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	Brick outer leaf
	Crown DriTherm Cavity Slabs
	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 DriTherm Cavity Slabs. Internal finish of plaster. For buildings from 12m to 25m high, British Board of Agrément Certificate No 95/3212 imposes additional requirements. Also the certification relates only to buildings where Knauf Insulation has given written approval for the use of  Crown 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 a thermal conductivity of between 0.032 and 0.037 W/mK 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

Crown DriTherm Cavity Slabs have a thermal conductivity between 0.032 and 0.037 W/mK.
The tables give U-values for typical constructions insulated with Crown DriTherm Cavity Slabs.

Insulation thickness (mm)	Brick outer leaf/cavity/100mm block inner leaf, plasterboard on dabs
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)
Block thickness (mm)	100		100		100		100		100
Crown DriTherm Cavity Slab
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
Crown DriTherm Cavity Slab 34
150 (75 +75)	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
65	0.40		0.39		0.37		0.34		0.32
Crown DriTherm Cavity Slab 32
200 (100+100)	0.15		0.15		0.15		0.14		0.14
175 (100+75)	0.17		0.17		0.17		0.16		0.16
150 (75+75)	0.20		0.19		0.19		0.18		0.18
130 (65+65)	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

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 for 100mm wide structural cavities and Ancon ST1 ties for larger cavities. The U-values have been calculated to BS EN ISO 6946: 1997.

Fire performance

Crown 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.

Moisture resistance

Crown DriTherm Cavity Slabs contain a water repellent silicone binder to ensure that no liquid water is able to pass through the slabs and reach the inner leaf of masonry. Tests by the British Board of Agrément confirm that Crown 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

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

 

Crown DriTherm Cavity Slabs are 3rd party certified by the British Board of Agrément.

 

The thickness of Crown 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 DriTherm Cavity Slabs should be chosen from the table.

It is not possible to permanently compress Crown DriTherm Cavity Slabs, either during or post installation because the products resilience is sufficient enough to dislodge bricks before the mortar has set.

Crown 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. 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.

Wall Ties

Crown 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 DriTherm Cavity Slabs halfway across their 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 spacings, Crown DriTherm Cavity Slabs should be cut to course, allowing an extra 5mm for compression to form closed butt joints. Where extra ties are required, e.g. at the side of openings, Crown DriTherm Cavity Slabs should be cut and fitted carefully around them. When off-cuts of Crown DriTherm Cavity Slabs are needed, the slabs can be cut with a long bladed knife or bricklayer's trowel.

 

Installation Sequence

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

 

 

 

 

 

 

 

 

 

2. Position the Crown DriTherm Cavity Slabs against the masonry, so that the wall tie drips are halfway across the top edge of the slabs. The Crown DriTherm Cavity Slabs should be cut to course if necessary. Crown 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 DriTherm Cavity Slabs do not not wick. Always bring Crown DriTherm Cavity Slabs to course with wall ties.

 

 

 

 

 

 

3. The following leaf is then built to the top level of the Crown DriTherm Cavity Slabs. Do not let the second leaf overtake the Crown 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 batten or board is 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 the Crown DriTherm Cavity Slabs. The mortar joint should be struck flush inside the cavity, and any mortar droppings must be cleaned off before the next  layer of Crown DriTherm Cavity Slabs are fitted.

 

 

 

 

 

Permitted deviations in cavity widths

Crown DriTherm Cavity Slab thickness (mm)	Nominal cavity width (mm)	Permitted deviation (mm)
200	200	20
175	175	20
150	150	20
130	130	15
125	125	15
100	100	15
85	85	15
75	75	15
65	65	10
50	50	10