External Masonry Cavity Walls - Technical Detail

External Masonry Cavity Walls

External Masonry Cavity Walls

External Masonry Cavity Wall Design

Masonry Cavity Walls

Full-fill with built in Glass Mineral Wool - DriTherm® Cavity Slabs


Slabs fully-fill the cavity and knit together at joints, preventing air movement and infiltration through or around the insulation. Slabs are installed under compression, preventing moisture penetration and cold bridging at joints and are low cost, quick and easy to install


  • BBA Certified for all exposure zones
  • Non-combustible Euroclass A1 reaction to fire classification
  • A+ Generic BRE Green Guide rating

Other Build Up Options


Full-fill with built-in glass mineral wool, extra wide cavities

Knauf-Insulation-Masonry-Cavity-MC100-Solution SF

Full-fill with injected glass mineral wool

Knauf Insulation Masonry Cavity Wall insulation Glass Mineral Wool

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 the performance of the cavity wall insulation regarding liquid water penetration.

Secondly, the U-value calculation method must take into consideration the number of wall ties per square metre as well as the cross-sectional area, and thermal performance of the wall tie. Stainless steel wall ties can be used for cavities up to 150mm and are recommended because they have a low cross- sectional area minimising their impact on the thermal performance of the wall. Where wider cavities are used a two-part wall tie is recommended. These are more robust and can have a significant impact on the U-value of the wall.

In all circumstances it is recommended that the designer seeks advice from the manufacturer of the wall tie to ensure that the tie selected meets the structural requirements of the building work. Low thermal conductivity wall ties are also available and are suitable for cavities up to 300mm wide, their thermal performance is such that they have negligible impact on the U-value of the wall.

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 barrier must provide at least 30 minutes fire resistance. If the cavity is fully-filled with a ‘built-in’ non-combustible mineral fibre product as constructed in accordance with Diagram 5.3 (Diagram 9.2 in Volume 2) of Approved Document B: 2019, Volume 1, ‘Dwelling houses’ cavity 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 jointing 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 cavity 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. Weep holes should be provided at no more than 900mm centres to drain each cavity tray with at least two weep holes per cavity tray.

Condensation risk

In a properly insulated masonry cavity wall, there is negligible risk of condensation forming on the inner block leaf. Condensation may have a detrimental effect on the thermal performance of a structure or cause damp on the inside face of the wall. Un-faced mineral wool products, being ‘vapour open’ offer negligible resistance to the passage of water vapour through the wall. The Knauf Insulation Technical Support Team can carry out condensation risk calculations if further reassurance is required.

Recommended solutions

The recommended masonry cavity wall solution is full-fill, either injected (Supafil®) or built-in slabs (DriTherm® Cavity Slabs).

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 good thermal performance in a manageable wall width.

The systems shown can be installed in all types of buildings across the built environment, as detailed in their British Board of Agrément (BBA) Certificates. With formal guarantees against liquid water penetration and a long history of use, they offer peace of mind for the specifier, builder, and client alike.

Full-fill: built-in

  • External brick outer leaf

  • DriTherm® Cavity Slab 

  • Blockwork inner leaf

  • Plasterboard on dabs or wet plaster finish

Full-fill: blown-in

  • External brick outer leaf

  • Supafil®

  • Blockwork inner leaf

  • Plasterboard on dabs or wet plaster finish

Solution Optimiser and Pathfinder

Knauf Insulation Solution Product Construction Code U-value Range (High) U-value Range (Low)
Full-fill with built-in Glass Mineral Wool

DriTherm® Cavity Slab 32

Mc01 0.30 0.09
Full-fill with built-in Glass Mineral Wool

DriTherm® Cavity Slab 34


0.27 0.10
Full-fill with built-in Glass Mineral Wool DriTherm® Cavity Slab 37 Mc01 0.30 0.11
Full-fill with blown Glass Mineral Wool Supafil® 34 Mc03 0.30 0.10
Full-fill with blown Glass Mineral Wool Supafil® 40 Mc03 0.34 0.11
Full-fill with blown Glass Mineral Wool Supafil® 34 Mc04 0.49 0.28
Full-fill with blown Glass Mineral Wool Supafil® 40 Mc04 0.55 0.31


DriTherm®Cavity Slabs are a range of lightweight semi rigid or rigid slabs of Glass Mineral Wool with a water repellent additive. They are manufactured to fit between wall ties at standard vertical spacings.

