action. Crown or Rocksilk DriTherm Cavity Slabs does not wick. Always bring Crown or Rocksilk DriTherm Cavity Slabs to course with wall ties.

 

 

 

 

 

 

 

 

 

3. The following leaf is then built to the top level of the Crown or Rocksilk DriTherm Cavity Slabs. Do not let the second leaf overtake the Crown or Rocksilk DriTherm Cavity Slabs so as to create a trough (but see 5 regarding choice of leading leaf).

 

 

 

 

 

 

 

 

 

 

 

 

 

4. Proceed similarly with successive stages of the wall. As with normal masonry cavity construction, no mortar should remain in the cavity. Particular care should be taken to keep slab joints closely butted and free from mortar. To facilitate keeping the top edges of slabs clean it is recommended that a cavity board be used.

 

 

 

 

 

 

 

 

 

 

 

 

 

5. Building may proceed leading with either the inner or the outer leaf. When leading with the inner leaf it is recommended to build a trough not more than one brick deep at horizontal joints in Crown or Rocksilk DriTherm Cavity Slabs. The mortar joint should be struck flush inside the cavity, and any mortar droppings must be cleaned off before the next Crown or Rocksilk DriTherm Cavity Slab is fitted.

 

 

 

 

 

 

 

 

 

 

 

 

 

Crown or Rocksilk DriTherm Cavity Slabs are 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 Crown or Rocksilk 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, Crown or Rocksilk DriTherm Cavity Slabs 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, Crown or Rocksilk DriTherm Cavity Slabs should be cut and fitted carefully around them. When off-cuts of Crown or Rocksilk DriTherm Cavity Slabs are needed, the slabs can be cut with a long bladed knife or bricklayer's trowel.

Permitted deviations in cavity widths

Crown or Rocksilk DriTherm Cavity Slabs thickness (mm)	Nominal cavity width (mm)	Permitted deviation (mm)
150	150	20
125	125	15
100	100	15
85	85	15
75	75	15
65	65	10
50	50	10

Where cut strips of Crown or Rocksilk DriTherm Cavity Slabs 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, exposed Crown or Rocksilk DriTherm Cavity Slabs should be covered.

 

If a Crown or Rocksilk 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.