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Cavity Trays CID Report Issues 5 A

Author/Issuer: Cavity Tray Standards    Added:

Upgraded Designs  – Part L and Part E

Cavity Trays Ltd has identified an opportunity to realign the specification of an existing product, so that the demands of Building Regulations Part L and Part E can be combined. In future data sheets of any realigned products will carry a ‘SAF’ marking.  SAF denotes sound and fire integrity.

This realignment refers to the Cavi 240 Type PWIB Party Wall Integrity Barrier. (Cavi 240 denotes 240 minutes fire integrity). Upgrading of this product permits it to be employed within the cavity to satisfy the total demands of both sound control and fire integrity – SAF, whilst maintaining the Cavi rating.

From January 2006 data sheets released bearing the SAF mark will indicate realigned and upgraded SAF products. At the same time we are releasing a number of new products, each of which offers advantages to the designer and the contractor in achieving quick and easy regulation compliance. Please request your copy now for early January release.


1Masonry Absorption Area Appraisal provides guidance on how much rainwater can permeate a given area of masonry. The purpose of applying MAAA is to determine just how much water is likely to enter the exterior skin of a cavity wall when severe weather is experienced. When saturation point is reached, penetrating water starts to flow down the inside face of the exterior skin. It is perhaps surprising how much water can permeate just one square metre of masonry – 2.25 litres per sq metre per hour in cement-based (reconstructed stone) material has been measured.

2In a typical new cavity wall there will be mortar debris and fines. These wash down with the water during the initialsaturations of the wall following completion. This is one of the reasons why new homes completed at the beginning of a summer will start to evacuate debris through weepholes later in the year as winter commences. It is simply full saturation being reached for the first time. The caviweeps within our range promote straight-flow routes with drop or sheltered outlets. These are purposely designed to function despite directly blowing wind and rain conditions. Importantly, where models incorporate screens to guard against insect infestation, the screens and / or baffles are so shaped not to hinder water and debris wash.

Cavity Trays Ltd assesses the optimum extent of weep evacuation by applying MAAA when appraising schemes for clients. When specifying, stipulate clearly the use of approved cavitrays and approved caviweeps so both the protection and evacuation are to the intended standard. Use of our approved cavitrays and approved caviweeps are necessary to provide functionality plus performance warranty for your benefit.

Weeps are essential to avoid water retention within a cavity wall as witnessed in these examples.

Cavity Trays Ltd is an Inaugural Partner

MMC Alliance stands for Modern Methods of Construction and is the name under which the recently launched Kingspan Off Site System Building operation is being promoted.

Cavity Trays Ltd is an Inaugural Partner of the Off-Site Partner Alliance. We are delighted our products have been identified as providing the functionality and long-term integrity sought by Kingspan. (Note: LABC Product Type Approval and European Technical Approval are two of the standards held by Cavity Trays Ltd).

Thin Joint Construction

With any thin joint block construction the internal skin is often raised first, and the external skin follows, albeit both skins are raised in stages. Conventional dpc, creased dpc or trays are commonly built into the inside skin (to support them), and are subsequently incorporated within the external skin when the external skin masonry reaches such levels.

The problem with this approach can be twofold:

Maintaining air tightness of the inner skin must be considered. (10m³/hr/m² is the worse standard that will be permitted under the new legislation). Whilst the inside skin must not be perforated it is obliged to accommodate numerous intrusions of dpc. This exercise is even more demanding when any dpc element is three-dimensional (stepping) as is encountered with a sloping intersection.

3Any protective shaped medium that originates in one skin and ends up in another is reliant on every course in both skins being in perfect harmony. If the course levels differ, the dpc element can be prevented from adopting the optimum shape.

The above scenario applies to all construction, but thin joint construction has smaller joints and accuracy is the keyword. There is considerable benefit therefore in adopting an approach that removes dependence on the inside skin to provide support.

The use of the right type of ready moulded dpc cavitray for sloping intersections is the key. The use of adjustable cavity upstand models where the upstand is also self-supporting means there is no need to build into the inner skin. Thus air tightness concerns need not arise. The trays are only built into the external skin and therefore the problem of inner and outer skin levels not being perfectly aligned is not an issue. At all times the adjustable cavity upstand protects the entire cavity, but without interfering with the inner skin status.

See Type X for sloping abutments. Type G for horizontal abutments.
Type C for common openings. Arresting Barriers for location protection. 

OPC Mortars and Lime Based Mortars

Lime based mortars have improved ‘breathability’ compared with OPC mortars. Thus they are more tolerant to thermal and moisture driven movement and less likely to suffer frost failure. Lime mortars are now being used on more new-build projects as well as on traditional restoration and conservation work. The petheleyne and polypropylene used in the manufacture of our preformed cavitrays is effectively inert. Thus correctly specified OPC or lime mortars cannot decay the material. Carbonation and sulphate attack associated with lime mortar do not attack or alter the protective cavitray material used in the manufacture of our products.

Combined Cavity Fire Stop and DPC Protection

Cavity Trays Type CFIS (Cavity Fire Integrity Stop) offers a longer fire integrity rating than is commonly offered within the industry. (CFIS is designated Cavi 240, signifying 240 minutes/four hours rating, available at no greater price than alternatives offering one hour). This product has also been available since November 2004 attached to a protective cloak so that fire integrity and damp protection are addressed at the same time. Specify clearly if you require this style. For options see individual entries of Type CFIS and Type CD and request the combined format.

 Part L Emerges as a Four Parter

The new ADs for Part L have been released after much waiting. The previous documents L1 and L2 have been replaced with four separate documents: L1a and L1b deal with dwellings. L2a and L2b deal with buildings other than dwellings. The four are technically issued as interim documents, meaning the proposals could change again. It is anticipated they will come into force on 6th April 2006. Whilst previous revisions of Part L addressed U-values, the elemental approach is now superseded. Carbon emissions calculation (SAP) is the new method of compliance demonstration. 

Part L – Conservatories and Concern over Regulations and Standards

The regulations are not demanding improvements of the design or manufacture of conservatories under 30m² to address improved energy efficiency and the control of passive temperature variables. Currently the new legislation key objective to improve summer comfort of dwellings (and thus minimise growth of domestic air conditioning) has overlooked such structures. However, regulations do demand extensions of any living area must be built to a standard that prevents dampness from permeating / affecting those parts of the structure that should be kept dry. Regardless whether an extension to a property is conventionally constructed of masonry, or is a predominately glass conservatory, the Building Regulations still apply. The determining factors with conservatories include the proposed use of the area and its relationship and configuration with the original property. Preformed cavitrays, vertical closers and damp courses are available to permit easy compliance.

Site Preparation – Resistance to Contaminants and Moisture

The ODPM has issued a new version of Approved Document C and BS 5390 provides the guidance on how to conduct identification and preparation of a construction site. The investigation requirement includes identification of solid, liquid or gaseous contaminants that may be present.  We recommend close study of our oversite membranes and cavity barriers to prevent methane and radon gases from entering the building envelope. These are listed within our new manual number 16.


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