Typical construction

Brick or block outer leaf (which may be rendered), brick or block inner leaf with cavity fully-filled with DriTherm® Cavity Slabs. Internal finish of 12.5mm standard plasterboard on dabs. For buildings from 12m to 25m high, British Board of Agrèment (BBA) Certificate

No 95/3212 imposes additional requirements. For additional information see the BBA Certificate or contact our Technical Support Team on 01744 766666.


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 mineral wool which comprise the product. Damp proof courses should be installed to ensure that penetrating water is directed only to the outer leaf. 


Thermal performance
  • DriTherm® Cavity Slab 37 has a thermal conductivity of 0.037 W/mK.

  • DriTherm® Cavity Slab 34 has a thermal conductivity of 0.034 W/mK.

  • DriTherm® Cavity Slab 32 has a thermal conductivity of 0.032 W/mK.

Fire performance

DriTherm® Cavity Slabs are classified as Euroclass A1 to BS EN 13501-1, which means that they are non-combustible and will not contribute to the development of a fire or flame spread.

Moisture resistance

DriTherm® Cavity Slabs contain a water-repellent silicone additive to ensure that no liquid water can pass through the slabs and reach the inner leaf of masonry.

Tests by the British Board of Agrément confirm that 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

DriTherm® Cavity Slabs have negligible water vapour resistance, allowing water vapour to pass freely through the slabs.

Typical wall/ground floor junction

Typical specification

All external walls to be insulated during construction by completely filling the cavity with DriTherm® Cavity Slab 37 */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 determined 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: 2005.

Alternatively, consult the National Building Specifications, Standard version clause/clauses…F30/10 and 150……………

Knauf Insulation specification clauses can be downloaded here.

Typical U-values for fully-filled masonry cavity walls

Cavity width (mm) Product

Medium block (l = 0.45)

High strength aircrete (l = 0.19) Standard aircrete (l = 0.15) Lightweight aircrete (l = 0.11)
300 DriTherm® Cavity Slab 32 0.11 0.10 0.10 0.10
DriTherm® Cavity Slab 34 0.11 0.11 0.11 0.10
DriTherm® Cavity Slab 37 0.12 0.12 0.11 0.11
200 DriTherm® Cavity Slab 32 0.15 0.15 0.14 0.14
DriTherm® Cavity Slab 34 0.16 0.15 0.15 0.15
DriTherm® Cavity Slab 37 0.17 0.16 0.16 0.16
150 DriTherm® Cavity Slab 32 0.19 0.18 0.17 0.17
DriTherm® Cavity Slab 34 0.20 0.19 0.18 0.18
DriTherm® Cavity Slab 37 0.21 0.20 0.20 0.19
125 DriTherm® Cavity Slab 32 0.22 0.21 0.20 0.20
DriTherm® Cavity Slab 34 0.23 0.22 0.21 0.20
DriTherm® Cavity Slab 37 0.25 0.23 0.23 0.22
100 DriTherm® Cavity Slab 32 0.26 0.25 0.24 0.23
DriTherm® Cavity Slab 34 0.27 0.26 0.25 0.24
DriTherm® Cavity Slab 37 0.29 0.27 0.27 0.25
85 DriTherm® Cavity Slab 32 0.30 0.28 0.27 0.26
DriTherm® Cavity Slab 34 0.31 0.29 0.28 0.27
DriTherm® Cavity Slab 37 0.33 0.31 0.30 0.28

Note: The Uvalues have been calculated assuming that all walls are lined with 12.50mm standard plasterboard (0.190W/mK) on dabs on standard blocks with 10mm mortar joints. Wall ties assumed to be stainless steel at 2.5 per m2 with a cross-sectional area of no more than 12.5mm2 for structural cavities up to 100mmwide. For cavities up to 150mm, the cross-sectional area of wall ties is assumed to be24mm2. For cavities greater than 150mm, the cross-sectional area of wall ties is assumed to be 60mm2.Air gap correction level is zero. Multiple layers are required for several of the solutions detailed above.


The thickness of DriTherm® Cavity Slab and the cavity width should be designed to suit the thickness of the proposed insulation material. 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 glass/rock mineral wool which comprise the material. Cavity trays and damp-proof courses (DPC`s) should be installed to ensure that penetrating water is directed only to the outer leaf.

The illustrations below 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 Technical Services Team. 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 and during rainy periods, any exposed DriTherm® Cavity Slabs 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.

Problems to avoid

Contact our Technical Support Team